title: The casein kinases Yck1p and Yck2p act in the secretory pathway, in part, by regulating the Rab exchange factor Sec2p.
abstract: Sec2p is a guanine nucleotide exchange factor that activates Sec4p, the final Rab GTPase of the yeast secretory pathway. Sec2p is recruited to secretory vesicles by the upstream Rab Ypt32p acting in concert with phosphatidylinositol-4-phosphate (PI(4)P). Sec2p also binds to the Sec4p effector Sec15p, yet Ypt32p and Sec15p compete against each other for binding to Sec2p. We report here that the redundant casein kinases Yck1p and Yck2p phosphorylate sites within the Ypt32p/Sec15p binding region and in doing so promote binding to Sec15p and inhibit binding to Ypt32p. We show that Yck2p binds to the autoinhibitory domain of Sec2p, adjacent to the PI(4)P binding site, and that addition of PI(4)P inhibits Sec2p phosphorylation by Yck2p. Loss of Yck1p and Yck2p function leads to accumulation of an intracellular pool of the secreted glucanase Bgl2p, as well as to accumulation of Golgi-related structures in the cytoplasm. We propose that Sec2p is phosphorylated after it has been recruited to secretory vesicles and the level of PI(4)P has been reduced. This promotes Sec2p function by stimulating its interaction with Sec15p. Finally, Sec2p is dephosphorylated very late in the exocytic reaction to facilitate recycling.
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093.
authors.forename: Danièle
authors.initials: D
authors.lastname: Stalder
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093 pnovick@ucsd.edu.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Novick
mesh_terms: D047389:Casein Kinase I / Q000235:genetics / Q000378:metabolism*; D004693:Glucan Endo-1,3-beta-D-Glucosidase / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D009154:Mutation; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D055571:Secretory Pathway; D022142:Secretory Vesicles / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000378:metabolism
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D018129:Phosphatidylinositol Phosphates; C060862:SEC15 protein, S cerevisiae; C064456:SEC2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; C037178:phosphatidylinositol 4-phosphate; D047389:Casein Kinase I; C479664:YCK1 protein, S cerevisiae; C479666:YCK2 protein, S cerevisiae; C080389:BGL2 protein, S cerevisiae; D004693:Glucan Endo-1,3-beta-D-Glucosidase; C121633:YPT32 protein, S cerevisiae; D020691:rab GTP-Binding Proteins