title: Slow translocon gating causes cytosolic exposure of transmembrane and lumenal domains during membrane protein integration.
abstract: Integral membrane proteins are cotranslationally inserted into the endoplasmic reticulum via the protein translocation channel, or translocon, which mediates the transport of lumenal domains, retention of cytosolic domains and integration of transmembrane spans into the phospholipid bilayer. Upon translocon binding, transmembrane spans interact with a lateral gate, which regulates access to membrane phospholipids, and a lumenal gate, which controls the translocation of soluble domains. We analyzed the in vivo kinetics of integration of model membrane proteins in Saccharomyces cerevisiae using ubiquitin translocation assay reporters. Our findings indicate that the conformational changes in the translocon that permit opening of the lumenal and lateral channel gates occur less rapidly than elongation of the nascent polypeptide. Transmembrane spans and lumenal domains are therefore exposed to the cytosol during integration of a polytopic membrane protein, which may pose a challenge to the fidelity of membrane protein integration.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 364 Plantation Street, Worcester, Massachusetts 01605-2324, USA.
authors.forename: Zhiliang
authors.initials: Z
authors.lastname: Cheng
authors.forename: Reid
authors.initials: R
authors.lastname: Gilmore
mesh_terms: D002135:Calcium-Binding Proteins / Q000737:chemistry; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D026901:Membrane Transport Proteins; D008958:Models, Molecular; D010539:Permeability; D017510:Protein Folding; D021382:Protein Sorting Signals; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry; D018000:Receptors, Peptide / Q000737:chemistry; D000069816:SEC Translocation Channels; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D014178:Translocation, Genetic; D025801:Ubiquitin / Q000378:metabolism; D043324:beta-Fructofuranosidase / Q000737:chemistry
chemical_list: D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D021382:Protein Sorting Signals; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D000069816:SEC Translocation Channels; C499383:SEC61 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; C066973:signal sequence receptor; C468437:SUC2 protein, S cerevisiae; D043324:beta-Fructofuranosidase