Exo84p is an exocyst protein essential for secretion

The exocyst is a multiprotein complex that plays an important role in secretory vesicle targeting and docking at the plasma membrane. Here we report the identification and characterization of a new component of the exocyst, Exo84p, in the yeast Saccharomyces cerevisiae. Yeast cells depleted of Exo84p cannot survive. These cells are defective in invertase secretion and accumulate vesicles similar to those in the late sec mutants. Exo84p co-immunoprecipitates with the exocyst components, and a portion of the Exo84p co-sediments with the exocyst complex in velocity gradients. The assembly of Exo84p into the exocyst complex requires two other subunits, Sec5p and Sec10p, Exo84p interacts with both Sec5p and Sec10p in a two-hybrid assay. Overexpression of Exo84p selectively suppresses the temperature sensitivity of a secb mutant. Exo84p specifically localizes to the bud tip or mother/daughter connection, sites of polarized secretion in the yeast S. cerevisiae. Exo84p is mislocalized in a secb mutant. These studies suggest that Exo84p is an essential protein that plays an important role in polarized secretion.