HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 AND TYPE-2 AND SIMIAN IMMUNODEFICIENCY VIRUS NEF USE DISTINCT BUT OVERLAPPING TARGET SITES FOR DOWN-REGULATION OF CELL-SURFACE CD4

Although the Nef proteins encoded by human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are known to induce the efficient internalization and degradation of cell surface CD4, it remains unclear whether this process involves a direct interaction be tween Nef and CD4. Here, we report that CD4 downregulation by HIV-1 an d SIV Nef requires distinct but overlapping target sites within the CD 4 intracytoplasmic domain, In particular, mutation of a glutamic acid residue located at CD4 residue 405 or of arginine and methionine resid ues located, respectively, at residues 406 and 407 results in a mutant CD4 protein that is efficiently downregulated by HIV-1 Nef but refrac tory to downregulation by SIV Nef. However, both HIV-1 and SIV Nef req uire an isoleucine located at residue 410 and the dileucine motif Foun d at CD4 residues 413 and 414. CD4 downregulation induced by the Nef p rotein encoded by HIV-2 is shown to require a CD4 target sequence that is similar to, but distinct from, that observed with SIV Nef, These d ata explain the previous finding that the murine CD4 protein, which ha s an alanine at residue 405, is refractor to downregulation by SN, but not HIV-1, Nef (J. L. Foster, S. J. Anderson, A. L. B. Frazier, and J . V. Garcia, Virology 201:373-379, 1994), In addition, these observati ons provide strong genetic support for the hypothesis that the Nef-med iated downregulation of cell surface CD4 requires a direct Nef-CD4 int eraction.