Structural interactions between retroviral Gag proteins examined by cysteine cross-linking

We have examined structural interactions between Gag proteins within Moloney murine leukemia virus (M-MuLV) particles by making use of the cysteine-specific cross-linking agents iodine and bis-maleimido hexane. Virion-associated wild-type M-MuLV Pr65(Gag) proteins in immature particles were intermolecularly cross-linked at cysteines to form Pr65(Gag) oligomers, from dimers to pentamers or hexamers. Following a systematic approach of cysteine-to-serine mutagenesis, we have shown that cross-linking of Pr65(Gag) occurred at cysteines of the nucleocapsid (NC) Cys-His motif, suggesting that the Cys-His motifs within virus particles are packed in close proximity. The M-MuLV Pr65(Gag) protein did not cross-link to the human immunodeficiency virus Pr55(Gag) protein when the two molecules were coexpressed, indicating either that they did not coassemble or that heterologous Gag proteins were not in close enough proximity to be cross-linked. Using an assembly- competent, protease-minus, cysteine-minus Pr65(Gag) protein as a template, novel cysteine residues were generated in the M-MuLV capsid domain major homology region (MHR). Cross-linking of proteins containing MHR cysteines showed above-background levels of Gag-Gag dimers but also identified a novel cellular factor, present in virions, that cross-linked to MHR residues. Although the NC cysteine mutation was compatible with M-MuLV particle assembly, deletions of the NC domain were not tolerated. These results suggest that the Cys-His motif is held in close proximity within immature M- MuLV particles by interactions between CA domains and/or non-Cys-His motif domains of the NC.