Presented here is a procedure to express and purify myosin 5a followed by a discussion of its characterization, using both ensemble and single molecule in vitro fluorescence microscopy-based assays, and how these methods can be modified for the characterization of nonmuscle myosin 2b.
Myosin proteins bind and interact with filamentous actin (F-actin) and are found in organisms across the phylogenetic tree. Their structure and enzymatic properties are adapted for the particular function they execute in cells. Myosin 5a processively walks on F-actin to transport melanosomes and vesicles in cells. Conversely, nonmuscle myosin 2b operates as a bipolar filament containing approximately 30 molecules. It moves F-actin of opposite polarity toward the center of the filament, where the myosin molecules work asynchronously to bind actin, impart a power stroke, and dissociate before repeating the cycle. Nonmuscle myosin 2b, along with its other nonmuscle myosin 2 isoforms, has roles that include cell adhesion, cytokinesis, and tension maintenance. The mechanochemistry of myosins can be studied by performing in vitro motility assays using purified proteins. In the gliding actin filament assay, the myosins are bound to a microscope coverslip surface and translocate fluorescently labeled F-actin, which can be tracked. In the single molecule/ensemble motility assay, however, F-actin is bound to a coverslip and the movement of fluorescently labeled myosin molecules on the F-actin is observed. In this report, the purification of recombinant myosin 5a from Sf9 cells using affinity chromatography is outlined. Following this, we outline two fluorescence microscopy-based assays: the gliding actin filament assay and the inverted motility assay. From these assays, parameters such as actin translocation velocities and single molecule run lengths and velocities can be extracted using the image analysis software. These techniques can also be applied to study the movement of single filaments of the nonmuscle myosin 2 isoforms, discussed herein in the context of nonmuscle myosin 2b. This workflow represents a protocol and a set of quantitative tools that can be used to study the single molecule and ensemble dynamics of nonmuscle myosins.
Myosins are motor proteins that exert force on actin filaments using the energy derived from adenosine triphosphate (ATP) hydrolysis1. Myosins contain a head, neck, and tail domain. The head domain contains the actin-binding region as well as the site of ATP binding and hydrolysis. The neck domains are composed of IQ motifs, which bind to light chains, calmodulin, or calmodulin-like proteins2,3. The tail region has several functions specific to each class of myosins, including but not limited to the dimerization of two heavy chains, binding of cargo molecules, and regulation of the myosin via autoinhibitory interactions with the head domains1.
The motile properties of myosin vary greatly between classes. Some of these properties include duty ratio (the fraction of myosin's mechanical cycle in which the myosin is bound to actin) and processivity (the ability of a motor to make multiple steps on its track before detachment)4. The over 40 classes of myosins were determined based on sequence analyses5,6,7,8. The class 2 myosins are classified as "conventional" since they were the first to be studied; all other classes of myosins are, therefore, classified as "unconventional."
Myosin 5a (M5a) is a class 5 myosin and is a processive motor, meaning that it can take multiple steps along actin before dissociating. It has a high duty ratio, indicating that it spends a large part of its mechanical cycle bound to actin9,10,11,12,13,14. In common with other myosins, the heavy chain contains an N-terminal motor domain that includes both an actin-binding and an ATP hydrolysis site followed by a neck region that serves as a lever-arm, with six IQ motifs that bind to essential light chains (ELC) and calmodulin (CaM)15. The tail region contains α-helical coiled-coils, which dimerize the molecule, followed by a globular tail region for binding cargo. Its kinetics reflect its involvement in the transport of melanosomes in melanocytes and of the endoplasmic reticulum in Purkinje neurons16,17. M5a is considered the prototypical cargo transport motor18.
Class 2 myosins, or the conventional myosins, include the myosins that power contraction of skeletal, cardiac, and smooth muscle in addition to the nonmuscle myosin 2 (NM2) isoforms, NM2a, 2b, and 2c19. The NM2 isoforms are found in the cytoplasm of all cells and have shared roles in cytokinesis, adhesion, tissue morphogenesis, and cell migration19,20,21,22. This paper discusses conventional myosin protocols in the context of nonmuscle myosin 2b (NM2b)23. NM2b, in comparison to M5a, has a low duty ratio and is enzymatically slower with a Vmax of 0.2 s-1 23 compared to M5a's Vmax of ≈18 s-1 24. Notably, truncated NM2b constructs with two heads do not readily move processively on actin; rather, each encounter with actin results in a power stroke followed by dissociation of the molecule25.
NM2b contains two myosin heavy chains, each with one globular head domain, one lever-arm (with one ELC and one regulatory light chain (RLC)), and an α-helical coiled-coil rod/tail domain, approximately 1,100 amino acids long, that dimerizes these two heavy chains. The enzymatic activity and structural state of NM2b are regulated by phosphorylation of the RLC23. Unphosphorylated NM2b, in the presence of ATP and physiological ionic strengths (approximately 150 mM salt), adopts a compact conformation wherein the two heads make participate in an asymmetric interaction and the tail folds back over the heads in two places23. In this state, the myosin does not interact strongly with actin and has very low enzymatic activity. Upon RLC phosphorylation by calmodulin-dependent myosin light chain kinase (MLCK) or Rho-associated protein kinase, the molecule extends and associates with other myosins through the tail region to form bipolar filaments of approximately 30 myosin molecules23. The aforementioned phosphorylation of the RLC also leads to increased actin-activated ATPase activity of NM2b by approximately four times26,27,28. This bipolar filament arrangement, featuring many myosin motors at each end, is optimized for roles in contraction and tension maintenance, where actin filaments with opposing polarities can be moved relative to each other23,29. Accordingly, NM2b has been shown to act as an ensemble of motors when interacting with actin. The large number of motors within this filament allow NM2b filaments to move processively on actin filaments, making in vitro filament processivity possible to characterize29.
While progress has been made in understanding the role of myosins in the cell, there is a need to understand their individual characteristics at the protein level. To understand actomyosin interactions at a simple protein-protein interaction level, rather than inside of a cell, we can express and purify recombinant myosins for use in in vitro studies. The results of such studies then inform mechanobiologists about the biophysical properties of specific myosins that ultimately drive complex cellular processes12,13,14,25,29. Typically, this is accomplished by adding an affinity tag to a full-length or truncated myosin construct and purifying via affinity chromatography29,30,31. Additionally, the construct can be engineered to include a genetically encodable fluorophore or a tag for protein labeling with a synthetic fluorophore. By adding such a fluorescent label, single molecule imaging studies can be performed to observe myosin mechanics and kinetics.
