title: A novel Cdc42-interacting domain of the yeast polarity establishment protein Bem1. Implications for modulation of mating pheromone signaling.
abstract: In Saccharomyces cerevisiae, the Rho-type small GTPase Cdc42 is activated by its guanine-nucleotide exchange factor Cdc24 to polarize the cell for budding and mating. A multidomain protein Bem1 interacts not only with Cdc42 but also with Cdc24 and the effectors of Cdc42, including the p21-activated kinase Ste20, to function as a scaffold for cell polarity establishment. Although Bem1 interacts with Cdc24 and Ste20 via its PB1 and the second SH3 domains (SH3b), respectively, it is unclear how Bem1 binds Cdc42. Here we show that a region comprising the SH3b and its C-terminal flanking segment termed CI (SH3b-CI) directly interacts with Cdc42. A dual-bait reverse two-hybrid approach revealed that the CI is critical to the interaction: N253D substitution in the CI abolishes the binding of the SH3b-CI to Cdc42 but not to the proline-rich region of Ste20, whereas W192K substitution in the SH3b has the opposite effect. Nevertheless, the SH3b-CI interacts with Ste20 proline-rich region and Cdc42 in a mutually exclusive manner. The N253D substitution renders cellular growth temperature-sensitive and suppresses mating. The W192K-induced mating defect is exacerbated by the N253D substitution and suppressed by increasing the dosage of Ste20 provided that the CI is intact. Intriguingly, Cdc42 can mediate an indirect interaction of the SH3b-CI to the CRIB domain of Ste20. These results suggest that the SH3b and the CI collaborate in tethering of Ste20 to Bem1 to ensure efficient mating pheromone signaling.
authors.affiliation: Department of Computational Biology, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Yamaguchi
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Ota
authors.forename: Takashi
authors.initials: T
authors.lastname: Ito
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000737:chemistry*; D000595:Amino Acid Sequence; D005982:Glutathione Transferase / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins; D020930:MAP Kinase Kinase Kinases; D008954:Models, Biological; D008969:Molecular Sequence Data; D010675:Pheromones / Q000737:chemistry / Q000378:metabolism; D011392:Proline / Q000737:chemistry; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D013696:Temperature; D020798:Two-Hybrid System Techniques; D020764:cdc42 GTP-Binding Protein / Q000737:chemistry*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D047908:Intracellular Signaling Peptides and Proteins; D010675:Pheromones; D029701:Saccharomyces cerevisiae Proteins; C073129:BEM1 protein, S cerevisiae; D011392:Proline; D005982:Glutathione Transferase; D017346:Protein Serine-Threonine Kinases; D020930:MAP Kinase Kinase Kinases; C078754:STE20 protein, S cerevisiae; D020764:cdc42 GTP-Binding Protein