A regulatory circuit links protein translocation at ER to exocytosis in Saccharomyces cerevisiae

The exocyst is a conserved protein complex proposed to mediate vesicle tethering at plasma membrane. Previously, we identified the SEBI/SBHI gene, encoding the beta subunit of the Sec61p ER translocation complex, as a multicopy suppressor of sec15-1 mutant that is defective for one subuunit of the exocyst complex. Here we show functional and physical interaction between components of the ER translocon and the exocytosis machinery. We show that overexpression of the SEB1 gene suppresses the growth defects in all exocyst sec mutants. In addition. overexpression of SEC6I or SSS1 encoding the other two components of the Sec6lp complex suppressed the growth defects of several exocyst mutants. In contrary, overexpression of exocytosis genes did not suppress defects in the translocation complex. Seblp was coimmunoprecipitated from yeast cell lysates with Sec15p and Sec8p, components of the exocyst complex, and with Sec4p, a secretory vesicle associated rab GTPase that binds to Sec15p and is essential for exocytosis. The interaction between Seblp and Sec15p was abolished in sec15-1 mutant and was restored upon SEB1 overexpression. Furthermore, in wild type cells overexpression of SEB1 as well as that of SEC4 increased the yield of secreted proteins. These findings propose a novel regulatory circuit in production of secreted proteins mediated by functional and physical link between the ER translocation complex and the exocyst