title: Integration of cytochrome b5 into endoplasmic reticulum membrane: participation of carboxy-terminal portion of the transmembrane domain.
abstract: Integration of cytochrome b(5) (b5), a tail-anchored protein located in the endoplasmic reticulum (ER) membrane, into the membrane was studied. Mutation of three amino acids, -Leu-Met-Tyr, at the carboxy-terminal end of the transmembrane segment of b5 to alanines resulted in localization of the mutated protein, b5LMY/AAA, in the cytosol as well as in the ER membrane. When an N-glycosylation site was introduced at the carboxy-terminal end of b5LMY/AAA, a substantial amount of the glycosylated form of the mutant protein was recovered in the cytosol fraction. A portion of the mutant protein recovered in the ER was released from the membrane by incubation with the cytosol fraction, but no further release was observed in the second incubation, suggesting that b5 is present in two different states, loosely-bound and firmly-integrated forms, in the ER membrane. These results suggest that b5 is integrated into the ER membrane via the loosely bound state, in which the carboxy-terminal end of the molecule is inserted into the luminal side of the vesicle but is easily translocated back to the cytosol, and that the three amino acids are important for conversion of the loosely-bound state to the firmly-integrated state.
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812-8581, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Tanaka
authors.forename: Jun-ya
authors.initials: JY
authors.lastname: Kinoshita
authors.forename: Rieko
authors.initials: R
authors.lastname: Kuroda
authors.forename: Akio
authors.initials: A
authors.lastname: Ito
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015786:Cytochromes b5 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D023302:Protein Footprinting; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D051381:Rats
chemical_list: D008565:Membrane Proteins; D015786:Cytochromes b5