Expression, purification, and characterization of full-length bovine leukemia virus Gag protein from bacterial culture

• Full-length BLV Gag protein was cloned into a pET45b vector.
• Expression of full-length protein requires induction at room temperature.
• Ammonium sulfate precipitation is necessary to remove truncated proteins.
• BLV Gag is primarily monomeric at lower concentrations.
• BLV Gag’s conformation is more flexible than MLV Gag but less so than HIV-1 Gag.

In retroviruses, the Gag protein is a precursor from which the mature proteins matrix, capsid, and nucleocapsid are derived. Gag plays an important structural role in the assembly of virions at the plasma membrane. While Gag proteins from several different retroviruses have been purified for study in vitro, there has yet to be a report of successful purification of deltaretroviral Gag. In this paper, we report the cloning, expression and purification of full-length bovine leukemia virus (BLV) Gag from Escherichia coli using a combination of polyethyleneimine precipitation, ammonium sulfate precipitation, and affinity chromatography. Experiments using size-exclusion chromatography were also performed to analyze the oligomeric state of the Gag protein in solution, and results suggest that it exists primarily as a monomer but may oligomerize into higher-order complexes to a small extent. Molecular weight estimation by comparison of elution volume to a set of protein standards supports the hypothesis that BLV Gag adopts a slightly extended conformation in solution. The results are discussed in comparison to the solution structure and assembly pathways of other retrovirus genera.