Multimerizable HIV Gag derivative binds to the liquid-disordered phase in model membranes

During HIV assembly, a protein coat on the inner leaflet of the plasma membrane drives the formation of virus particles, and appears to induce the preferential accumulation of 'raft' lipids in the viral envelope, although the lipid raft concept mainly proposes microdomains of these lipids in the outer leaflet. The common hypothesis is that Gag preferentially associates with, and thereby probably induces, raft-like domains, because the protein is multimerized and specifically linked to two saturated acyl chains. To test this hypothesis, we constructed a minimal in vitro system in which we analysed the interaction of a Gag derivative, which could be triggered to multimerize, with a domain-forming model membrane resembling the inner leaflet of the plasma membrane. Confirming studies with authentic Gag, this Gag derivative only bound to membranes when it was multimerized, myristoylated and when phosphatidylinositol 4,5-bisphosphate was present in the membrane. Unexpectedly, however, the multimerized Gag derivative was largely excluded from ordered domains in model membranes. This suggests that the mechanism of membrane reorganization during HIV assembly does not simply result from a higher affinity of the clustered Gag membrane binding domain to ordered membrane domains, but involves more complex biophysical interactions or possibly also an additional protein machinery.