Murine retrovirus Pr65gag forms a 130K dimer in the absence of disulfide reducing agents

Gazdar-murine sarcoma virus (Gz-MSV) particles, obtained from tissue culture fluids of chronically infected HTG-2 hamster cells are immature in morphology and contain uncleaved Pr65gag as the predominant protein (greater than 95% Coomassie blue stain) (A. Pinter and E. deHarven, 1979, Virology 99, 103-110; Y. Yoshinaka and R. B. Luftig, 1982, Virology 118, 380-388). When Gz-MSV particles are disrupted in 1% sodium dodecyl sulfate (SDS) and then analyzed by SDS-polyacrylamide gel electrophoresis (PAGE) in the absence of reducing agents, such as beta-mercaptoethanol (beta-MSH) almost half of the Pr65gag Coomassie blue-stained band is detected as a band at a Mr of 130K. Electrophoretic blotting studies with monospecific antisera against MuLV p30, p15, p12, and p10 showed that the 130K band cross-reacted with all four antigens suggesting that it was a dimer of Pr65gag. Two-dimensional (2D) SDS-PAGE where the first dimension was run under nonreducing conditions and the second with beta-MSH, supported the contention that the 130K band was a dimeric complex of Pr65gag. One also saw minor amounts of a 260K and higher polymeric forms of Pr65gag on the SDS gels, suggesting that polymeric forms may exist as well. When 32P-labeled Gz-MSV particles obtained by in vivo labeling of infected HTG-2 cells with [32P]PPi were electrophoresed on SDS-PAGE, only 10% of the 32P label was detected at the 130K position. In contrast, 30% of the Coomassie blue-stained Pr65gag material was found at 130K on the 2D gels. This suggests that unphosphorylated Pr65gag is more likely to participate in dimer formation than phosphorylated Pr65gag. Pr65gag of Moloney murine leukemia virus (M-MuLV), which is present as a minor (5% of stain) protein band on SDS-PAGE also showed 130K dimers. Further, in beta-MSH-deficient SDS preparations of Gz-MSV, electrophoresed after trypsin treatment, a 32K band that stained with p15, but not p10, p12, nor p30, antisera was observed. If beta-MSH was added, this band was no longer present. Thus Pr65gag dimerization in immature MuLV particles appears to at least involve the p15 region of the polyprotein. Since p15 is an extremely hydrophobic protein, formation of Pr65gag dimers may occur when virion precursor proteins are brought to the cell membrane during virus assembly.