The yeast Par-1 homologs Kin1 and Kin2 show genetic and physical interactions with components of the exocytic machinery

Kin1 and Kin2 are Saccharomyces cerevisiae counterparts of Par-1, the Caenorhabditis elegans kinase essential for the establishment of polarity in the one cell embryo. Here, we present evidence for a novel link between Kin1, Kin2, and the secretory machinery of the budding yeast. We isolated KIN1 and KIN2 as suppressors of a mutant form of Rho3, a Rho-GTPase acting in polarized trafficking. Genetic analysis suggests that KIN1 and KIN2 act downstream of the Rab-GTPase Sec4, its exchange factor Sec2, and several components of the vesicle tethering complex, the Exocyst. We show that Kin1 and Kin2 physically interact with the t-SNARE Sec9 and the Lg1 homologue Sro7, proteins acting at the final stage of exocytosis. Structural analysis of Kin2 reveals that its catalytic activity is essential for its function in the secretory pathway and implicates the conserved 42-amino acid tail at the carboxy terminal of the kinase in autoinhibition. Finally, we find that Kin1 and Kin2 induce phosphorylation of t-SNARE Sec9 in vivo and stimulate its release from the plasma membrane. In summary, we report the finding that yeast Par-1 counterparts are associated with and regulate the function of the exocytic apparatus via phosphorylation of Sec9.