CD4 downregulation by the HIV-1 protein Nef reveals distinct roles for the γ1 and γ2 subunits of the AP-1 complex in protein trafficking

The HIV accessory protein Nef is a major determinant of viral pathogenesis that facilitates viral particle release, prevents viral antigen presentation and increases infectivity of new virus particles. These functions of Nef involve its ability to remove specific host proteins from the surface of infected cells, including the CD4 receptor. Nef binds to the adaptor protein 2 (AP-2) and CD4 in clathrin-coated pits, forcing CD4 internalization and its subsequent targeting to lysosomes. Herein, we report that this lysosomal targeting requires a variant of AP-1 containing isoform 2 of gamma-adaptin (AP1G2, hereafter gamma 2). Depletion of the gamma 2 or mu 1A (AP1M1) subunits of AP-1, but not of gamma 1 (AP1G1), precludes Nef-mediated lysosomal degradation of CD4. In gamma 2-depleted cells, CD4 internalized by Nef accumulates in early endosomes and this alleviates CD4 removal from the cell surface. Depletion of gamma 2 also hinders EGFR-EGF-complex targeting to lysosomes, an effect that is not observed upon gamma 1 depletion. Taken together, our data provide evidence that the presence of gamma 1 or gamma 2 subunits delineates two distinct variants of AP-1 complexes, with different functions in protein sorting.