title: The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally.
abstract: Opsin, a member of the G-protein-coupled receptor family, is a polytopic membrane protein that does not encode a cleaved amino-terminal signal sequence. The amino terminus of opsin precedes the first known targeting information, suggesting that it translocates across the endoplasmic reticulum (ER) membrane after synthesis, uncoupled from translation. However, translocation across the mammalian ER is believed to be coupled to protein synthesis. In this study we show that opsin, within a range of nascent peptide lengths, targets and translocates equally efficiently co- and posttranslationally. Longer nascent opsin peptides have a lower efficiency of cotranslational translocation but an even lower efficiency of posttranslational translocation. We also show that SRP is required for both co- and posttranslational targeting.
authors.affiliation: Laboratory of Cellular Biophysics, The Rockefeller University, New York, New York 10021, USA.
authors.forename: Elliott M
authors.initials: EM
authors.lastname: Kanner
authors.forename: Irene K
authors.initials: IK
authors.lastname: Klein
authors.forename: Martin
authors.initials: M
authors.lastname: Friedlander
authors.forename: Sanford M
authors.initials: SM
authors.lastname: Simon
mesh_terms: D000818:Animals; D002417:Cattle; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D010446:Peptide Fragments / Q000096:biosynthesis / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D012270:Ribosomes / Q000378:metabolism; D017299:Rod Opsins / Q000096:biosynthesis / Q000378:metabolism*; D018271:Signal Recognition Particle / Q000378:metabolism
chemical_list: D010446:Peptide Fragments; D017299:Rod Opsins; D018271:Signal Recognition Particle