Following purification, the myosin can be characterized in several ways. ATPase activity can be measured by colorimetric methods, providing insight into the overall energy consumption and actin affinity of the motor under different conditions32. To learn about the mechanochemistry of its motility, further experiments are required. This paper details two in vitro fluorescence microscopy-based methods that can be used to characterize the motile properties of a purified myosin protein.
The first of these methods is the gliding actin filament assay, which can be used to quantitatively study the ensemble properties of myosin motors, as well as qualitatively study the quality of a batch of purified protein33. Although this paper discusses the use of total internal reflection fluorescence (TIRF) microscopy for this assay, these experiments can be effectively performed using a wide-field fluorescence microscope equipped with a digital camera, commonly found in many labs34. In this assay, a saturating layer of myosin motors is attached to a coverslip. This can be accomplished using nitrocellulose, antibodies, membranes, SiO2-derivatized surfaces (such as trimethylchlorosilane), among others29,33,35,36,37,38. Fluorescently labeled actin filaments are passed through the coverslip chamber, upon which the actin binds to the myosin attached to the surface. Upon addition of ATP (and kinases in the study of NM2), the chamber is imaged to observe the translocation of actin filaments by the surface-bound myosins. Tracking software can be used to correlate the velocity and length of each gliding actin filament. Analysis software can also provide a measure of the number of both moving and stationary actin filaments, which can be useful to determine the quality of a given myosin preparation. The proportion of stalled filaments can also be intentionally modulated by surface tethering of actin to other proteins and measured to determine the load dependence of the myosin39. Because each actin filament can be propelled by a large number of available motors, this assay is very reproducible, with the final measured velocity being robust to perturbations such as alterations in the starting myosin concentration or the presence of additional factors in the solution. This means it can be easily modified to study myosin activity under different conditions, such as altered phosphorylation, temperature, ionic strength, solution viscosity, and the effects of load induced by surface tethers. Although factors such as strong-binding myosin "dead heads" incapable of ATP hydrolysis can cause stalled actin filaments, multiple methods exist to mitigate such issues and allow for accurate measurements. The kinetic properties of myosin vary greatly across classes and, depending on the specific myosin used, the speed of actin filament gliding in this assay can vary from under 20 nm/s (myosin 9)40,41, and up to 60,000 nm/s (Characean myosin 11)42.
The second assay inverts the geometry of the gliding actin filament assay12. Here, the actin filaments are attached to the coverslip surface and the movement of single molecules of M5a or of individual bipolar filaments of NM2b are visualized. This assay can be used to quantify the run lengths and velocities of single myosin molecules or filaments on actin. A coverslip is coated with a chemical compound that blocks non-specific binding and simultaneously functionalizes the surface, such as biotin-polyethylene glycol (biotin-PEG). The addition of modified avidin derivatives then primes the surface and biotinylated actin is passed through the chamber, resulting in a layer of F-actin stably bound to the bottom of the chamber. Finally, activated and fluorescently labeled myosin (typically 1-100 nM) is flowed through the chamber, which is then imaged to observe myosin movement over the stationary actin filaments.
These modalities represent fast and reproducible methods that can be employed to examine the dynamics of both nonmuscle and muscle myosins. This report outlines the procedures to purify and characterize both M5a and NM2b, representing unconventional and conventional myosins, respectively. This is followed by a discussion of some of the myosin-specific adaptations, which can be performed to achieve successful capturing of motion in the two types of the assay.
Expression and molecular biology
The cDNA for the myosin of interest must be cloned onto a modified pFastBac1 vector that encodes for either a C-terminal FLAG-tag (DYKDDDDK) if expressing M5a-HMM, or an N-terminal FLAG-tag if expressing the full-length molecule of NM2b23,43,44,45,46. C-terminal FLAG-tags on NM2b results in a weakened affinity of the protein for the FLAG-affinity column. In contrast, the N-terminally FLAG-tagged protein usually binds well to the FLAG-affinity column23. The N-terminally tagged protein retains enzymatic activity, mechanical activity and phosphorylation-dependent regulation23.
In this paper, a truncated mouse M5a heavy meromyosin (HMM)-like construct with a GFP between the FLAG-tag and the C-terminus of the myosin heavy chain was used. Note that unlike NM2b, M5a-HMM can be successfully tagged and purified with either N- or C-terminal FLAG tags and in both cases the resulting construct will be active. The M5a heavy chain was truncated at amino acid 1090 and contains a three amino acid linker (GCG) between the GFP and the coiled-coil region of the M5a47. No additional linker was added between the GFP and FLAG-tag. M5a-HMM was co-expressed with calmodulin. The full-length human NM2b construct was co-expressed with ELC and RLC. The N-termini of the RLC was fused with a GFP via a linker of five amino acids (SGLRS). Directly attached to the FLAG-tag was a HaloTag. Between the HaloTag and the N-terminus of the myosin heavy chain was a linker made of two amino acids (AS).
Both myosin preparations were purified from one liter of Sf9 cell culture infected with baculovirus at a density of approximately 2 x 106 cells/mL. The volumes of the baculovirus for each subunit depended on the virus's multiplicity of infection as determined by the manufacturer's instructions. In the case of M5a, cells were co-infected with two different baculoviruses-one for calmodulin, and one for M5a heavy chain. In the case of the NM2b, cells were co-infected with three different viruses-one for ELC, one for RLC, and one for NM2b heavy chain. For labs working with a diversity of myosins (or other multi-complex proteins), this approach is efficient since it allows for many combinations of heavy and light chains and commonly used light chains such as calmodulin can be co-transfected with many different myosin heavy chains. All cell work was completed in a biosafety cabinet with proper sterile technique to avoid contamination.
For the expression of both M5a and NM2b, the Sf9 cells producing the recombinant myosins were collected 2-3 days post-infection, via centrifugation, and stored at -80 °C. Cell pellets were obtained by centrifuging the co-infected Sf9 cells at 4 °C for 30 min at 2,800 x g. The protein purification process is detailed below.
