Structural and functional correlates between HIV-1 and SIV Nef isolates.

The nef genes of HIV-1 and SIV encode 27-34 kDa myristoylated proteins which have been shown to induce cell surface CD4 downregulation and bind to a cellular protein kinase. To identify regions of Nef important for function, structure-function correlates of HIVSF2 nef (Nef) and SIVmm239open nef (SNef) were sought by constructing Nef/SNef hybrids. Metabolic labeling with 35[S]methionine/cysteine demonstrated similar amounts of 35[S] incorporation into all but one hybrid, SNeftll, in which the C-terminus of SNef was replaced by that of Nef. The weak protein expression of SNeftll was attributable to its short half-life of approximately 45 min. Nef, SNef, and SHSNef, a hybrid containing the internal sequences of Nef and the N- and C-terminal sequences from SNef, downregulated CD4 in human CEM cells. Only Nef and SHSNef downregulated CD4 in mouse AKR1-G1 cells. Nef, SNef, and SHSNef also effectively bound phosphoproteins of MW = 62,000 and 78,000 in CEM cells. Two additional hybrids, in which the Nef sequences of SHSNef were replaced with additional SNef sequences, were essentially ineffective in both assays. Thus, in two different assays of Nef function, swapping the SIV and HIV internal nef sequences were shown to be greatly deleterious to Nef function while SHSNef remained functional.