Rous Sarcoma Virus Gag Has No Specific Requirement for Phosphatidylinositol-(4,5)-Bisphosphate for Plasma Membrane Association In Vivo or for Liposome Interaction In Vitro

The MA domain of the retroviral Gag protein mediates interactions with the plasma membrane, which is the site of productive virus release. HIV-1 MA has a phosphatidylinositol-(4,5)-bisphosphate [ PI(4,5)P(2)] binding pocket; depletion of this phospholipid from the plasma membrane compromises Gag membrane association and virus budding. We used multiple methods to examine the possible role of PI(4,5)P(2) in Gag-membrane interaction of the alpharetrovirus Rous sarcoma virus (RSV). In contrast to HIV-1, which was tested in parallel, neither membrane localization of RSV Gag-GFP nor release of virus-like particles was affected by phosphatase-mediated depletion of PI(4,5)P(2) in transfected avian cells. In liposome flotation experiments, RSV Gag required acidic lipids for binding but showed no specificity for PI(4,5)P(2). Mono-, di-, and triphosphorylated phosphatidylinositol phosphate (PIP) species as well as high concentrations of phosphatidylserine (PS) supported similar levels of flotation. A mutation that increases the overall charge of RSV MA also enhanced Gag membrane binding. Contrary to previous reports, we found that high concentrations of PS, in the absence of PIPs, also strongly promoted HIV-1 Gag flotation. Taken together, we interpret these results to mean that RSV Gag membrane association is driven by electrostatic interactions and not by any specific association with PI(4,5)P(2).