In this article, we describe the protocols of protein expression, purification, crystallization and structure determination of the N-terminal domain of ryanodine receptor from diamondback moth (Plutella xylostella).
Development of potent and efficient insecticides targeting insect ryanodine receptors (RyRs) has been of great interest in the area of agricultural pest control. To date, several diamide insecticides targeting pest RyRs have been commercialized, which generate annual revenue of 2 billion U.S. dollars. But comprehension of the mode of action of RyR-targeting insecticides is limited by the lack of structural information regarding insect RyR. This in turn restricts understanding of the development of insecticide resistance in pests. The diamondback moth (DBM) is a devastating pest destroying cruciferous crops worldwide, which has also been reported to show resistance to diamide insecticides. Therefore, it is of great practical importance to develop novel insecticides targeting the DBM RyR, especially targeting a region different from the traditional diamide binding site. Here, we present a protocol to structurally characterize the N-terminal domain of RyR from DBM. The x-ray crystal structure was solved by molecular replacement at a resolution of 2.84 Å, which shows a beta-trefoil folding motif and a flanking alpha helix. This protocol can be adapted for the expression, purification and structural characterization of other domains or proteins in general.
Ryanodine receptors (RyRs) are specific ion channels, which mediate the permeation of Ca2+ ions across the sarcoplasmic reticulum (SR) membranes in muscle cells. Therefore, they play an important role in the excitation contraction coupling process. In its functional form, RyR assembles as a homo-tetramer with a molecular mass of >2 MDa, with each subunit comprising of ~5000 amino acid residues. In mammals, there are three isoforms: RyR1- skeletal muscle type, RyR2- cardiac muscle type and RyR3- ubiquitously expressed in different tissues1.
In insects there is only one type of RyR, which is expressed in muscular and nervous tissue2. Insect RyR is more similar to mammalian RyR2 with a sequence identity of about 47%3. Diamide insecticides targeting RyR of Lepidoptera and Coleoptera have been developed and marketed by major companies like Bayer (flubendiamide), DuPont (chlorantraniliprole) and Syngenta (cyantraniliprole). Since its relatively recent launch, diamide insecticides have become one of the fastest growing class of insecticides. Currently, the sales of these three insecticides annually have crossed 2 billion U.S. dollars with a growth rate of more than 50% since 2009 (Agranova).
Recent studies have reported the development of resistance in insects after a few generations of usage of these insecticides4,5,6,7,8. The resistance mutations in the transmembrane domain of RyRs from the diamondback moth (DBM), Plutella xylostella (G4946E, I4790M) and the corresponding positions in tomato leafminer, Tuta absoluta (G4903E, I4746M) show that the region might be involved in diamide insecticide binding as this region is known to be critical for gating of the channel4,8,9. Despite extensive research in this area, the exact molecular mechanisms of diamide insecticides remain elusive. Moreover, it is unclear whether the resistance mutations affect the interactions with diamides directly or allosterically.
Earlier studies have reported the structure of several RyR domains from mammalian species and the structure of full-length mammalian RyR1 and RyR2 by x-ray crystallography and cryo-electron microscopy, respectively10,11,12,13,14,15,16,17,18,19,20,21. But so far, no structure of insect RyR has been reported, which prohibits us from understanding the molecular intricacies of the receptor function as well as the molecular mechanisms of insecticide action and development of insecticide resistance.
In this manuscript, we present a generalized protocol for the structural characterization of N-terminal β-trefoil domain of ryanodine receptor from the diamondback moth, a destructive pest infecting cruciferous crops worldwide22. The construct was designed according to the published rabbit RyR1 NTD crystal structures23,24and the cryo-EM structural models16,17,18,19,20,21. This is the first high-resolution structure reported for insect RyR, which reveals the mechanism for channel gating and provides an important template for the development of species-specific insecticides using structure-based drug design. For structure elucidation, we employed x-ray crystallography, which is considered as the 'gold standard' for protein structure determination at near atomic resolution. Although the crystallization process is unpredictable and labor intensive, this step-by-step protocol will help researchers to express, purify and characterize other domains of insect RyR or any other proteins in general.
