Differential activities of the human immunodeficiency virus type 1-encoded Vpu protein are regulated by phosphorylation and occur in different cellular compartments

Abstract

The human immunodeficiency virus type 1 (HIV-1)-specific Vpu is an 81-amino-acid amphipathic integral membrane protein with at least two different biological functions: (i) enhancement of virus particle release from the plasma membrane of HIV-1-infected cells and (ii) degradation of the virus receptor CD4 in the endoplasmic reticulum (ER). We have previously found that Vpu is phosphorylated in infected cells at two seryl residues in positions 52 and 56 by the ubiquitous casein kinase 2. To study the role of Vpu phosphorylation on its biological activity, a mutant of the vpu gene lacking both phosphoacceptor sites was introduced into the infectious molecular clone of HIV-1, pNL4-3, as well as subgenomic Vpu expression vectors. This mutation did not affect the expression level or the stability of Vpu but had a significant effect on its biological activity in infected T cells as well as transfected HeLa cells. Despite the presence of comparable amounts of wild-type and nonphosphorylated Vpu, decay of CD4 was observed only in the presence of phosphorylated wild-type Vpu. Nonphosphorylated Vpu was unable to induce degradation of CD4 even if the proteins were artificially retained in the ER. In contrast, Vpu-mediated enhancement of virus secretion was only partially dependent on Vpu phosphorylation. Enhancement of particle release by wild-type Vpu was efficiently blocked when Vpu was artificially retained in the ER, suggesting that the two biological functions of Vpu are independent, occur at

Solution structure of the X4 protein coded by the SARS related coronavirus reveals an immunoglobulin like fold and suggests a binding activity to integrin I domains

“…Prominent examples of viral accessory proteins with such functions are known from other viruses, e.g. HIV-1 Vpu binds to CD4 and prevents CD4 delivery to the cell surface and induces even its degradation [38,39]. LFA-1 is exclusively expressed on the surface of leukocytes including T cells and dendritic cells.…”

NMR structural characterization of HIV‐1 virus protein U cytoplasmic domain in the presence of dodecylphosphatidylcholine micelles

“…The most well-defined function of VpU is downregulation of CD4 in the endoplasmic reticulum, which is mediated by the cytoplasmic region of the protein [1,2]. This function involves binding and recruitment of the b-transducin repeat-containing protein (b-TrCP) [3,4] The HIV-1 encoded virus protein U (VpU) is required for efficient viral release from human host cells and for induction of CD4 degradation in the endoplasmic reticulum.…”

Viral Takeover of the Host Ubiquitin System

“…The ability of the HIV-1 accessory protein Vpu to enhance viral release has been known for quite some time, however the www.frontiersin.org mechanism for this function was not clear. Viral egress appeared to be separable from Vpu's prominent role in inducing the ERassociated proteasomal degradation of the HIV-1 receptor, CD4 (Schubert and Strebel, 1994;Schubert et al, 1996), although some viral release enhancement has been attributed to CD4 depletion (Bour et al, 1999). However, recent evidence suggests that, similar to the situation with CD4, Vpu acts as a substrate recognition factor for an SCF Ub ligase complex to target Tetherin for downregulation (Van Damme et al, 2008;Douglas et al, 2009;Goffinet et al, 2009;Iwabu et al, 2009;Mangeat et al, 2009;Mitchell et al, 2009).…”