1. Protein purification
2. Gliding actin filament assay
3. Single molecule TIRF assay
4. Image analysis
The purification of myosin can be evaluated by performing reducing sodium dodecyl sulfate-polyacrylamide (SDS-PAGE) gel-electrophoresis as shown in Figure 2. While this figure represents the final, post-dialyzed myosin, SDS-PAGE can be performed on aliquots from the various stages of the purification procedure to identify any products lost to the supernatant. Myosin 5a HMM has a band in the 120-130 kDa range and the full-length nonmuscle myosin 2b has a band in the 200-230 kDa range, corresponding to heavy chains29,44. The myosin 5a also has a band near the 17 kDa mark, marking calmodulin. The nonmuscle myosin 2b has a band at approximately 17 kDa, denoting the ELC. Because a GFP-tagged RLC is present in this NM2b preparation, the RLC appears at approximately 47 kDa; however, an unlabeled RLC will be present at approximately 20 kDa if not tagged with a GFP.
The gliding actin filament assay shown in Video 1 and Figure 3 represents the characteristics of an ideal and trackable movie. This gliding actin filament assay features the smooth movement of labeled actin filaments. The black actin wash ensures that the dead myosin heads are removed from the measurement field, further contributing to the overall smooth movement of the actin filaments. The fluorescently labeled filaments are short enough that a single filament does not cross over on itself, which is more optimal for the tracking program. Actin filaments that are too long will cross over other filaments, which can present difficulties to the gliding actin filament assay tracking program. This problem can be avoided by pipetting up and down 10-20 times to shear the actin filaments before loading onto the coverslip.
In the case of NM2b, the use of methylcellulose can significantly improve the quality of the recorded movies as it reduces the diffusion of the actin away from the imaging surface. This is not necessary for M5a because its higher duty ratio allows for a stronger attachment of the actin to the myosin-coated surface. If methylcellulose is used, wicking the solution through the chamber is necessary to ensure the solution flows through. As shown in Video 2, when all other conditions are identical except for the exclusion of methylcellulose, the actin filaments do not remain as closely associated with the myosin-coated surface.
Conversely, the goal for the inverted motility assay shown in Video 3 and Figure 4 is to introduce surface-tethered fluorescent actin filaments upon which myosin movement can be observed. An important requirement of the inverted assay is to ensure that the myosin movement is consistently observed across the FOV, as shown. The use of a mixture of DTT, glucose, catalase, and glucose oxidase can minimize photobleaching to allow for longer measurements52. Furthermore, if the acquisition rate for the assay is low, shuttering the illumination light off between acquiring frames can help with excessive photobleaching. Shuttering of the excitation light can be done via a mechanical shutter, or an acousto-optic tunable filter (AOTF).
Figure 1: Preparation of functionalized flow-cell chambers. (A) Begin with a cleaned microscope slide, two pieces of double-sided tape cut to approximately 2 cm, and a functionalized coverslip. (B) Add the tape to the center of the microscope slide. (C) Attach the coverslip to the tape with the coating (i.e., nitrocellulose) facing down and gently press on the overlapping regions with the tape using a plastic pipette tip to ensure that the coverslip has adhered to the chamber. Please click here to view a larger version of this figure.
Figure 2: SDS polyacrylamide gel electrophoresis of expressed NM2b and M5a-HMM. (A) A representative SDS PAGE gel image for a full-length NM2b heavy chain (≈230 kDa) and GFP-RLC (≈47 kDa) and ELC (≈17 kDa). Gel image reproduced and modified from Melli et al. (2018)29. (B) A representative SDS PAGE gel image for an M5a-HMM-like heavy chain (≈120 kDa) and calmodulin (≈17 kDa). Note that the gel in this image does not have a GFP inserted to the C-terminal end. A GFP inserted in the myosin heavy chain increases the molecular weight by ≈27 kDa. Gel image reproduced and modified was originally published in the Journal of Biological Chemistry44. Please click here to view a larger version of this figure.
Figure 3: Gliding actin filament assay results acquired via TIRF illumination. (A) Example frame from a movie showing translocation of rhodamine-phalloidin labeled actin filaments (in red) on 0.2 µM NM2b in the presence of 0.7% methylcellulose at 30 °C. Scale bar = 10 µm. (B) Filament tracking image output from the FASTrack program for NM2b for the same FOV as shown in (A) Scale bar = 10 µm. (C) Representative histogram of the acto-NM2b gliding velocity, showing that this sample of NM2b can generate an actin gliding velocity of 77 ± 15 nm/s (mean ± standard deviation; number of tracks = 550). (D) Example frame from a movie showing translocation of rhodamine-phalloidin labeled actin filaments (in red) on 75 nM M5a-HMM. Scale bar = 10 µm. (E) Filament tracking image output by the FASTrack program for M5a-HMM for the same FOV as shown in (D) Scale bar = 10 µm. (F) A representative histogram of the acto-M5a-HMM gliding velocity, showing that this sample of M5a can generate an actin gliding velocity of 515 ± 165 nm/s (mean ± standard deviation; number of tracks = 25098). Please click here to view a larger version of this figure.
Figure 4: Inverted assay results acquired via TIRF illumination. (A) A representative FOV from a two-channel merged image stack showing the movement of NM2b filaments (displayed in green) on biotinylated actin filaments labeled with AF647-phalloidin (displayed in blue) at 30 °C. Polymerized filaments of recombinantly expressed and purified NM2b co-expressed with ELC and GFP-RLC are observed as the green, elongated particles in the FOV. Scale bar = 10 µm. (B) Representative histogram of the velocity of NM2b filaments. Analysis was performed using the image analysis software described in the Table of Materials. Single NM2b filaments have a velocity of 84 ± 22 nm/s (mean ± standard deviation; number of particles tracked = 133), when moving along single actin filaments. (C) Example kymograph of the NM2b filament motion along a single actin filament. Note that some of the regions of the particle shows a "rotation" of the NM2b filament, along the actin filament, which most likely represents the time when one side of the bipolar NM2b filament detaches from the actin filament, as shown previously in Melli et al.29. (D) A representative FOV of the single molecule movement M5a-HMM (displayed in green) on biotinylated actin filaments labeled with rhodamine-phalloidin (displayed in red). Scale bar = 10 µm. (E) Representative histogram of run length of M5a-HMM, fit to a single exponential. Analysis was performed using the image analysis software described in the List of Materials. The characteristic run length is 1.3 µm with a 95% confidence interval of 1.23-1.42 µm in this example. (F) Representative histogram of single molecule M5a-HMM velocity on single actin filaments. Analyzed data output from image analysis shows a mean velocity of 668 ± 258 nm/s (mean ± standard deviation; number of particles tracked = 684). (G) Example kymograph of single molecules of M5a-HMM motion along a single actin filament. Please click here to view a larger version of this figure.