1. Gene Cloning, Protein Expression, and Purification
2. Protein Preparation and Crystallization
3. Crystal Mounting, X-Ray Data Collection, and Structure Determination
Purification
The N-terminal domain of DBM RyR was expressed as a fusion protein with a hexahistidine tag, a MBP tag and a TEV protease cleavage site. We followed a five-step purification strategy to obtain a highly pure protein, suitable for crystallization purpose. At first, the fusion protein was purified from the soluble fraction of cell lysate by Ni-NTA column (HisTrap HP). Next, the fusion protein was subjected to TEV protease cleavage and the hexahistidine-MBP moiety was removed by amylose resin column followed by Ni-NTA column. Further, the protein was purified by anion exchange column (Q Sepharose HP) and finally by gel-filtration column (Superdex 200 26/600). The final yield of purified protein from 1 L bacterial culture using 2YT media was ~4 mg. The purified protein showed a single band on SDS-PAGE at ~21 kDa (Figure 1). The elution volume from gel-filtration column confirmed the purified RyR NTD to be a monomer (Figure 1).
Crystallization
Initial crystallization screening in 96-well plates yielded crystals in several conditions. These conditions were selected for crystal optimization in 24 well plates. The most optimal condition where high quality plate-shape crystals were formed was 0.1 M HEPES pH 7.0 and 1.6 M ammonium sulfate (Figure 2).
Structure determination
Crystals obtained was diffracted to 2.84 Å on beamline BL17U1 at Shanghai Synchrotron Radiation Facility. The crystal was indexed in space group P61 with unit-cell parameters a = 170.13, b = 170.13, c = 51.763 Å, α = β = 90.00°, γ = 120°. For structure determination, molecular replacement was employed using rabbit RyR1 NTD as a search model in PHENIX. Further refinement of the structure was done in PHENIX to a final Rwork and Rfree of 21.63 and 24.52%, respectively. The data collection and refinement statistics are listed in Table 1. The solved structure of DBM RyR NTD covering the residues 1-205 (PDB ID 5Y9V) is shown in Figure 3.
Figure 1: SDS-PAGE and gel filtration chromatogram representing purification of DBM RyR NTD35. SDS PAGE (15%) in the inset shows purified DBM RyR NTD as a single band after the five step purification strategy. The left lane shows standard protein marker (PM). The gel filtration chromatogram obtained using a Superdex 200 26/600 column shows the elution peak at 240 mL, which corresponds to the monomeric form of the protein. Please click here to view a larger version of this figure.
Figure 2. Crystallization of DBM RyR NTD35. Crystals of DBM RyR NTD produced by vapor-diffusion method as seen under a light microscope. The crystallization condition was 0.1 M HEPES pH 7.0 and 1.6 M ammonium sulfate. Scale bar = 200 µm. Please click here to view a larger version of this figure.
Figure 3: Structure of DBM RyR NTD (PDB ID 5Y9V)35. Solved structure of the protein shown from two views. Secondary structure elements are labeled. β strands are shown in purple. α helix and a 310 helix are shown in blue. Loops are shown in white. Nt and Ct represent the N-terminal and C-terminal of the protein, respectively. Please click here to view a larger version of this figure.
Table 1: Data collection and refinement statistics for the DBM RyR NTD crystal35. Please click here to download this table.
In this paper, we describe the procedure to recombinantly express, purify, crystallize and determine the structure of DBM RyR NTD. For crystallization, a crucial requirement is to obtain proteins with high solubility, purity and homogeneity. In our protocol, we chose to use pET-28a-HMT vector as it contains a hexahistidine tag and MBP tag, both of which could be utilized for purification to obtain a higher fold purity. Additionally, the MBP tag aids in the solubility of the target protein. We purified the protein by five consecutive steps which yielded protein that was highly pure and suitable for crystallization. Crystallization screening was performed using automated liquid handling system. Compared to traditional screening by manual drop setting, automated system uses very small volumes of sample, saves time and energy. It also enhances reproducibility due to accuracy of liquid handling. Crystals were diffracted in-house for screening and at synchrotron for data collection as it provides x-ray with higher intensity and less divergence, thereby yielding high quality data. Molecular replacement was our first choice as similar structural templates were available in the database. The alternative way is to use experimental phasing, including MAD, SAD, MIR, etc.
