VPU-binding protein (UBP) and its role in human immunodeficiency virus type 1 (HIV-1) particle release

Vpu plays two roles in HIV-1 replication: the degradation of the HIV-1 receptor, CD4, and the enhancement of virion release. A yeast GAL-4 two-hybrid system was used to screen a lymphocyte a cDNA library for cDNAs that encode proteins that interact with Vpu. This analysis resulted in the identification of a novel cDNA sequence and protein that we have termed UBP for "Vpu&barbelow b&barbelowinding p&barbelowrotein." UBP is a novel member of the tetratricopeptide repeat (TPR) protein family. The ubp RNA is approximately 2600 nucleotides in length and is expressed in many human tissues. Western blot analysis using UBP-specific polyclonal rabbit antiserum detects a protein of approximately 34 kDa in HeLa cells. UBP was also found to interact with HIV-1 Gag only when Gag was not co-expressed with Vpu. The co-expression of Vpu resulted in the loss of the ability of Gag to bind to UBP. The data suggest that the presence of Vpu in Gag-expressing cells results in a modification of Gag which renders Gag incapable of binding to UBP. Overexpression of UBP in virus-producing HeLa cells led to a marked decrease in virion release, indicating that UBP likely plays a role in the virus release activity of Vpu. Mutagenesis of Vpu was conducted in an attempt to correlate UBP binding with virus release enhancement. The results indicate that the central one-third of Vpu is involved in the interaction with UBP, and the transmembrane domain and the C-terminal one-third of Vpu are involved in the enhancement of virus release. The results also suggest that the enhancement of particle release mediated by Vpu is independent of the ability of Vpu to interact with UBP.