title: Protein disulphide isomerase and a lumenal cyclophilin-type peptidyl prolyl cis-trans isomerase are in transient contact with secretory proteins during late stages of translocation.
abstract: The transport of a presecretory protein into the mammalian endoplasmic reticulum can be divided into early translocation events which include specific targeting of the presecretory protein to and insertion into the endoplasmic reticulum membrane and late translocation events, comprising signal sequence cleavage, completion of translocation and folding of the secretory protein into a functional conformation. The microsomal membrane proteins Sec61 alpha p and translocating-chain-associating membrane protein were previously identified as being in close contact with a nascent presecretory protein at an early step of translocation. Here, we investigated whether additional microsomal proteins are in contact with translocating chains during or immediately after transit. This was addressed by crosslinking after release of the nascent chain from Sec61 alpha p. We observed two additional membrane proteins interacting with the nascent precursor in the early stages of translocation and three lumenal proteins interacting with the processed polypeptide chain in the late stages of translocation. One of the lumenal proteins was identified as protein disulphide isomerase by immunoprecipitation. Another of the lumenal proteins was suggested to be a lumenal cyclophilin-type peptidyl prolyl cis-trans isomerase by the effect of cyclosporin A. We propose that molecular chaperones, such as protein disulphide isomerase and cyclophilin may represent two of the lumenal proteins which are involved in completion of translocation.
authors.affiliation: Institut für Biochemie und Molekulare Zellbiologie, Universität Göttingen, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Klappa
authors.forename: R B
authors.initials: RB
authors.lastname: Freedman
authors.forename: R
authors.initials: R
authors.lastname: Zimmermann
mesh_terms: D000591:Amino Acid Isomerases / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D003432:Cross-Linking Reagents / Q000494:pharmacology; D016572:Cyclosporine / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007301:Insect Hormones / Q000737:chemistry; D019476:Insect Proteins; D007535:Isomerases / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism*; D008861:Microsomes / Q000378:metabolism; D018832:Molecular Chaperones / Q000378:metabolism; D008969:Molecular Sequence Data; D019696:Peptidylprolyl Isomerase; D011233:Precipitin Tests; D011388:Prolactin / Q000737:chemistry / Q000378:metabolism*; D019704:Protein Disulfide-Isomerases; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism*; D011691:Puromycin / Q000737:chemistry; D000069816:SEC Translocation Channels; D013997:Time Factors
chemical_list: D002352:Carrier Proteins; C059037:CecA2 protein, Bombyx mori; D003432:Cross-Linking Reagents; D007301:Insect Hormones; D019476:Insect Proteins; D008565:Membrane Proteins; D018832:Molecular Chaperones; D011498:Protein Precursors; D000069816:SEC Translocation Channels; D011691:Puromycin; C017671:preprolactin; D016572:Cyclosporine; D011388:Prolactin; D007535:Isomerases; D000591:Amino Acid Isomerases; D019696:Peptidylprolyl Isomerase; D019704:Protein Disulfide-Isomerases