Assembly and Purification of Prototype Foamy Virus Intasomes
Summary
Recombinant retroviral integrase and DNA oligomers mimicking viral DNA ends can form an enzymatically active complex known as an intasome. Intasomes may be used for biochemical, structural, and kinetic studies. This protocol details how to assemble and purify prototype foamy virus intasomes.
Abstract
A defining feature and necessary step of the retrovirus life cycle is the integration of the viral genome into the host genome. All retroviruses encode an integrase (IN) enzyme that catalyzes the covalent joining of viral to host DNA, which is known as strand transfer. Integration may be modeled in vitro with recombinant retroviral IN and DNA oligomers mimicking the ends of the viral genome. In order to more closely recapitulate the integration reaction that occurs in vivo, integration complexes are assembled from recombinant IN and synthetic oligomers by dialysis in a reduced salt concentration buffer. The integration complex, called an intasome, may be purified by size exclusion chromatography. In the case of prototype foamy virus (PFV), the intasome is a tetramer of IN and two DNA oligomers and is readily separated from monomeric IN and free oligomer DNA. The integration efficiency of PFV intasomes may be assayed under a variety of experimental conditions to better understand the dynamics and mechanics of retroviral integration.
Introduction
Integration of the viral genome into the host genome is a mandatory step in the life cycle of all retroviruses1. The viral enzyme integrase (IN) catalyzes the covalent joining of each end of the viral DNA genome to the host DNA. During a cellular infection, IN is part of a pre-integration complex that mediates integration. Recombinant IN complexed with double stranded DNA oligomers mimicking the viral DNA ends can also perform integration into a target DNA in vitro2. A common integration assay in vitro utilizes a supercoiled plasmid as the target DNA. Integration of both viral DNA oligomers (vDNA) to the plasmid results in a linear product and is termed concerted integration (Figure 2A). The integration assay in vitro may also yield products with only one vDNA covalently joined to the target plasmid resulting in a relaxed circle. This half-site integration product appears to be an artefact of the assay in vitro.
Recombinant IN and vDNA may perform integration in vitro, but they are not ideal reagents for the study of the dynamics or structure of integration complexes when monomeric IN would obscure relevant visualization. Purified integration complexes, or "intasomes," are required for dynamic single molecule analysis or structural studies. PFV IN and vDNAs may be assembled by dialysis from a relatively high salt concentration buffer to a lower salt concentration3,4. During dialysis, a precipitate forms. This precipitate is removed from dialysis and the salt concentration is increased. The higher salt concentration solubilizes the precipitate containing PFV intasomes. The intasomes are then purified by size exclusion chromatography (SEC). Recombinant prototype foamy virus (PFV) IN has been shown to exist as a monomer in solution at concentrations up to 225 µM5. SEC fractionation effectively separates the PFV intasomes (225.5 kDa), which includes a tetramer of PFV IN and two vDNAs, from monomeric PFV IN (44.4 kDa) and free vDNA (24.0 kDa). The PFV intasomes may be frozen and retain integration activity for at least six months of storage at -80 ˚C.
Recombinant PFV intasomes may also be modified to include IN amino acid substitutions or truncation mutations or vDNAs labeled with fluorophores or biotin4,6. The purified PFV intasomes readily perform integration into a supercoiled plasmid target DNA in vitro. Bulk biochemical integration assays with intasomes may test the effects of IN mutations, IN inhibitors, or other chemical additives. Biotinylated intasomes can be used to probe affinity with nucleic acids or proteins. PFV intasomes are functional at ambient temperature allowing for single molecule microscopy analysis by magnetic tweezers to measure the time between joining of the two vDNA ends or total internal reflection fluorescence to visualize the intasome search on target DNA6. In addition, PFV intasomes were the first to be structurally characterized significantly impacting the field of retroviral integration3.
Protocol
Representative Results
Discussion
Retroviral INs form a multimeric complex with viral genomic DNA to perform integration and continue the viral life cycle. The number of IN monomers per intasome may be tetramers, octamers, or possibly higher order multimers11,12,13,14. PFV intasomes are a tetramer of recombinant IN with double-stranded DNA oligomers that mimic viral genomic DNA ends3. These inta…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was supported by NIH AI099854 and AI126742 to KEY.
Materials
| DNA Oligomers | IDT | N/A | Custom DNA Oligos |
| Tris Ultra Pure | Gojira Fine Chemicals | UTS1003 | |
| NaCl | P212121 | RP-S23020 | |
| UltraPure EDTA | Invitrogen/Gibco | 15575 | |
| Amicon Ultra 0.5 mL centrifugal filters | Sigma-Aldrich | Z677094-24EA | 3 kDa MWCO |
| DTT | P212121 | SV-DTT | |
| BIS-TRIS propane,>=99.0% (titration) | Sigma-Aldrich | B6755-500G | |
| ZnCl2 | Sigma-Aldrich | 208086 | |
| MgSO4 | Amresco | 0662 | |
| Glycerol | Thermo Fisher Scientific | G37-20 | |
| Gel-loading tips, 1 – 200 μL | Corning | CLS4853-400EA | |
| Razor blade; Single-edged; 100/Pk.; Pack of 100 | Fisher Scientific | 12-640 | |
| Sterile Disposable Filter Units with PES Membrane > 250mL | Thermo Fisher Scientific | 568-0020 | |
| Dialysis tubing clips | Spectrum Labs | 132734 | |
| 6-8 kDa 10 mm Dialysis Tubing | Spectrum Medical | 132645 | |
| Superose 6 10/300 GL | GE Healthcare Life Sciences | 17517201 | |
| Hi-Res Standard Agarose | AGTC Bioproducts | AG500D1 | |
| Ethidium bromide | Thermo Fisher Scientific | BP1302 | |
| Orange G | Fisher Scientific | 0-267 | |
| Hyladder 10kb, 500 lanes | Denville Scientific | CB4225-4 |
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