Binding of human immunodeficiency virus type 1 Gag to membrane: Role of the matrix amino terminus

Binding of the human immunodeficiency virus type 1 (HIV-1) Gag protein prec ursor, Pr55(Gag), to membrane is an indispensable step in virus assembly. P reviously, we reported that a matrix (MA) residue 6 substitution (6VR) impo sed a virus assembly defect similar to that observed with myristylation-def ective mutants, suggesting that the 6VR change impaired membrane binding. I ntriguingly, the 6VR mutation had no effect on Gag myristylation, The defec tive phenotype imposed by 6VR was reversed by changes at other positions in MA, including residue 97, In this study, we use several biochemical method s to demonstrate that the residue 6 mutation, as well as additional substit utions in MA amino acids 7 and 8, reduce membrane binding without affecting N-terminal myristylation. This effect is observed in the context of Pr55(G ag), a truncated Gag containing only MA and CA, and in MA itself. The membr ane binding defect imposed by the 6VR mutation is reversed by second-site c hanges in MA residues 20 and 97, both of which, when present alone, increas e membrane binding to levels greater than those for the wild type. Both red uced and enhanced membrane binding imposed by the MA substitutions depend u pon the presence of the N-terminal myristate. The results support the myris tyl switch model recently proposed for the regulation of Gag membrane bindi ng, according to which membrane binding is determined by the degree of expo sure or sequestration of the N-terminal myristate moiety. Alternatively, in sertion of the myristate into the lipid bilayer might be a prerequisite eve nt for the function of other distinct MA-encoded membrane binding domains.