title: The sterol-binding protein Kes1/Osh4p is a regulator of polarized exocytosis.
abstract: Oxysterol-binding protein (OSBP)-related protein Kes1/ Osh4p is implicated in nonvesicular sterol transfer between membranes in Saccharomyces cerevisiae. However, we found that Osh4p associated with exocytic vesicles that move from the mother cell into the bud, where Osh4p facilitated vesicle docking by the exocyst tethering complex at sites of polarized growth on the plasma membrane. Osh4p formed complexes with the small GTPases Cdc42p, Rho1p and Sec4p, and the exocyst complex subunit Sec6p, which was also required for Osh4p association with vesicles. Although Osh4p directly affected polarized exocytosis, its role in sterol trafficking was less clear. Contrary to what is predicted for a sterol-transfer protein, inhibition of sterol binding by the Osh4p Y97F mutation did not cause its inactivation. Rather, OSH4(Y97F)  is a gain-of-function mutation that causes dominant lethality. We propose that in response to sterol binding and release Osh4p promotes efficient exocytosis through the co-ordinate regulation of Sac1p, a phosphoinositide 4-phosphate (PI4P) phosphatase, and the exocyst complex. These results support a model in which Osh4p acts as a sterol-dependent regulator of polarized vesicle transport, as opposed to being a sterol-transfer protein.
authors.affiliation: Department of Molecular Biology and Biochemistry, 8888 University Drive, Simon Fraser University, Burnaby, BC, Canada.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Alfaro
authors.forename: Jesper
authors.initials: J
authors.lastname: Johansen
authors.forename: Shubha A
authors.initials: SA
authors.lastname: Dighe
authors.forename: Giselle
authors.initials: G
authors.lastname: Duamel
authors.forename: Keith G
authors.initials: KG
authors.lastname: Kozminski
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Beh
mesh_terms: D001692:Biological Transport; D002352:Carrier Proteins; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis / Q000235:genetics / Q000502:physiology*; D020558:GTP Phosphohydrolases / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D011987:Receptors, Steroid / Q000235:genetics* / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism*; D015398:Signal Transduction; D013261:Sterols / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000378:metabolism; D000098825:Oxysterol Binding Proteins
chemical_list: D002352:Carrier Proteins; D020558:GTP Phosphohydrolases; D008565:Membrane Proteins; D010744:Phosphoric Monoester Hydrolases; D011987:Receptors, Steroid; D029701:Saccharomyces cerevisiae Proteins; D013261:Sterols; D033921:Vesicular Transport Proteins; C087901:KES1 protein, S cerevisiae; D000098825:Oxysterol Binding Proteins; C490684:SEC6 protein, S cerevisiae; C081864:SAC1 protein, S cerevisiae