While solving the structure of DBM RyR NTD, the Matthews coefficients obtained from Xtriage30 suggested that the asymmetric unit (ASU) most likely contained four monomers with 46% solvent content, which has a probability of 49%. However, we could not find the right solution with four molecules in ASU. Subsequent runs to look for two, three, five, six molecules also failed. Eventually we found the right solution with only one molecule in ASU, which only has 4% probability and over 86% solvent content. The high water content was confirmed after we solved the structure. Thus, the extreme high solvent content does exist depending on the intrinsic way in which protein packs.
Although X-ray crystallography is a gold-standard in protein structure determination, proteins with large disorder or flexible regions and some large protein complexes with weak affinity are challenging to crystallize. Protein engineering methods, including loop-truncation, surface entropy reduction and cross-linking, might improve the chance of obtaining better protein crystals. Besides the revealing of high-resolution protein structures, X-ray crystallography can also be used to study the protein-pesticide interactions, which would help us on structure-based pesticide design. Qi et al. found that different families of diamides might bind to distinct sites that are different across species36. Using our strategy, one can determine the RyR structures from multiple species and identify the unique elements responsible for the observed species-specificity. Overall, this protocol can be adapted for expression, purification, crystallization and structure determination of any proteins or protein domains by themselves or in complex with small molecule drugs.
The authors have nothing to disclose.
Funding for this research was provided by: National Key Research and Development Program of China (2017YFD0201400, 2017YFD0201403), National Nature Science Foundation of China (31320103922, 31230061), and Project of National Basic Research (973) Program of China (2015CB856500, 2015CB856504). We are grateful to the staff on the beamline BL17U1 at Shanghai Synchrotron Radiation Facility (SSRF).
pET-28a-HMT vector | This modified pET vector contains a hexahistidine tag, an MBP fusion protein and a TEV protease cleavage site at the N-terminus (Lobo and Van Petegem, 2009) | ||
E. coli BL21 (DE3) strain | Novagen | 69450-3CN | |
HisTrapHP column (5 mL) | GE Healthcare | 45-000-325 | |
Amylose resin column | New England Biolabs | E8021S | |
Q Sepharose high-performance column | GE Healthcare | 17-1154-01 | |
Amicon concentrators (10 kDa MWCO) | Millipore | UFC901008 | |
Superdex 200 26/600 gel-filtration column | GE Healthcare | 28-9893-36 | |
Automated liquid handling robotic system | Art Robbins Instruments | Gryphon | |
96 Well CrystalQuick | Greiner bio-one | 82050-494 | |
Uni-Puck | Molecular Dimensions | MD7-601 | |
Mounted CryoLoop – 20 micron | Hampton Research | HR4-955 | |
CryoWand | Molecular Dimensions | MD7-411 | |
Puck dewar loading tool | Molecular Dimensions | MD7-607 | |
Nano drop | Thermo Scientific | NanoDrop One | |
Crystal incubator | Molecular Dimensions | MD5-605 | |
X-Ray diffractor | Rigaku | FRX | |
PCR machine | Eppendorf | Nexus GX2 | |
Plasmid mini-prep kit | Qiagen | 27104 | |
Gel extraction kit | Qiagen | 28704 | |
SspI restriction endonuclease | NEB | R0132S | |
T4 DNA polymerase | Novagen | 2868713 | |
Kanamycin | Scientific Chemical | 25389940 | |
IPTG | Genview | 367931 | |
HEPES | Genview | 7365459 | |
β-mercaptoethanol | Genview | 60242 | |
Centrifuge | Thermo Scientific | Sorvall LYNX 6000 | |
Sonnicator | Scientz | II-D | |
Protein purification system | GE Healthcare | Akta Pure | |
Light microscope | Nikon | SMZ745 | |
IzIt crystal dye | Hampton Research | HR4-710 | |
Electrophoresis unit | Bio-Rad | 1658005EDU | |
Shaker Incubator | Zhicheng | ZWYR-D2401 | |
Index crystal screen | Hampton Research | HR2-144 | |
Structure crystal screen | Molecular Dimensions | MD1-01 | |
ProPlex crystal screen | Molecular Dimensions | MD1-38 | |
PACT premier crystal screen | Molecular Dimensions | MD1-29 | |
JCSG-plus crystal screen | Molecular Dimensions | MD1-37 |