Abstract
BackgroundThe Pr55gag (Gag) polyprotein of HIV serves as a scaffold for virion assembly and is thus essential for progeny virion budding and maturation. Gag localizes to the plasma membrane (PM) and membranes of late endosomes, allowing for release of infectious virus directly from the cell membrane and/or upon exocytosis. The host factors involved in Gag trafficking to these sites are largely unknown. Upon activation, CD4+ T cells, the primary target of HIV infection, express the class II transcriptional activator (CIITA) and therefore the MHC class II isotype, HLA-DR. Similar to Gag, HLA-DR localizes to the PM and at the membranes of endosomes and specialized vesicular MHC class II compartments (MIICs). In HIV producer cells, transient HLA-DR expression induces intracellular Gag accumulation and impairs virus release.Methodology/Principal FindingsHere we demonstrate that both stable and transient expression of CIITA in HIV producer cells does not induce HLA-DR-associated intracellular retention of Gag, but does increase the infectivity of virions. However, neither of these phenomena is due to recapitulation of the class II antigen presentation pathway or CIITA-mediated transcriptional activation of virus genes. Interestingly, we demonstrate that CIITA, apart from its transcriptional effects, acts cytoplasmically to enhance Pr160gag-pol (Gag-Pol) levels and thereby the viral protease and Gag processing, accounting for the increased infectivity of virions from CIITA-expressing cells.Conclusions/SignificanceThis study demonstrates that CIITA enhances HIV Gag processing, and provides the first evidence of a novel, post-transcriptional, cytoplasmic function for a well-known transactivator.
Highlights
HIV polyprotein Gag serves as a scaffold to promote assembly of progeny virions at cellular membranes [1] and recruits components of the vesicular protein sorting pathway to facilitate virus budding [2,3,4]
Flow cytometric analysis confirmed these findings, as cells transfected with HLA-DR heterodimers and/or co-transfected with some or all of the components of the class II antigen presentation pathway stained as Gaghi, indicating Gag accumulation (Figure 1B and C)
Upon investigation of class II transcriptional activator (CIITA)-mediated alleviation of HLA-DR-induced Gag retention, we found that this effect was not due to the lack of invariant chain (Ii) or HLA-DM, as had been expected, nor was it a consequence of Rab4B expression in these cells
Summary
HIV polyprotein Gag serves as a scaffold to promote assembly of progeny virions at cellular membranes [1] and recruits components of the vesicular protein sorting pathway to facilitate virus budding [2,3,4]. Host factors which participate in targeting Gag trafficking to particular membranes are largely unknown. The first model proposes that following synthesis, Gag traffics to endosomal membranes, and upon exocytosis is deposited on the PM, where it serves as the site for productive virus assembly [14,17]. The Pr55gag (Gag) polyprotein of HIV serves as a scaffold for virion assembly and is essential for progeny virion budding and maturation. Gag localizes to the plasma membrane (PM) and membranes of late endosomes, allowing for release of infectious virus directly from the cell membrane and/or upon exocytosis. In HIV producer cells, transient HLA-DR expression induces intracellular Gag accumulation and impairs virus release
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