Differential activities of the human immunodeficiency virus type 1-encoded Vpu protein are regulated by phosphorylation and occur in different cellular compartments.
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 different sites within a cell, and exhibit different sensitivity to phosphorylation.
Schubert
U
U
Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.
Strebel
K
K
D001483:Base Sequence; D001692:Biological Transport; D015704:CD4 Antigens / Q000378:metabolism; D002451:Cell Compartmentation; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015497:HIV-1 / Q000254:growth & development*; D006367:HeLa Cells / Q000382:microbiology; D054298:Human Immunodeficiency Virus Proteins; D006801:Humans; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011499:Protein Processing, Post-Translational / Q000235:genetics; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics; D013601:T-Lymphocytes / Q000382:microbiology; D054334:Viral Regulatory and Accessory Proteins / Q000235:genetics / Q000378:metabolism*; D000086082:Viroporin Proteins
D015704:CD4 Antigens; D054298:Human Immunodeficiency Virus Proteins; D011494:Protein Kinases; D054334:Viral Regulatory and Accessory Proteins; C514859:vpu protein, Human immunodeficiency virus 1; D000086082:Viroporin Proteins