title: Protein translocation across the endoplasmic reticulum membrane in cold-adapted organisms.
abstract: Secretory proteins enter the secretory pathway by translocation across the membrane of the endoplasmic reticulum (ER) via a channel formed primarily by the Sec61 protein. Protein translocation is highly temperature dependent in mesophilic organisms. We asked whether the protein translocation machinery of organisms from extremely cold habitats was adapted to function at low temperature and found that post-translational protein import into ER-derived microsomes from Antarctic yeast at low temperature was indeed more efficient than into mesophilic yeast microsomes. Analysis of the amino-acid sequences of the core component of the protein translocation channel, Sec61p, from Antarctic yeast species did not reveal amino-acid changes potentially adaptive for function in the cold, because the sequences were too divergent. We therefore analyzed Sec61alpha (vertebrate Sec61p) sequences and protein translocation into the ER of Antarctic and Arctic fishes and compared them to Sec61alpha and protein translocation into the ER of temperate-water fishes and mammals. Overall, Sec61alpha is highly conserved amongst these divergent taxa; a number of amino-acid changes specific to fishes are evident throughout the protein, and, in addition, changes specific to cold-water fishes cluster in the lumenal loop between transmembrane domains 7 and 8 of Sec61alpha, which is known to be important for protein translocation across the ER membrane. Secretory proteins translocated more efficiently into fish microsomes than into mammalian microsomes at 10 degrees C and 0 degrees C. The efficiency of protein translocation at 0 degrees C was highest for microsomes from a cold-water fish. Despite substantial differences in ER membrane lipid composition, ER membrane fluidity was identical in Antarctic fishes, mesophilic fishes and warm-blooded vertebrates, suggesting that membrane fluidity, although typically important for the function of the transmembrane proteins, is not limiting for protein translocation across the ER membrane in the cold. Collectively, our data suggest that the limited amino-acid changes in Sec61alpha from fishes may be functionally significant and represent adaptive changes that enhance channel function in the cold.
authors.affiliation: University of Cambridge, CIMR, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, UK. kbr20@cam.ac.uk
authors.forename: Karin
authors.initials: K
authors.lastname: Römisch
authors.forename: Nicola
authors.initials: N
authors.lastname: Collie
authors.forename: Nelyn
authors.initials: N
authors.lastname: Soto
authors.forename: James
authors.initials: J
authors.lastname: Logue
authors.forename: Margaret
authors.initials: M
authors.lastname: Lindsay
authors.forename: Wiep
authors.initials: W
authors.lastname: Scheper
authors.forename: Chi-Hing C
authors.initials: CH
authors.lastname: Cheng
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D003080:Cold Temperature; D003454:Cryptococcus / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005399:Fishes; D005656:Fungal Proteins / Q000737:chemistry; D007425:Intracellular Membranes / Q000378:metabolism*; D008055:Lipids / Q000737:chemistry; D008099:Liver / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry / Q000502:physiology*; D026901:Membrane Transport Proteins; D008854:Microscopy, Electron; D008861:Microsomes / Q000378:metabolism; D008862:Microsomes, Liver / Q000737:chemistry; D008954:Models, Biological; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D012313:RNA / Q000737:chemistry; D000069816:SEC Translocation Channels; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D013696:Temperature
chemical_list: D018076:DNA, Complementary; D005656:Fungal Proteins; D008055:Lipids; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; C499383:SEC61 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D012313:RNA