title: Spontaneous release of cytosolic proteins from posttranslational substrates before their transport into the endoplasmic reticulum.
abstract: In posttranslational translocation in yeast, completed protein substrates are transported across the endoplasmic reticulum membrane through a translocation channel formed by the Sec complex. We have used photo-cross-linking to investigate interactions of cytosolic proteins with a substrate synthesized in a reticulocyte lysate system, before its posttranslational translocation through the channel in the yeast membrane. Upon termination of translation, the signal recognition particle (SRP) and the nascent polypeptide-associated complex (NAC) are released from the polypeptide chain, and the full-length substrate interacts with several different cytosolic proteins. At least two distinct complexes exist that contain among other proteins either 70-kD heat shock protein (Hsp70) or tailless complex polypeptide 1 (TCP1) ring complex/chaperonin containing TCP1 (TRiC/CCT), which keep the substrate competent for translocation. None of the cytosolic factors appear to interact specifically with the signal sequence. Dissociation of the cytosolic proteins from the substrate is accelerated to the same extent by the Sec complex and an unspecific GroEL trap, indicating that release occurs spontaneously without the Sec complex playing an active role. Once bound to the Sec complex, the substrate is stripped of all cytosolic proteins, allowing it to subsequently be transported through the membrane channel without the interference of cytosolic binding partners.
authors.affiliation: Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.
authors.forename: K
authors.initials: K
authors.lastname: Plath
authors.forename: T A
authors.initials: TA
authors.lastname: Rapoport
mesh_terms: D056404:Chaperonin Containing TCP-1; D018833:Chaperonins / Q000378:metabolism; D003432:Cross-Linking Reagents; D003600:Cytosol / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005656:Fungal Proteins / Q000378:metabolism; D008954:Models, Biological; D018832:Molecular Chaperones; D014176:Protein Biosynthesis; D011498:Protein Precursors / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D012270:Ribosomes / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D018271:Signal Recognition Particle / Q000378:metabolism; D015534:Trans-Activators / Q000378:metabolism
chemical_list: D003432:Cross-Linking Reagents; D005656:Fungal Proteins; C048632:MF(ALPHA)1 protein, S cerevisiae; D018832:Molecular Chaperones; D011498:Protein Precursors; D029701:Saccharomyces cerevisiae Proteins; D018271:Signal Recognition Particle; D015534:Trans-Activators; C096099:nascent-polypeptide-associated complex; D056404:Chaperonin Containing TCP-1; D018833:Chaperonins