The transmembrane domain is sufficient for Sbh1p function, its association with the Sec61 complex, and interaction with Rtn1p

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.