PROLINE-RICH (PXXP) MOTIFS IN HIV-1 NEF BIND TO SH3 DOMAINS OF A SUBSET OF SRC KINASES AND ARE REQUIRED FOR THE ENHANCED GROWTH OF NEF(+) VIRUSES BUT NOT FOR DOWN-REGULATION OF CD4

Human immunodeficiency virus (HIV) and simian immunodeficiency virus Nef proteins contain a conserved motif with the minimal consensus (PxxP) site for Src homology region 3 (SH3)-mediated protein-protein interactions. Nef PxxP motifs show specific binding to biotinylated SH3 domains of Hck and Lyn, but not to those of other tested Src family kinases or less related proteins. A unique cooperative role of a distant proline is also observed. Endogenous Hck of monocytic U937 cells can be specifically precipitated by matrix-bound HIV-1 Nef, but not by mutant protein lacking PxxP. Intact Nef PxxP motifs are dispensable for Nef-induced CD4 down-regulation, but are required for the higher in vitro replicative potential of Nef(+) viruses. Thus, CD4 down-regulation and promotion of viral growth are two distinct functions of Nef, and the latter is mediated via SH3 binding.