Functional characterization of the trans-membrane domain interactions of the Sec61 protein translocation complex beta-subunit.
BACKGROUND
In eukaryotic cells co- and post-translational protein translocation is mediated by the trimeric Sec61 complex. Currently, the role of the Sec61 complex beta-subunit in protein translocation is poorly understood. We have shown previously that in Saccharomyces cerevisiae the trans-membrane domain alone is sufficient for the function of the beta-subunit Sbh1p in co-translational protein translocation. In addition, Sbh1p co-purifies not only with the protein translocation channel subunits Sec61p and Sss1p, but also with the reticulon family protein Rtn1p.


RESULTS
We used random mutagenesis to generate novel Sbh1p mutants in order to functionally map the Sbh1p trans-membrane domain. These mutants were analyzed for their interactions with Sec61p and how they support co-translational protein translocation. The distribution of mutations identifies one side of the Sbh1p trans-membrane domain alpha-helix that is involved in interactions with Sec61p and that is important for Sbh1p function in protein translocation. At the same time, these mutations do not affect Sbh1p interaction with Rtn1p. Furthermore we show that Sbh1p is found in protein complexes containing not only Rtn1p, but also the two other reticulon-like proteins Rtn2p and Yop1p.


CONCLUSIONS
Our results identify functionally important amino acids in the Sbh1p trans-membrane domain. In addition, our results provide additional support for the involvement of Sec61beta in processes unlinked to protein translocation.
Research Program in Cell and Molecular Biology, Institute of Biotechnology, P,O, Box 56, 00014 University of Helsinki, Finland. xqzhao@genetics.ac.cn
Xueqiang
X
Zhao
Jussi
J
Jäntti
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D047468:Immunoprecipitation; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D017433:Protein Structure, Secondary; 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*; D033921:Vesicular Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D026901:Membrane Transport Proteins; C505725:Rtn1 protein, S cerevisiae; D000069816:SEC Translocation Channels; C499383:SEC61 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C101188:Sbh1 protein, S cerevisiae; D033921:Vesicular Transport Proteins; C426246:YOP1 protein, S cerevisiae