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Vpu
Vpu is a protein that in HIV is encoded by the ''vpu'' gene. Vpu stands for "Viral Protein Unique". 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 in order 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, SIV, and SIV, 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, M2 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.
== Expression ==
Vpu and Env are expressed from the same bicistronic mRNA in a Rev-dependent manner, presumably by leaky scanning of ribosomes through the vpu initiation codon. In fact Vpu gene overlaps at its3′-end with the env gene. Several HIV-1 isolates were found to carry point mutations in the Vpu translation initiation codon but have otherwise intact vpu genes. Since removal of the Vpu initiation codon results in increased expression of the downstream env gene, it is possible that HIV-1 actually uses this mechanism as a molecular switch to regulate the relative expression of Vpu or Env in infected cells. The possible benefits of such a regulation are unclear.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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