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.
Highlights
Protein secretion is an essential process in prokaryotes and eukaryotes (Matlack et al, 1998)
Several factors required for protein translocation across the endoplasmic reticulum (ER) membrane may be cold sensitive, we focused our subsequent analysis on the pore-forming component of the protein translocation channel, Sec61p or Sec61α, because channel opening is essential for protein entry into the secretory pathway and has recently been shown to involve a conformational change in the channel (Beckmann et al, 2001)
Translocation into S. cerevisiae microsomes is most efficient at 20°C; translocation at 10°C is approximately 1.8fold lower, and translocation at 0°C is 3.1-fold lower than at 10°C, suggesting that the activity of the Sec61 channel in S. cerevisiae is optimal at its growth temperature in the wild
Summary
Protein secretion is an essential process in prokaryotes and eukaryotes (Matlack et al, 1998). The protein translocation channel in the ER membrane is composed of three subunits: Sec61α, Sec61β and Sec61γ (Sec61p, Sbh1p, and Sss1p in yeast) (Johnson and van Waes, 1999). These proteins form heterotrimers, several of which assemble in the ER membrane in response to the presence of a functional signal sequence to form the protein translocation channel for secretory proteins (Matlack et al, 1998). The principal channel subunit, Sec61α, contains 10 transmembrane domains, which line the channel (Johnson and van Waes, 1999; Matlack et al, 1998) It is currently unknown how the channel opens and closes. The mechanism of channel opening from the lumenal side for export is not understood
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
