title: Phosphatidylinositol 4-phosphate controls both membrane recruitment and a regulatory switch of the Rab GEF Sec2p.
abstract: Sec2p is the guanine nucleotide exchange factor (GEF) that activates the Rab GTPase Sec4p on secretory vesicles. Sec2p also binds a Rab acting earlier in the secretory pathway, Ypt32-GTP, forming a Rab GEF cascade. Ypt32p and the Sec4p effector Sec15p (a component of the exocyst complex) compete for binding to Sec2p. Indeed Ypt32p initially recruits Sec2p, but subsequently allows a handoff of active Sec2p/Sec4p to Sec15p. Intriguingly, Golgi-associated phosphatidylinositol 4-phosphate (PI4P) works together with Ypt32-GTP in this context. PI4P inhibits Sec2p-Sec15p interactions, promoting recruitment of Sec2p by Ypt32p as secretory vesicles form. However, PI4P levels appear to decline as vesicles reach secretory sites, allowing Sec15p to replace Ypt32p as vesicles mature. In this way, the regulation of PI4P levels may switch Sec2p/Sec4p function during vesicle maturation, from a Rab GEF recruitment cascade involving Ypt32p to an effector positive feedback loop involving Sec15p.
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: Emi
authors.initials: E
authors.lastname: Mizuno-Yamasaki
authors.forename: Martina
authors.initials: M
authors.lastname: Medkova
authors.forename: Jeff
authors.initials: J
authors.lastname: Coleman
authors.forename: Peter
authors.initials: P
authors.lastname: Novick
mesh_terms: D019870:1-Phosphatidylinositol 4-Kinase / Q000235:genetics / Q000378:metabolism; D005089:Exocytosis / Q000502:physiology; D006056:Golgi Apparatus / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics* / Q000378:metabolism; D008856:Microscopy, Fluorescence; D009154:Mutation; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics* / Q000254:growth & development / Q000502:physiology; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D014617:Vacuoles / Q000502:physiology; D020691:rab GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D020696:rab5 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D018129:Phosphatidylinositol Phosphates; C064456:SEC2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C037178:phosphatidylinositol 4-phosphate; D019870:1-Phosphatidylinositol 4-Kinase; C086867:YPT52 protein, S cerevisiae; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins