Sbh1p, a subunit of the Sec61 translocon, interacts with the chaperone calnexin in the yeast Yarrowia lipolytica

The core component of the translocation apparatus, Sec61p or a, was previously cloned in Yarrowia lipolytica. Using anti-Sec61p antibodies, we showed that most of the translocation sites are devoted to co-translational translocation in this yeast, which is similar to the situation in mammalian cells but in contrast to the situation in Saccharomyces cerevisiae, where post-translational translocation is predominant. In order to characterize further the minimal translocation apparatus in Y lipolytica, the beta Sec61 complex subunit, Sbh1p, was cloned by functional complementation of a Deltasbh1, Deltasbh2 S. cerevisiae mutant. The secretion of the reporter protein is not impaired in the Y lipolytica sbh1 inactivated strain. We screened the E lipolytica two-hybrid library to look for partners of this translocon component. The ER-membrane chaperone protein, calnexin, was identified as an interacting protein. By a co-immunoprecipitation approach, we confirmed this association in Yarrowia and then showed that the S. cerevisiae Sbh2p protein was a functional homologue of YISbh1p. The interaction of Sbh1p with calnexin was shown to occur between the lumenal domain of both proteins. These results suggest that the P subunit of the Sec61 translocon may relay folding of nascent proteins to their translocation.