title: A gating motif in the translocation channel sets the hydrophobicity threshold for signal sequence function.
abstract: A critical event in protein translocation across the endoplasmic reticulum is the structural transition between the closed and open conformations of Sec61, the eukaryotic translocation channel. Channel opening allows signal sequence insertion into a gap between the N- and C-terminal halves of Sec61. We have identified a gating motif that regulates the transition between the closed and open channel conformations. Polar amino acid substitutions in the gating motif cause a gain-of-function phenotype that permits translocation of precursors with marginally hydrophobic signal sequences. In contrast, hydrophobic substitutions at certain residues in the gating motif cause a protein translocation defect. We conclude that the gating motif establishes the hydrophobicity threshold for functional insertion of a signal sequence into the Sec61 complex, thereby allowing the wild-type translocation channel to discriminate between authentic signal sequences and the less hydrophobic amino acid segments in cytosolic proteins. Bioinformatic analysis indicates that the gating motif is conserved between eubacterial and archaebacterial SecY and eukaryotic Sec61.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
authors.forename: Steven F
authors.initials: SF
authors.lastname: Trueman
authors.forename: Elisabet C
authors.initials: EC
authors.lastname: Mandon
authors.forename: Reid
authors.initials: R
authors.lastname: Gilmore
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D057927:Hydrophobic and Hydrophilic Interactions; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017017:Methanococcus / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D000069816:SEC Translocation Channels; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D021382:Protein Sorting Signals; D000069816:SEC Translocation Channels; C499383:SEC61 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins