Vpu can bind to newly synthesized CD4 in the endoplasmic reticulum. In addition, Vpu has a binding domain for TrCP. This domain includes two conserved serine residues in the cytoplasmic domain of Vpu that are constitutively phosphorylated by casein kinase 2 (CK-2). Formation of multiprotein complexes consisting of TrCP, Vpu, and CD4 results in the TrCP-dependent ubiquitination of CD4 and subsequent degradation by cytosolic proteasomes.
In unstimulated cells, NF-kB resides in the cytoplasm in an inactive complex with its inhibitor IkB. Upon stimulation of cells by cytokines such as TNF-a (1), IkB is rapidly phosphorylated by an IkB specific kinase (2), which results in the rapid degradation of IkB via a TrCP-dependent pathway (3). Infection of cells by HIV-1 results in the gradual intracellular accumulation of Vpu. Because of its constitutively active TrCP-binding motif and the fact that it is not sensitive to TrCP-mediated proteolysis, Vpu functions as a competitive inhibitor of TrCP. This results in the gradual accumulation of IkB and the progressive impairment of the cell’s ability to activate NF-kB (4). The inhibition of NF-kB blocks the synthesis of anti-apoptotic proteins such as the Bcl-2 family proteins (e.g. Bcl-xL and A1/Bfl-1) or TNFR complex proteins (e.g., TRAF1) (5). TRAF1 is induced by TNF-a treatment and normally inhibits activation of caspase-8 (6). In Vpu-expressing cells, the levels of TRAF1, in response to TNF stimulation, are reduced and no longer sufficient to inhibit the cytokine-induced activation of caspase-8 (6). Activated caspase-8 in turn induces the release of cytochrome c from the mitochondria (7). Release of cytochrome c is normally inhibited by the Bcl-2 family of proteins. However, in Vpu-expressing cells the levels of Bcl-2 proteins are limiting and no longer sufficient to block cytochrome c release (8). After its release from the mitochondria, cytochrome c forms ternary complexes with Apaf-1 and caspase-9 (9), resulting in the activation of caspase-3 (10). Active caspase-3 finally triggers a reaction that results in the cleavage of a number of target proteins including Bcl-2 family proteins (11) and leads to cell death (12).
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Last Updated July 05, 2006