Relationships between MA-RNA binding in cells and suppression of HIV-1 Gag mislocalization to intracellular membranes

Abstract

The HIV-1 Gag matrix (MA) domain mediates localization of Gag to the plasma membrane (PM), the site for infectious virion assembly. The MA highly basic region (HBR) interacts with phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2], a PM-specific acidic lipid. MA-HBR also binds RNAs. To test whether acidic lipids alone determine PM-specific localization of Gag or whether MA-RNA binding also plays a role, we compared a panel of MA-HBR mutants that contain two types of substitutions at MA residues 25/26 or 29/31: Lys->Arg (KR) (25/26KR and 29/31KR) and Lys->Thr (KT) (25/26KT and 29/31KT). Consistent with the importance of the HBR charge in RNA binding, both KT mutants failed to bind RNA via MA efficiently unlike the corresponding KR mutants. Both 25/26KT Gag-YFP and 29/31KT Gag-YFP bound non-specifically to PM and intracellular membranes, presumably via the myristoyl moiety and remaining MA basic residues. In contrast, 25/26KR Gag-YFP bound specifically to the PM, suggesting a role for the total positive charge and/or MA-bound RNA in navigating Gag to the PM. Unlike 29/31KT Gag-YFP, 29/31KR Gag-YFP was predominantly cytosolic and showed little intracellular membrane binding despite having a higher HBR charge. Therefore, it is likely that MA-RNA binding blocks promiscuous Gag membrane binding in cells. Notably, introduction of a heterologous multimerization domain restored PI(4,5)P2-dependent PM-specific localization for 29/31KR Gag-YFP, suggesting that the blocking of PM binding is more readily reversed than that of intracellular membrane binding. Altogether, these cell-based data support a model in which MA-RNA binding ensures PM-specific localization of Gag via suppression of non-specific membrane binding.\n\nIMPORTANCEThe PM-specific localization of HIV-1 Gag is a crucial early step in the infectious progeny production. The interaction between the MA highly basic region (HBR) of Gag and the PM-specific lipid PI(4,5)P2 is critical for Gag localization to the PM. Additionally, in vitro evidence has indicated that MA-RNA binding prevents non-specific binding of Gag to non-PI(4,5)P2-containing membranes. However, cell-based evidence supporting a role for HIV-1 MA-RNA binding in PM-specific subcellular localization has been scarce; thus, it remained possible that in cells, just the high basic charge or the PI(4,5)P2-binding ability is sufficient for MA to direct Gag specifically to the PM. The current study revealed for the first time an excellent correlation between RNA binding of MA-HBR and inhibition of promiscuous Gag localization, both within the cells, and thereby provided cell-based evidence supporting a mechanism in which HIV-1 MA binding to RNA ensures specific localization of Gag to the PM.