Video 1: Comparison of NM2b and M5a-HMM gliding actin filament assay. The NM2b gliding actin filament assay was performed in the presence of methylcellulose (left; video panel A) and the M5a-HMM in the absence of methylcellulose (right; video panel B) at 30 °C. Note that the time stamp advances faster in the NM2b video panel, compared to the M5a-HMM video panel to show the movement of the rhodamine-labeled actin filaments (red) that is approximately the same. This is since the actual actin translocation velocity of NM2b is close to 7 times slower than that for M5a-HMM (77 nm/s, versus 515 nm/s, respectively, extracted from the Gaussian peak fit to the histogram in Figure 3). Scale bar = 10 µm in both video panels. NM2b data acquired at 0.33 frames per second with 200 ms exposure. M5a-HMM data acquired at 5 frames per second with 200 ms exposure (continuous) and subsequently down-sampled to 1 frame per second. Timestamps were added using the plugin described in the List of Materials. Please click here to download this video.
Video 2: Gliding actin filament assay of NM2b in the absence of methylcellulose. When all other conditions are the same except for the absence of methylcellulose, the actin filaments sometimes do not stick well to the coverslip coated with 0.2 µM NM2b, leading to lower-quality movies with actin filaments "flopping" close to the surface of the NM2b coated coverslip. Scale bar = 10 µm. This can be resolved by introducing methylcellulose to show the smooth motion of the actin filaments, as shown in the left video panel of Video 1 (NM2b gliding actin filament assay). Another alternative is to increase the NM2b concentration to ≈1 µM. This movie was acquired at 0.33 frames per second with 200 ms exposure. Please click here to download this video.
Video 3: Comparison of NM2b and M5a-HMM inverted motility assay. The NM2b inverted motility assay was performed in the presence of methylcellulose and recorded at a rate of 0.33 frames per second with the use of a shutter (left; video panel A), Video panel C shows the same FOV as A, but with particles are identified and tracked using image analysis software. Similarly, the inverted motility assay for M5a-HMM in the absence of methylcellulose was recorded at a rate of 5 frames per second (right; video panel B). Video panel D shows the same FOV as B, but with particles identified and tracked using image analysis software. Scale bars = 10 µm in all video panels. The two lasers were toggled back and forth with the use of a single camera for acquisition. Please click here to download this video.
Buffer Name | Composition | Step(s) Used | Comments |
M5a Extraction Buffer | 0.3 M NaCl | 1.1 | Keep on ice. |
15 mM MOPS, pH 7.2 | |||
15 mM MgCl2 | |||
1.5 mM EGTA | |||
4.5 mM NaN3 | |||
NM2b Extraction Buffer | 0.5 M NaCl | 1.1 | Keep on ice. |
15 mM MOPS, pH 7.2 | |||
15 mM MgCl2 | |||
1.5 mM EGTA | |||
4.5 mM NaN3 | |||
Buffer A | 0.5 M NaCl | 2.2 | Keep on ice. |
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
3 mM NaN3 | |||
1 mM ATP | |||
1 mM DTT | |||
5 mM MgCl2 | |||
Buffer B | 0.5 M NaCl | 2.3 | Keep on ice. |
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
3 mM NaN3 | |||
1 mM DTT | |||
Elution Buffer | 0.5 M NaCl | 3.1 | Keep on ice. |
0.5 mg/mL FLAG peptide | |||
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
3 mM NaN3 | |||
pH 7.2 | |||
M5a Dialysis Buffer | 500 mM KCl | 4.1 | Use cold dH2O to bring to volume. |
10 mM MgCl2 | |||
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
1 mM DTT | |||
NM2b Dialysis Buffer | 25 mM NaCl | 4.1 | Use cold dH2O to bring to volume. |
10 mM MgCl2 | |||
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
1 mM DTT | |||
NM2b Storage Buffer | 0.5 M NaCl | 5.1 | Keep on ice. |
10 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | |||
3 mM NaN3 |
Table 1: Buffers used in protein purification.
Buffer Name | Composition (M5a) | Composition (NM2b) | Step(s) Used (M5a/NM2b) | Comments |
4X Motility Buffer (4X MB) | 80 mM MOPS, pH 7.2 | 80 mM MOPS, pH 7.2 | Vacuum filter and store in 4°C | |
20 mM MgCl2 | 20 mM MgCl2 | |||
0.4 mM EGTA | 0.4 mM EGTA | |||
pH 7.4 | pH 7.4 | |||
50 mM Salt Motility Buffer (50 mM MB) | 25% v/v 4X MB | 25% v/v 4X MB | Vacuum filter and store in 4°C | |
50 mM KCl | 50 mM NaCl | |||
Raise to volume with dH2O | Raise to volume with dH2O | |||
500 mM Salt Motility Buffer (500 mM MB) | N/A | 25% v/v 4X MB | Vacuum filter and store in 4°C | |
500 mM NaCl | ||||
Raise to volume with dH2O | ||||
Myosin | 0.05-0.1 µM myosin | 0.2 µM myosin | 4.2/5.2 | Keep on ice. |
1 mM DTT | 1 mM DTT | |||
Dilute in 50 mM MB | Dilute in 500 mM MB | |||
1 mg/mL Bovine Serum Albumin (BSA) | 1 mg/mL BSA | 1 mg/mL BSA | 4.3/5.3 | Keep on ice. |
Dilute in 50 mM MB | Dilute in 500 mM MB | |||
1 mM DTT | 1 mM DTT | |||
5 µM Unlabeled F-actin in 50 mM MB (black actin) | 5 µM unlabeled F-actin | 5 µM unlabeled F-actin | 4.5/5.5 | Keep on ice. Shear actin by pipetting up and down 5-10 times, or by using a syringe. |
1 μM calmodulin (CaM) | 1 mM ATP | |||
1 mM ATP | 0.2 mM CaCl2 | |||
Dilute in 50 mM MB | 1 μM CaM | |||
1–10 nM myosin light chain kinase (MLCK) | ||||
Dilute in 50 mM MB | ||||
MB with 1 mM DTT and 1 mM ATP | 1 mM DTT | 1 mM DTT | 4.6/5.6 | Keep on ice. |
1 mM ATP | 1 mM ATP | |||
Dilute in 50 mM MB | Dilute in 50 mM MB | |||
MB with DTT | 1 mM DTT | 1 mM DTT | 4.4, 4.7, 4.9/5.4, 5.7, 5.9 | Keep on ice. |
Dilute in 50 mM MB | Dilute in 50 mM MB | |||
20 nM Rhodamine-Phalloidin F-actin (Rh-Actin) | 20 nM Rhodamine-phalloidin F-actin | 20 nM Rhodamine-phalloidin F-actin | 4.8/5.8 | Keep on ice. Do not vortex. |
1 mM DTT | 1 mM DTT | |||
Dilute in 50 mM MB | Dilute in 50 mM MB | |||
Final Buffer | 50 mM KCl | 0.7% methylcellulose (optional) | 4.10/5.10 | Add in the glucose, glucose oxidase, and catalase immediately before performing the experiment. Keep on ice. |
20 mM MOPS, pH 7.2 | 50 mM NaCl | |||
5 mM MgCl2 | 20 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | 5 mM MgCl2 | |||
1 mM ATP | 0.1 mM EGTA | |||
50 mM DTT | 1 mM ATP | |||
1 μM calmodulin | 50 mM DTT | |||
2.5 mg/mL glucose | 1–10 nM MLCK | |||
100 μg/mL glucose oxidase | 0.2 mM CaCl2 | |||
40 μg/mL catalase | 1 μM calmodulin | |||
2.5 mg/mL glucose | ||||
100 μg/mL glucose oxidase | ||||
40 μg/mL catalase |
Table 2: Buffers used in gliding assay.
