The effect of point mutations within the N-terminal domain of Mason-Pfizer monkey virus capsid protein on virus core assembly and infectivity.

Retroviral capsid protein (CA) mediates protein interactions driving the assembly of both immature viral particles and the core of the mature virions. Structurally conserved N-terminal domains of several retroviruses refold after proteolytic cleavage into a beta-hairpin, stabilized by a salt bridge between conserved N-terminal Pro and Asp residues. Based on comparison with other retroviral CA, we identified Asp50 and Asp57 as putative interacting partners for Pro1 in Mason-Pfizer monkey virus (M-PMV) CA. To investigate the importance of CA Pro1 and its interacting Asp in M-PMV core assembly and infectivity, P1A, P1Y, D50A, T54A and D57A mutations were introduced into M-PMV. The P1A and D57A mutations partially blocked Gag processing and the released viral particles exhibited aberrant cores and were non-infectious. These data indicate that the region spanning residues Asp50-Asp57 plays an important role in stabilization of the beta-hairpin and that Asp57 likely forms a salt-bridge with P1 in M-PMV CA.