Gag Localization and Virus-Like Particle Release Mediated by the Matrix Domain of Human T-Lymphotropic Virus Type 1 Gag Are Less Dependent on Phosphatidylinositol-(4,5)-Bisphosphate than Those Mediated by the Matrix Domain of HIV-1 Gag

The human immunodeficiency virus type 1 (HIV-1) Gag matrix (MA) domain facilitates Gag targeting and binding to the plasma membrane (PM) during virus assembly. Interaction with a PM phospholipid, phosphatidylinositol-(4,5)-bisphosphate [ PI(4,5)P-2], plays a key role in these MA functions. Previous studies showed that overexpression of polyphosphoinositide 5-phosphatase IV (5ptaseIV), which depletes cellular PI(4,5)P-2, mislocalizes HIV-1 Gag to the cytosol and greatly reduces HIV-1 release efficiency. In this study, we sought to determine the role of the MA-PI(4,5)P-2 interaction in Gag localization and membrane binding of a deltaretrovirus, human T-lymphotropic virus type 1 (HTLV-1). We compared the chimeric HIV-1 Gag (HTMA), in which MA was replaced with HTLV-1 MA, with wild-type HIV-1 and HTLV-1 Gag for PI(4,5)P-2 dependence. Our results demonstrate that, unlike HIV-1 Gag, subcellular localization of and VLP release by HTLV-1 and HTMA Gag were minimally sensitive to 5ptaseIV overexpression. These results suggest that the interaction of HTLV-1 MA with PI(4,5)P-2 is not essential for HTLV-1 particle assembly. Furthermore, liposome-binding analyses showed that both HTLV-1 and HTMA Gag can bind membrane efficiently even in the absence of PI(4,5)P-2. Efficient HTLV-1 Gag binding to liposomes was largely driven by electrostatic interaction, unlike that of HIV-1 Gag, which required specific interaction with PI(4,5)P-2. Furthermore, membrane binding of HTLV-1 Gag in vitro was not suppressed by RNA, in contrast to HIV-1 Gag. Altogether, our data suggest that Gag targeting and membrane binding mediated by HTLV-1 MA does not require PI(4,5)P-2 and that distinct mechanisms regulate HIV-1 and HTLV-1 Gag membrane binding.

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