Vpu protein

Vpu is an accessory protein that in HIV is encoded by the vpu gene. Vpu stands for "Viral Protein U". The Vpu protein acts in the degradation of CD4 in the endoplasmic reticulum and in the enhancement of virion release from the plasma membrane of infected cells. Vpu induces the degradation of the CD4 viral receptor and therefore participates in the general downregulation of CD4 expression during the course of HIV infection. Vpu-mediated CD4 degradation is thought to prevent CD4-Env binding in the endoplasmic reticulum to facilitate proper Env assembly into virions. It is found in the membranes of infected cells, but not the virus particles themselves.

The Vpu gene is found exclusively in HIV-1 and some HIV-1-related simian immunodeficiency virus (SIV) isolates, such as SIV_cpz, SIV_gsn, and SIV_mon, but not in HIV-2 or the majority of SIV isolates. Structural similarities between Vpu and another small viral protein, M2, encoded by influenza A virus were first noted soon after the discovery of Vpu. Since then, Vpu has been shown to form cation-selective ion channels when expressed in Xenopus oocytes or mammalian cells and also when purified and reconstituted into planar lipid bilayers. Vpu also permeabilizes membranes of bacteria and mammalian cells to small molecules. Therefore, it is considered a member of the Viroporins family.