Analysis of Human Immunodeficiency Virus Type 1 Matrix Binding to Membranes and Nucleic Acids

The human immunodeficiency virus type 1 (HIV-1) matrix (MA) protein targets HIV-1 precursor Gag (PrGag) proteins to assembly sites at plasma membrane (PM) sites that are enriched in cholesterol and phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P-2]. MA is myristoylated, which enhances membrane binding, and specifically binds PI(4,5)P-2 through headgroup and 2' acyl chain contacts. MA also binds nucleic acids, although the significance of this association with regard to the viral life cycle is unclear. We have devised a novel MA binding assay and used it to examine MA interactions with membranes and nucleic acids. Our results indicate that cholesterol increases the selectivity of MA for PI(4,5)P-2-containing membranes, that PI(4,5)P-2 binding tolerates 2' acyl chain variation, and that the MA myristate enhances membrane binding efficiency but not selectivity. We also observed that soluble PI(4,5)P-2 analogues do not compete effectively with PI(4,5)P-2-containing liposomes for MA binding but surprisingly do increase nonspecific binding to liposomes. Finally, we have demonstrated that PI(4,5)P-2-containing liposomes successfully outcompete nucleic acids for MA binding, whereas other liposomes do not. These results support a model in which RNA binding protects MA from associating with inappropriate cellular membranes prior to PrGag delivery to PM assembly sites.