title: Vps9 family protein Muk1 is the second Rab5 guanosine nucleotide exchange factor in budding yeast.
abstract: VPS9 domains can act as guanosine nucleotide exchange factors (GEFs) against small G proteins of the Rab5 family. Saccharomyces cerevisiae vps9Δ mutants have trafficking defects considerably less severe than multiple deletions of the three cognate Rab5 paralogs (Vps21, Ypt52, and Ypt53). Here, we show that Muk1, which also contains a VPS9 domain, acts as a second GEF against Vps21, Ypt52, and Ypt53. Muk1 is partially redundant with Vps9 in vivo, with vps9Δ muk1Δ double mutant cells displaying hypersensitivity to temperature and ionic stress, as well as profound impairments in endocytic and Golgi endosome trafficking, including defects in sorting through the multivesicular body. Cells lacking both Vps9 and Muk1 closely phenocopy double and triple knock-out strains lacking Rab5 paralogs. Microscopy and overexpression experiments demonstrate that Vps9 and Muk1 have distinct localization determinants. These experiments establish Muk1 as the second Rab5 GEF in budding yeast.
authors.affiliation: Department of Biochemistry, University of Washington, Seattle, Washington 98195-7350, USA.
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Paulsel
authors.forename: Alexey J
authors.initials: AJ
authors.lastname: Merz
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Nickerson
mesh_terms: D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D057146:Multivesicular Bodies / Q000378:metabolism; D009154:Mutation; D021381:Protein Transport / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D014617:Vacuoles / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*; 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; C583270:Muk1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C099285:VPS9 protein, S cerevisiae; D033921:Vesicular Transport Proteins; D049452:Green Fluorescent Proteins; C086866:VPS21 protein, S cerevisiae; C086867:YPT52 protein, S cerevisiae; C086868:YPT53 protein, S cerevisiae; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins
keywords: Endocytosis; G Proteins; Guanine Nucleotide Exchange Factor (GEF); Luciferase Reporter of Intraluminal Deposition (LUCID); MVB; Membrane Fusion; Rab; Vps21; Ypt52; Ypt53