VPU-INDUCED DEGRADATION OF CD4 - REQUIREMENT FOR SPECIFIC AMINO-ACID-RESIDUES IN THE CYTOPLASMIC DOMAIN OF CD4

Two functions have been attributed to the product of the human immunod eficiency virus type 1 vpu open reading frame: it increases virion rel ease from infected cells and induces rapid degradation of CD4 shortly after its synthesis. In the absence of Vpu, newly synthesized gp160 an d CD4 associate in the endoplasmic reticulum (ER), forming a complex w hose further maturation is blocked and which is eventually degraded. I n studies using NL4-3-based expression vectors, it has been previously shown that Vpu induces the release of gp160 from the complex that it forms with CD4 in the ER. This release, which appears to be due to the rapid degradation of CD4 induced by Vpu, allows gp160 to transit to t he Golgi, where it matures further. We investigated which regions of C D4 are important for its susceptibility to Vpu-induced degradation by transfecting HeLa cells with isogenic vpu-positive and vpu-negative pr oviruses and vectors expressing various truncated or mutated CD4 molec ules. The results suggested that the cytoplasmic domain of CD4 contain s a determinant lying within amino acids 418 to 425 that is critical f or susceptibility to Vpu-induced degradation. Neither the phosphorylat ion sites in the cytoplasmic domain nor the Lck interaction region was required for the effect. Vpu-induced degradation was specific for CD4 , since CD8, even when retained in the ER, was not degraded. In additi on, under conditions of high-level Vpu expression, CD4 degradation cou ld be observed in the absence of gp160 or other means of retaining CD4 in the ER.