| HIV-1 Nef, a small accessory protein, is an important pathogenic factor for HIV virus infection and AIDS disease progression. Although the exact mechanism of its pathogenicity is not fully understood by far, at least four aspects of its in vitro functions are found to be involved: (a) Nef-mediated CD4 downregulation; (b) Nef-mediated MHC-I downregulation; (c) activation of cell signaling pathways; (d) enhancement of viral infectivity. Among these functions, CD4 and MHC-I downregulation are the most extensively studied. Acting as an adaptor protein, Nef binds CD4 at the plasma membrane and MHC-I molecules in the trans-Golgi Network (TGN) respectively, utilizes cellular adaptor protein complexes (AP-2 and AP-1 respectively) and reroutes both proteins to lysosome for degradation. However, many mechanism details still remain unclear. Our work revealed that: (1) Nef-mediated CD4 downregulation is AP-2 dependent; (2) An optimal downstream context is required for the AP-2 binding motif, a dileucine motif in the Nef C-terminal loop region to downregulate CD4 properly; (3) HIV-1 Nef is ubiquitinated and lysine 144, a ubiquitin binding site in two Nef alleles investigated, pNL4-3 and NA7, is required for Nef-mediated CD4 and MHC-I downregulation. All these findings contribute to further understand mechanisms of CD4 and MHC-I downregulation mediated by Nef and provide more information to assess Nef as a potential molecular target for anti-viral therapy. |