Buffer Name | Composition (M5a) | Composition (NM2b) | Step(s) Used (M5a/NM2b) | Comments |
4X Motility Buffer (4X MB) | 80 mM MOPS, pH 7.2 | 80 mM MOPS, pH 7.2 | Vacuum filter and store in 4°C | |
20 mM MgCl2 | 20 mM MgCl2 | |||
0.4 mM EGTA | 0.4 mM EGTA | |||
pH 7.4 | pH 7.4 | |||
50 mM salt Motility Buffer (50 mM MB) | 25% v/v 4X MB | 25% v/v 4X MB | Vacuum filter and store in 4°C | |
50 mM KCl | 50 mM NaCl | |||
Raise to volume with dH2O | Raise to volume with dH2O | |||
150 mM salt Motility Buffer (150 mM MB) | 25% v/v 4X MB | Vacuum filter and store in 4°C | ||
150 mM NaCl | ||||
Raise to volume with dH2O | ||||
Myosin | 30 nM myosin | See "Final Buffer" Recipe/4.12 | Keep on ice. | |
1 mM DTT | ||||
Dilute in 150 mM MB | ||||
2 mg/mL NeutrAvidin | 2 mg/mL NeutrAvidin | 2 mg/mL NeutrAvidin | 3.5/4.8 | Keep on ice. |
1 mM DTT | 1 mM DTT | |||
Dilute in 50 mM MB | Dilute in 150 mM MB | |||
1 mg/mL bovine serum albumin (BSA) | 1 mg/mL BSA | 1 mg/mL BSA | 3.3/4.6 | Keep on ice. |
1 mM DTT | 1 mM DTT | |||
Dilute in 50 mM MB | Dilute in 150 mM MB | |||
200 nM rhodamine-phalloidin biotinylated F-actin (bRh-Actin) | 200 nM rhodamine-phalloidin biotinylated F-actin | 200 nM rhodamine-phalloidin biotinylated F-actin | 3.7/4.10 | Avoid shearing by not vortexing or pipetting up and down. To mix, gently invert. |
1 mM DTT | 1 mM DTT | |||
Dilute in 50 mM MB | Dilute in 150 mM MB | |||
MB with DTT | 50 mM DTT | 50 mM DTT | 3.2, 3.4, 3.6, 3.8/4.5, 4.7, 4.9, 4.11, 4.13 | Keep on ice. |
Dilute in 50 mM MB | Dilute in 150 mM MB | |||
Final Buffer | 50 mM KCl | 0.7% methylcellulose (optional) | 3.9/4.14 | Add in the glucose, glucose oxidase, and catalase immediately before performing the experiment. Keep on ice. |
20 mM MOPS, pH 7.2 | 50 mM NaCl | |||
5 mM MgCl2 | 20 mM MOPS, pH 7.2 | |||
0.1 mM EGTA | 5 mM MgCl2 | |||
1 mM ATP | 0.1 mM EGTA | |||
50 mM DTT | 1 mM ATP | |||
1 μM calmodulin | 50 mM DTT | |||
2.5 mg/mL glucose | 1–10 nM MLCK | |||
100 μg/mL glucose oxidase | 0.2 mM CaCl2 | |||
40 μg/mL catalase | 1 μM calmodulin | |||
10 nM myosin | 2.5 mg/mL glucose | |||
100 μg/mL glucose oxidase | ||||
40 μg/mL catalase |
Table 3: Buffers used in TIRF assay.
Presented here is a workflow for the purification and in vitro characterization of myosin 5a and nonmuscle myosin 2b. This set of experiments is useful for quantifying the mechanochemical properties of purified myosin constructs in a fast and reproducible manner. Although the two myosins shown here are just two specific examples out of the many possibilities, the conditions and techniques can be applied, with some tailoring, to most myosins and to many other motor proteins.
The protocols discussed here are subject to variations depending on the individual needs of the lab and experiments. For example, as discussed in the Expression and Molecular Biology section, the proteins used in this paper were generated from a co-infection of two or more viruses; however, successful protein expression can also be achieved with multi-expression vectors such as the p-FastBac dual expression vector or the BiGBac expression system53.
Several factors can hamper the successful production of the recombinant protein. To prevent protein degradation, it is imperative that every step of the protein purification is completed at the appropriate temperature, with all centrifugation steps occurring at 4 °C and all other steps being performed on ice. Excess bands may be apparent in the gel of the dialyzed protein product. This could be indicative of degradation or contamination. Subsequent purification via size-exclusion or ionic-exchange chromatography can enhance the purity of the samples54. To that effect, it is recommended to save aliquots at each step of the protein purification process for troubleshooting, should there be a low yield of myosin or suspected protein contamination. Sometimes, there can be inadequate binding of the lysate to the resin in step 1.8, which can result in the loss of the myosin to the supernatant in subsequent centrifugations. This can be resolved by varying the duration of resin binding during this step, even leaving it to bind overnight, if necessary. Longer incubation times introduce a greater risk of protein degradation if contaminant proteases are not sufficiently inhibited, and myosins with proteolytically sensitive regions will be adversely affected. Additionally, improper washing of the resin both before and after use may result in the elution of an undesired protein product, so it is imperative that the proper protocols are followed before and after using the FLAG-affinity resin. If the resin is washed immediately after use and stored appropriately, it can be reused up to 20 times.
