The Sec61 protein translocation complex in the endoplasmic reticulum (ER) membrane is composed of three subunits: The a-subunit, called Sec61p in yeast, is a multi-spanning membrane protein that forms the protein conducting channel. The functions of the smaller, carboxy-terminally tail-anchored ß subunit Sbh1p, its close homologue Sbh2p, and the subunit Sss1p are not well understood. Here we show that co-translational protein translocation into the ER is reduced in sbh1 sbh2 cells, whereas there is a limited reduction of post-translational tranlocation and no effect on export of a mutant form of alfa-factor precursor for ER-associated degradation in the cytosol. The translocation defect and the temperature-sensitive growth phenotype of sbh1 sbh2 cells were rescued by expression of the trans-membrane domain of Sbh1p alone and the Sbh1p trans-membrane domain was sufficient for coimmunoprecipitation with Sec61p and Sss1p. Furthermore, we show that Sbh1p co-precipitates with the ER trans-membrane protein Rtn1p. Sbh1p-Rtn1p complexes appear not to contain Sss1p and Sec61p. Our results define the trans-membrane domain as the minimal functional domain of the Sec61ß homologue Sbh1p in ER translocation, identify a novel interaction partner for Shb1p and imply that Sbh1p has additional functions that are not directly linked to protein translocation in association with the Sec61-complex.
- Sec proteins
- gene expression