title: Specific transmembrane segments are selectively delayed at the ER translocon during opsin biogenesis.
abstract: A site-specific cross-linking approach was used to study the integration of TM (transmembrane) segments 4-7 of the polytopic membrane protein, opsin, at the ER (endoplasmic reticulum). We found that although TM4 exits the ER translocon rapidly, TM segments 5, 6 and 7 are all retained at the translocon until opsin biosynthesis is terminated. Furthermore, although artificial extension of the nascent chain is not sufficient to release the C-terminal region of opsin from the translocon, substitution of the native TM segment 7 with a more hydrophobic TM segment results in its rapid lateral exit into the lipid bilayer. We conclude that the intrinsic properties of a TM segment determine the timing of its membrane integration rather than its relative location within the polypeptide chain. A pronounced and prolonged association of opsin TM5 with the translocon-associated component PAT-10 was also observed, suggesting that PAT-10 may facilitate the assembly of distinct opsin subdomains during membrane integration. The results of the present study strongly support a model in which the ER translocon co-ordinates the integration of selected TM segments in response to the specific requirements of the precursor being synthesized.
authors.affiliation: Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester M13 9PT, UK.
authors.forename: Nurzian
authors.initials: N
authors.lastname: Ismail
authors.forename: Samuel G
authors.initials: SG
authors.lastname: Crawshaw
authors.forename: Benedict C S
authors.initials: BC
authors.lastname: Cross
authors.forename: Anna C
authors.initials: AC
authors.lastname: Haagsma
authors.forename: Stephen
authors.initials: S
authors.lastname: High
mesh_terms: D000818:Animals; D002417:Cattle; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D017353:Gene Deletion; D057927:Hydrophobic and Hydrophilic Interactions; D008565:Membrane Proteins / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D021381:Protein Transport; D017299:Rod Opsins / Q000096:biosynthesis* / Q000235:genetics; D000069816:SEC Translocation Channels
chemical_list: D008565:Membrane Proteins; D017299:Rod Opsins; D000069816:SEC Translocation Channels