It is important to note that protein degradation also occurs during the expression stage and shortening expression times may be advantageous in terms of degradation, although this can be at the expense of the total yield. Following the procedures outlined here to monitor the protein sample at different stages of the protocol (i.e., before, during, and after purification) will help to determine the stages necessary for optimization. For myosins that repeatedly resist successful expression and purification, common problems can be co-expression with insufficient or inappropriate light chains as well as improper folding during overexpression. Appropriate light chains must be selected based on known interactions when possible and heavy chain to light chain baculovirus ratios must be tested in small-scale experiments to determine the optimum. For myosins that aggregate or yield little or no soluble active product, co-expression with chaperones can aid in successfully obtaining active protein54,55.
Purified myosin products inevitably contain a small population of damaged myosin, referred to as "dead heads," which can be addressed in two ways. One method, outlined in this protocol, involves flowing unlabeled, or "black", actin through the chamber in the gliding actin filament assay. Subsequently washing with ATP causes functional myosins to dissociate from the black actin while dead heads will remain bound to this unlabeled actin due to their inability to hydrolyze ATP and due to their high affinity. While performing the black actin wash, a syringe can be used to shear the actin effectively. Additional shearing can be accomplished by vortexing, provided that any resultant bubbles are removed by centrifugation. An alternative method is to selectively pellet the dead heads from the myosin sample by mixing myosin with F-actin and Mg-ATP at high salt (0.5 M) concentrations and sedimenting in a table-top ultracentrifuge. The myosins capable of hydrolyzing ATP under these conditions do not stay bound to the actin due to their low affinity for actin under these high salt conditions and are found in the supernatant, whereas the myosin dead heads remain bound to the actin in the pellet56. Similarly, a sedimentation with actin and resuspension of the pellet can also be used to remove myosins which are incapable of binding to actin in the absence of ATP. Note that a small proportion of this type of dead heads will have less of an impact on these types of assays. By doing a nucleotide-free sedimentation and resuspension followed by an ATP-bound centrifugation and resuspension, myosins that are competent for both actin-binding and ATP-dependent release from actin can be isolated.
Motility assays can also be modified in several ways. For example, in the gliding actin filament assay for the NM2b, the NM2b is phosphorylated in the chamber via the addition of MLCK, calmodulin, calcium, and ATP in the black actin step, as well as in the Final Buffer. However, the NM2b can also be phosphorylated in a tube, before performing the assay. By doing so, the percent of phosphorylated NM2b can be quantified by running a native gel with a urea-containing sample buffer or performing mass spectrometry57,58. The effect of temperature on myosin activity can also be investigated. This can be accomplished by employing an objective heating system on the microscope or an environmental enclosure, so that the flow cell is maintained at constant temperature. Ionic strength is another important consideration. For many myosins, actin affinity and enzymatic activity will be increased at lower ionic strength; for others, higher ionic strength is necessary59. In addition to providing valuable information about the myosin mechanism, lowering the ionic strength can enhance motility and make myosin more accessible to investigation with many assays. In contrast, some motors will exhibit electrostatic tethering effects, which will slow motility at lower ionic strengths. Finally, when assaying the movement of NM2b filaments, it is crucial to maintain ionic strength within a narrow range (150-200 mM ionic strength), approximating those found in most cell types. The use of lower ionic strengths results in aggregation of the myosin filaments, while the filaments depolymerize at higher ionic strengths.
With many myosins, particularly those with low duty ratios, the conditions of the final buffer given for M5a would result in the fluorescently labeled actin filaments being only loosely bound to the surface or dissociating altogether. This results in erratic movements that complicate quantification. Better quality movement can often be obtained using methylcellulose (0.7%) in the Final Buffer. Methylcellulose is a viscous crowding agent and forces actin filaments to remain close to the surface even when the density of attached myosin motors is sparse60. Similarly, it has been observed that the inclusion of methylcellulose in the final buffer of the single filament motility assay is necessary to observe movement with NM2a, and the same phenomenon was reported for smooth muscle myosin filaments29,61. This also increases the processivity of NM2b filaments. One potentially unwanted side effect of using methylcellulose in this assay is that the crowding agent properties can promote the lateral association of myosin filaments into bundles. Alternatives to methylcellulose when troubleshooting a lack of movement or loosely bound actin filaments in the gliding actin filament assay are to lower the salt concentration in the motility buffers or to increase the myosin surface density. As stated above, a high ionic strength in the motility buffers has been shown to lower the ability of some myosins to bind to actin29,34,62.
Another variation of the gliding actin filament assay is the use of antibodies to anchor the myosin onto the glass coverslip. For example, if a GFP is present at the C-terminal end of the myosin construct, an anti-GFP antibody can be used to fix the GFP-myosin to the coverslip36,63,64. This can aid with obtaining successful motility in situations where the geometry of the system may otherwise hamper actin translocation, such as in the case of testing artificial or short lever arms54,64. Additionally, the effect of load on translocation velocities can be investigated in the gliding actin filament assay by employing actin-binding proteins such as α-actinin or utrophin39,50,65. Such a measurement can be useful to compare the effect of load on an ensemble of myosins versus the load-dependent kinetics of a single myosin that can be measured using an optical trapping assay66,67. This can be accomplished by adding increasing amounts of an actin-binding protein along with the myosin in the initial step. The actin-binding protein binds to the surface and exerts a frictional load on the actin filaments that are being moved by myosins, which results in a graded velocity as the concentration of actin-binding protein on the surface is increased39.
The single molecule/ensemble motility assay can also be adapted to investigate the effect of various actin structures on myosin movement. For example, rather than observing myosin movement on top of single actin filaments, fascin- or α-actinin-mediated actin bundles can be studied as an in vitro reconstitution of the actin filament network found in cells68,69. The effect of actin-binding proteins such as tropomyosin can also be studied65,70,71,72.
