title: BiP acts as a molecular ratchet during posttranslational transport of prepro-alpha factor across the ER membrane.
abstract: We have addressed the mechanism by which proteins are posttranslationally transported across the membrane of the yeast endoplasmic reticulum (ER). We demonstrate that BiP (Kar2p), a member of the Hsp70 family resident in the ER lumen, acts as a molecular ratchet during translocation of the secretory protein prepro-alpha factor through the channel formed by the Sec complex. Multiple BiP molecules associate with each translocation substrate following interaction with the J domain of the Sec63p component of the Sec complex. Bound BiP minimizes passive backward movements of the substrate through the channel, and BiP's subsequent dissociation results in a free polypeptide in the ER lumen. Antibodies against the substrate can replace BiP, indicating that a Brownian ratchet is sufficient to achieve translocation.
authors.affiliation: Howard Hughes Medical Institute, Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115-6091, USA.
authors.forename: K E
authors.initials: KE
authors.lastname: Matlack
authors.forename: B
authors.initials: B
authors.lastname: Misselwitz
authors.forename: K
authors.initials: K
authors.lastname: Plath
authors.forename: T A
authors.initials: TA
authors.lastname: Rapoport
mesh_terms: D000595:Amino Acid Sequence; D003001:Cloning, Molecular; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D004926:Escherichia coli; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins / Q000737:chemistry / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D007700:Kinetics; D000072235:Mating Factor; D008969:Molecular Sequence Data; D010455:Peptides / Q000235:genetics; D011485:Protein Binding; D014176:Protein Biosynthesis; D011498:Protein Precursors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D011994:Recombinant Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D005656:Fungal Proteins; D018840:HSP70 Heat-Shock Proteins; C060153:KAR2 protein, yeast; C048632:MF(ALPHA)1 protein, S cerevisiae; D010455:Peptides; D011498:Protein Precursors; D011994:Recombinant Proteins; D029701:Saccharomyces cerevisiae Proteins; D000072235:Mating Factor