Modulation of protein trafficking pathways by HIV-1 Nef
HIV-1 Nef is a viral pathogenic determinant that manipulates the host cell biology to promote the viral life cycle. HIV-1 Nef has been reported to alter the intracellular trafficking of important molecules that participate in the anti-HIV immune response, such as class I and II major histocompatibility complex molecules (MHC-I and MHC-II), CD4, CD28, and DC-SIGN.
The molecular details of MHC-I and CD4 disruption are poorly understood; however, it has been proposed that Nef affects intracellular trafficking pathways by acting as a molecular adaptor protein. In support of this theory, we determined that Nef interacted with the cytoplasmic domain of MHC-I both in vivo and in vitro, and required a highly conserved tyrosine-based sequence in MHC-I. Interestingly, MHC-I allotypes that harbored substitutions in this region were not susceptible to Nef. This differential regulation of MHC-I would allow the virus to simultaneously escape immune surveillance by both cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells.
To further define the exact mechanism of MHC-I disruption, we carefully examined MHC-I biology in T cells that expressed HIV-1 Nef. We found that MHC-I assembly, maturation, and transit to the trans-Golgi network (TGN) were not affected by HIV-1 Nef. However, instead of being exported to the cell surface, MHC-I was redirected to acidic compartments for degradation. The ability of Nef to alter MHC-I trafficking was dependent on expression of AP-1A, a cellular protein complex known to mediate TGN-to-endolysosomal vesicle transport. In HIV-infected primary T cells, Nef promoted a physical interaction between endogenous AP-1 and MHC-I, and we determined the recruitment of AP-1 required amino acids in both Nef and the MHC-I cytoplasmic tail. Finally, the Nef-dependent degradation of both MHC-I and CD4 was ultimately mediated through a late endosomal compartment, and required the expression of the small GTPase, Rab7. These molecular details reveal important clues about the biological activities of HIV-1 Nef and its role in the pathogenicity of HIV.