Of note is the versatility in choosing a label for the single molecule/ensemble motility assay. In this report, a GFP label was used on both the M5a-HMM and NM2b; however, many other labels can be used. Examples include HaloTag or SNAP-tag, which can be genetically fused to the myosin and covalently bind a synthetic dye. The benefit of HaloTag technology lies in its versatility for several experimental adaptations, such as labeling with different colors or adding a biotin affinity tag29,73. Additionally, the use of quantum dot technology can be employed to improve the resolution of single molecule fluorescence tracking, which also addresses the limitation of GFP's low brightness and the tendency for photobleaching74. Tags can be successfully attached to light chains as well as the heavy chain11,75,76.
For achieving success in the single molecule TIRF motility assay, a key factor is using a well blocked and functionalized surface. A simple method to achieve moderate blocking is to use biotinylated-BSA bound to a nitrocellulose surface. Although this will work well enough to characterize many motors, including M5a, the level of nonspecific binding on such a surface is prohibitive for reproducing clean movement with samples such as NM2b. A key breakthrough in this regard was the transition to PEGylated surfaces doped with biotin-PEG for functionalization77. The PEG surfaces provide a far superior level of surface blocking and a defect-free PEGylated surface can remain free of nonspecific binding for very long periods of time. The specific protocol detailed here allows the production of biotinylated PEG surfaces in a matter of hours and if immediately stored as described, the surfaces can be used for several weeks with only a marginal decline in quality.
A key consideration before collecting data for tracking is the acquisition frame rate. Movement between subsequent frames must be large enough to avoid oversampling errors. High sampling rates will yield overestimated velocities due to the division of localization errors by a small time interval and increase the apparent error of the measurement. In cases where the raw data is too finely sampled, the data can be down-sampled by taking every Nth frame to create a new stack and considering the change in frame rate that results. Subpixel movements between frames must be avoided and movements of several hundred nanometers are required to obtain accurate values. In all cases where a new sample is being characterized, the results generated by automated analysis must be compared to a small dataset of manually tracked filaments for consistency.
When analyzing data from single molecule motility experiments, care must be taken when choosing which parameters to measure, how to filter data, and how to fit data. As stated above, the sampling rate can be an important factor when analyzing velocity data. For many myosins, processive runs will be short and well approximated by a straight line. In such cases, the start to endpoint distance of the track may provide a good measure of the run length and this can be divided by the duration of the track to yield a good estimate of velocity. In cases where the tracks are very long and follow curved paths around bent filaments, this type of analysis will yield inaccurate results and a total distance traveled must be used, using an acquisition rate that allows for successive localization points to be sufficiently well spaced to avoid oversampling errors as described above, while being close enough together that the straight line distance between them remains a good approximation of the curve between those points. In addition, for motor proteins with long run lengths in relation to the length of the track, additional statistics such as the Kaplan-Meier estimator must be made when calculating run lengths78. The same is true for situations in which photobleaching is sufficiently likely to occur before the end of a processive run. Another phenomenon that can be observed in single molecule fluorescence studies is photoblinking, in which fluorophores switch between the on and off state rapidly and appear to blink. This typically does not occur in these motility experiments; however, if this does occur, the laser intensity and exposure times can be decreased which should minimize the effect. Several chemicals, including β-mercaptoethanol, Trolox, cyclooctateraene, n-propyl gallate, 4-nitrobenzyl alcohol, and 1,4-diazabicyclo[2.2.2]octane can be utilized to mitigate this as well79.
In summary, this article presents detailed protocols that are robust in their ability to quantify mechanochemical properties such as actin translocation velocity, myosin translocation velocity, and myosin run length. These assays are reproducible and can be used to determine the quality of the purified myosin even in situations where the motile characteristics are not the specific end goal of the study. In addition, changes such as pH, temperature, and chemical regulators can be introduced to these assays to examine how the mechanochemistry of the studied myosin is affected. Taken together, the actin gliding and inverted motility assays can allow for a better understanding of myosin ensemble behavior and intermolecular variations in molecular motor mechanics and kinetics. The fluorescence microscopy-based assays described here support a reductionist's approach to cytoskeletal research and can be a powerful tool to understand protein-protein dynamics in vitro. Together, data collected from these highly controlled experiments can be used to advise mechanobiologists of key actomyosin behaviors that may be relevant at the cell biological level, and beyond.
The authors have nothing to disclose.
We thank Dr. Fang Zhang for technical assistance with the preparation of the reagents used for collecting this data. This work was supported by the NHLBI/NIH Intramural Research Program funds HL001786 to J.R.S.
