title: CD4 and BST-2/tetherin proteins retro-translocate from endoplasmic reticulum to cytosol as partially folded and multimeric molecules.
abstract: CD4 and BST-2/Tetherin are cellular membrane proteins targeted to degradation by the HIV-1 protein Vpu. In both cases proteasomal degradation following recruitment into the ERAD pathway has been described. CD4 is a type I transmembrane glycoprotein, with four extracellular immunoglobulin-like domains containing three intrachain disulfide bridges. BST-2/Tetherin is an atypical type II transmembrane glycoprotein with an N-terminal transmembrane domain and a C-terminal glycophosphatidylinositol anchor, which dimerizes through three interchain bridges. We investigated spontaneous and Vpu-induced retro-translocation of CD4 and BST-2/Tetherin using our novel biotinylation technique in living cells to determine ER-to-cytosol retro-translocation of proteins. We found that CD4 retro-translocates with oxidized intrachain disulfide bridges, and only upon proteasomal inhibition does it accumulate in the cytosol as already reduced and deglycosylated molecules. Similarly, BST-2/Tetherin is first exposed to the cytosol as a dimeric oxidized complex and then becomes deglycosylated and reduced to monomers. These results raise questions on the required features of the putative retro-translocon, suggesting alternative retro-translocation mechanisms for membrane proteins in which complete cysteine reduction and unfolding are not always strictly required before ER to cytosol dislocation.
authors.affiliation: From the International Centre for Genetic Engineering and Biotechnology (ICGEB), Padriciano 99, 34149 Trieste, Italy and.
authors.forename: Gianluca
authors.initials: G
authors.lastname: Petris
authors.forename: Antonio
authors.initials: A
authors.lastname: Casini
authors.forename: Linda
authors.initials: L
authors.lastname: Sasset
authors.forename: Francesca
authors.initials: F
authors.lastname: Cesaratto
authors.forename: Marco
authors.initials: M
authors.lastname: Bestagno
authors.forename: Anna
authors.initials: A
authors.lastname: Cereseto
authors.forename: Oscar R
authors.initials: OR
authors.lastname: Burrone
mesh_terms: D015703:Antigens, CD / Q000235:genetics / Q000378:metabolism*; D015704:CD4 Antigens / Q000235:genetics / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D058851:GPI-Linked Proteins / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D015497:HIV-1 / Q000235:genetics / Q000378:metabolism; D054298:Human Immunodeficiency Virus Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction; D017510:Protein Folding; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000235:genetics; D054334:Viral Regulatory and Accessory Proteins / Q000235:genetics / Q000378:metabolism; D000074010:Bone Marrow Stromal Antigen 2; D000086082:Viroporin Proteins
chemical_list: D015703:Antigens, CD; D015704:CD4 Antigens; D058851:GPI-Linked Proteins; D054298:Human Immunodeficiency Virus Proteins; D046912:Multiprotein Complexes; D054334:Viral Regulatory and Accessory Proteins; C094572:BST2 protein, human; D000074010:Bone Marrow Stromal Antigen 2; C514859:vpu protein, Human immunodeficiency virus 1; D000086082:Viroporin Proteins
keywords: BST-2; Biotin; Biotinylation; CD4; Disulfide; ER-associated Degradation; ERAD; Oxidation-Reduction; Retro-translocation; Tetherin