title: A sugar chain at a specific position in the nascent polypeptide chain induces forward movement during translocation through the translocon.
abstract: Nascent polypeptide chains synthesized by membrane bound ribosomes are cotranslationally translocated through and integrated into the endoplasmic reticulum translocon. Hydrophobic segments and positive charges on the chain are critical to halt the ongoing translocation. A marginally hydrophobic segment, which cannot be inserted into the membrane by itself, can be a transmembrane segment depending on its downstream positive charges. In certain conditions, positive charges even 60 residues downstream cause the marginally hydrophobic segment to span the membrane by inducing the segment to slide back from the lumen. Here we systematically examined the effect of a core sugar chain on the fate of a marginally hydrophobic segment using a cell-free translation and translocation system. A sugar chain added within 12 residues upstream of the marginally hydrophobic segment prevents the sliding back and promotes forward movement of the polypeptide chain. The sugar chain apparently functions as a ratchet to keep the polypeptide chain in the lumen. We propose that the sugar chain is a third topology determinant of membrane proteins, in addition to a hydrophobic segment and positive charges of the nascent chain.
authors.affiliation: Graduate School of Life Science, University of Hyogo, Kouto Ako-gun, Hyogo 678-1297, Japan.
authors.forename: Marifu
authors.initials: M
authors.lastname: Yamagishi
authors.forename: Hidenobu
authors.initials: H
authors.lastname: Fujita
authors.forename: Fumiko
authors.initials: F
authors.lastname: Morimoto
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kida
authors.forename: Masao
authors.initials: M
authors.lastname: Sakaguchi
mesh_terms: D000818:Animals; D002241:Carbohydrates / Q000737:chemistry*; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D006031:Glycosylation; D010455:Peptides / Q000737:chemistry / Q000378:metabolism*; D014176:Protein Biosynthesis; D021381:Protein Transport / Q000502:physiology*; D011817:Rabbits; D012270:Ribosomes / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D002241:Carbohydrates; D010455:Peptides