1 mL Syringe | BD | 309628 | |
2 M CaCl2 Solution | VWR | 10128-558 | |
2 M MgCl2 Solution | VWR | 10128-298 | |
27 Gauge Needle | Becton Dickinson | 309623 | |
5 M NaCl Solution | KD Medical | RGE-3270 | |
Acetic Acid | ThermoFisher Scientific | 984303 | |
Amyl Acetate | Ladd Research Industries | 10825 | |
Anti-FLAG M2 Affinity Gel | Millipore Sigma | A2220 | https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Bulletin/a2220bul.pdf |
ATP | Millipore Sigma | A7699 | |
Biotinylated G-Actin | Cytoskeleton, Inc. | AB07 | |
Bovine Serum Albumin | Millipore Sigma | 5470 | |
bPEG-silane | Laysan Bio, Inc | Biotin-PEG-SIL-3400-1g | |
Bradford Reagent Concentrate | Bio-Rad | 5000006 | |
Calmodulin | PMID: 2985564 | ||
Catalase | Millipore Sigma | C40 | |
Cell Line (Sf9) in SF-900 II SFM | ThermoFisher Scientific | 11496015 | http://tools.thermofisher.com/content/sfs/manuals/bevtest.pdf https://tools.thermofisher.com/content/sfs/manuals/bactobac_man.pdf |
Circular Filter Paper – Gliding Assay | Millipore Sigma | WHA1001125 | |
Circular Filter Paper – Inverted Assay | Millipore Sigma | WHA1001090 | |
cOmplete, EDTA-Free Protease Inhibitor Tablets | Millipore Sigma | 5056489001 | This should be stored at 4 °C. The tablets can be used directly or can be reconstituted as a 25x stock solution by dissolving 1 tablet in 2 mL of distilled water. The resulting solution can be stored at 4 °C for 1-2 weeks or at least 12 weeks at -20 °C. |
Concentrating Tubes (100,000 MWCO) | EMD Millipore Corporation | UFC910024 | The MWCO of the tube is not necessarily "one size fits all," as long as the MWCO is less than the total molecular weight of the protein being purified. The NM2b herein was concentrated with a 100,000 MWCO tube and the M5a was concentrated with a 30,000 MWCO tube. |
Coomassie Brilliant Blue R-250 Dye | ThermoFisher Scientific | 20278 | |
Coverslip Rack | Millipore Sigma | Z688568-1EA | |
Coverslips: Gliding Acting Filament Assay | VWR International | 48366-227 | |
Coverslips: Inverted Motility Assay | Azer Scientific | ES0107052 | |
Dialysis Tubing (3500 Dalton MCWO) | Fischer Scientific | 08-670-5A | The diameter of the dialysis tube can vary, but the MWCO should be the same. The NM2b used herein was dialyzed in an 18 mm dialysis tube. The tubes can be stored in 20% alcohol solution at 4 °C. |
DL-Dithiothreitol | Millipore Sigma | D0632 | |
Double-Sided Tape | Office Depot | 909955 | |
DYKDDDDK Peptide | GenScript | RP10586 | This can be dissolved in a buffer containing 0.1 M NaCl, 0.1 mM EGTA, 3 mM NaN3, and 10 mM MOPS (pH 7.2) to a final concentration of 50 mg/mL. This can be stored at -20 °C as 300 µL aliquots. |
EGTA | Millipore Sigma | E4378 | |
Elution Column | Bio-Rad | 761-1550 | These can be reused. To clean, rinse the column with 2-3 column volumes of PBS and distilled water. Chill the column at 4° C before use. |
Ethanol | Fischer Scientific | A4094 | |
G-actin | PMID: 4254541 | G-actin stock can be stored at 200 μM in liquid N2. | |
Glucose | Millipore Sigma | G8270 | |
Glucose Oxidase | Millipore Sigma | G2133 | |
Glycine Buffer Solution, 100 mM, pH 2-2.5, 1 L | Santa Cruz Biotechnology | sc-295018 | |
HaloTag | Promega | G100A | |
HCl | Millipore Sigma | 320331 | |
KCl | Fischer Scientific | P217-500 | |
Large-Orifice Pipet Tips | Fischer Scientific | 02-707-134 | |
Leupeptin Protease Inhibitor | ThermoFisher Scientific | 78435 | |
Mark12 Unstained Standard Ladder | ThermoFisher Scientific | LC5677 | |
Methanol | Millipore Sigma | MX0482 | |
Methylcellulose | Millipore Sigma | M0512 | |
Microscope Slides | Fischer Scientific | 12-553-10 | |
MOPS | Fischer Scientific | BP308-100 | |
mPEG-silane | Laysan Bio, Inc | MPEG-SIL-2000-1g | |
Myosin Light Chain Kinase | PMID: 23148220 | FLAG-tagged MLCK can be purified the same way that the FLAG-tagged myosin was purified herein. | |
NaN3 | Millipore Sigma | S8032 | |
NeutrAvidin | ThermoFisher Scientific | 31050 | |
Nitrocellulose | Ladd Research Industries | 10800 | |
NuPAGE 4 to 12% Bis-Tris Mini Protein Gel, 15-well | ThermoFisher Scientific | NP0323PK2 | |
NuPAGE LDS Sample Buffer (4X) | ThermoFisher Scientific | NP0007 | |
Phosphate-Buffered Saline, pH 7.4 | ThermoFisher Scientific | 10010023 | |
PMSF | Millipore Sigma | 78830 | PMSF can be made as a 0.1 M stock solution in isopropanol and stored in 4 °C. Isopropanol addition results in crystal precipitation, which can be dissolved by stirring at room temperature. Immediately before use, PMSF can be added dropwise to a rapidly stirring solution to a final concentration of 0.1 mM. |
Razor Blades | Office Depot | 397492 | |
Rhodamine-Phalloidin | ThermoFisher Scientific | R415 | Stock can be diluted in 100% methanol to a final concentration of 200 μM. |
Sf9 Media | ThermoFisher Scientific | 12658-027 | This should be stored at 4° C. Its shelf life is 18 months from the date of manufacture. |
Tissue Culture Dish – Gliding Assay | Corning | 353025 | Each tissue culture dish can hold approximately nine coverslips. |
Tissue Culture Dish – Inverted Assay | Corning | 353003 | Each tissue culture dish can hold approximately four coverslips. |
Smooth-sided 200 µL Pipette Tips | Thomas Scientific | 1158U38 | |
EQUIPMENT | |||
Centrifuge | ThermoFisher Scientific | 75006590 | |
Microscope | Nikon | Model: Eclipse Ti with H-TIRF system with 100x TIRF Objective (N.A. 1.49) | |
Microscope Camera | Andor | Model: iXon DU888 EMCCD camera (1024 x 1024 sensor format) | |
Microscope Environmental Control Box | Tokai HIT | Custom Thermobox | |
Microscope Laser Unit | Nikon | LU-n4 four laser unit with solid state lasers for 405nm, 488nm, 561nm,and 640nm | |
Mid Bench Centrifuge | ThermoFisher Scientific | Model: CR3i | |
Misonix Sonicator | Misonix | XL2020 | |
Optima Max-Xp Tabletop Ultracentrifuge | Beckman Coulter | 393315 | |
Plasma-Cleaner | Diener electronic GmbH + Co. KG | System Type: Zepto | |
Sonicator Probe (3.2 mm) | Qsonica | 4418 | |
Standard Incubator | Binder | Model: 56 | |
Waverly Tube Mixer | Waverly | TR6E | |
SOFTWARE | |||
ImageJ FIJI | https://imagej.net/Fiji/Downloads | ||
FAST (Version 1.01) | http://spudlab.stanford.edu/fast-for-automatic-motility-measurements | FAST is available for Mac OSX and Linux based systems. | |
Image Stabilizer Plugin | https://imagej.net/Image_Stabilizer | ||
ImageJ TrackMate | https://imagej.net/TrackMate | ||
Imaging Software | NIS Elements (AR package) | ||
http://www.cs.cmu.edu/~kangli/code/Image_Stabilizer.html | |||
File:TrackMate-manual.pdf | |||
https://github.com/turalaksel/FASTrack | |||
https://github.com/turalaksel/FASTrack/blob/master/README.md |