HIV-1 Vpu

Enhances virion budding by targeting host CD4 and Tetherin/BST2 to proteasome degradation. Degradation of CD4 prevents any unwanted premature interactions between viral Env and its host receptor CD4 in the endoplasmic reticulum. Degradation of antiretroviral protein Tetherin/BST2 is important for virion budding, as BST2 tethers new viral particles to the host cell membrane. Mechanistically, Vpu bridges either CD4 or BST2 to BTRC, a substrate recognition subunit of the Skp1/Cullin/F-box protein E3 ubiquitin ligase, induces their ubiquitination and subsequent proteasomal degradation. The alteration of the E3 ligase specificity by Vpu seems to promote the degradation of host IKBKB, leading to NF-kappa-B down-regulation and subsequent apoptosis. Acts as a viroporin that forms an oligomeric ion channel in membranes. Modulates the host DNA repair mechanisms to promote degradation of nuclear viral cDNA in cells that are already productively infected in order to suppress immune sensing and proviral hyper-integration (superinfection). Manipulates PML-NBs and modulates SUMOylation of host BLM protein thereby enhancing its DNA-end processing activity toward viral unintegrated linear DNA. Also inhibits RAD52-mediated homologous repair of viral cDNA, preventing the generation of dead-end circular forms of single copies of the long terminal repeat and permitting sustained nucleolytic attack.