       Document 0621
 DOCN  M9630621
 TI    The human immunodeficiency virus type 1 Vpu protein: a potential
       regulator of proteolysis and protein transport in the mammalian
       secretory pathway.
 DT    9603
 AU    Vincent MJ; Abdul Jabbar M; Department of Molecular Biology, Cleveland
       Clinic Foundation,; Ohio 44195, USA.
 SO    Virology. 1995 Nov 10;213(2):639-49. Unique Identifier : AIDSLINE
       MED/96074538
 AB    HIV-1 Vpu is a small transmembrane phosphoprotein of 16 kDa which
       performs critical roles in CD4 proteolysis and virus release. Previous
       studies have demonstrated that Vpu-induced degradation of CD4 occurs in
       the endoplasmic reticulum (ER), and that the proteolytic process is
       sequence specific requiring both the transmembrane and cytoplasmic
       domains of CD4. In the present study, we investigated the effects of Vpu
       expression on the intracellular membrane trafficking pathway of
       mammalian cells. In singly transfected cells, the HIV envelope
       glycoproteins and vesicular stomatitis virus glycoprotein (VSV G) were
       properly transported to the cell surface undergoing oligosaccharide
       modifications characteristic of their movement through the Golgi
       complex. In contrast, the cell surface delivery of glycoproteins was
       severely impeded in cells expressing Vpu. Biochemical analyses revealed
       that Vpu expression blocked the transfer of proteins from the ER-Golgi
       complex to the plasma membrane in a dose- and protein-dependent manner.
       Soluble gp120 exhibited extreme transport defects in the presence of
       Vpu, whereas transmembrane proteins (e.g., gp160, VSV) responded only
       moderately to wild-type Vpu. To gain insight into Vpu-mediated transport
       inhibition, we performed mutational analysis of the CK-2 phosphorylation
       sites (serines at 52 and 56) in the Vpu protein. CK-2 phosphorylation of
       Vpu has been shown to regulate the activity of the protein in reactions
       that involve the proteolysis of CD4 in the ER. We demonstrate here that
       the phosphorylation mutant is defective in both sequence-specific
       degradation of VRE-containing substrates and the transport inhibition of
       gp120 and VSV-G in the secretory pathway. Thus, these experiments have
       revealed that Vpu-mediated proteolysis and transport inhibition are
       mechanistically coupled requiring the same structural elements of the
       Vpu protein in both processes. We propose that the primary effect of Vpu
       expression is to impede the secretion process and then access
       glycoproteins bearing the VRE for Vpu-mediated proteolysis in the ER of
       mammalian cells.
 DE    Base Sequence  Biological Transport  Cell Membrane/*METABOLISM  DNA
       Primers  Endoplasmic Reticulum/*METABOLISM  Gene Products,
       vpu/GENETICS/*METABOLISM  Golgi Apparatus/*METABOLISM  Hela Cells  Human
       HIV Envelope Protein gp120/*METABOLISM  *HIV-1  Intracellular
       Membranes/METABOLISM  Molecular Sequence Data  Phosphorylation
       Protein-Serine-Threonine Kinases/METABOLISM  Transfection  Viral
       Envelope Proteins/GENETICS/METABOLISM  JOURNAL ARTICLE

       SOURCE: National Library of Medicine.  NOTICE: This material may be
       protected by Copyright Law (Title 17, U.S.Code).

