The SH3 domain-binding surface and an acidic motif in HIV-1 Nef regulate trafficking of class I MHC complexes

Fief, a regulatory protein of human and simian immunodeficiency viruses, downregulates cell surface expression of both class I MHC and CD4 molecules in T cells by accelerating their endocytosis, Fibroblasts were used to study alterations in the traffic of class I MHC complexes induced by Fief. We found that Nef downregulates class I MHC complexes by a novel mechanism involving the accumulation of endocytosed class I MHC in the trans-Golgi, where it colocalizes with the adaptor protein-1 complex (AP-1), This effect of Fief on class I MHC traffic requires the SH3 domain-binding surface and a cluster of acidic amino acid residues in Fief, both of which are also required for Fief to downregulate class I MHC surface expression and to alter signal transduction in T cells. Downregulation of class I MHC complexes from the surface of T cells also requires a tyrosine residue in the cytoplasmic domain of the class I MHC heavy chain molecule. The requirement of the same surfaces of the Nef molecule for downregulation of surface class I MHC complexes in T cells and for their accumulation in the trans-Golgi of fibroblasts indicates that the two effects of Nef involve similar interactions with the host cell machinery and involve a molecular mechanism regulating class I MHC traffic that is common for both of these cell types. Interestingly, the downregulation of class I MHC does not require the ability of Nef to colocalize with the adaptor protein-2 complex (AP-2), We showed previously that the ability of Fief to colocalize with AP-2 correlates with the ability of Fief to downregulate CD4 expression. Our observations indicate that Nef downregulates class I MHC and CD4 surface expression via different interactions with the protein sorting machinery, and link the sorting and signal transduction machineries in the regulation of class I MHC surface expression by Nef.