title: Scaffold Protein Ahk1, Which Associates with Hkr1, Sho1, Ste11, and Pbs2, Inhibits Cross Talk Signaling from the Hkr1 Osmosensor to the Kss1 Mitogen-Activated Protein Kinase.
abstract: In the budding yeast Saccharomyces cerevisiae, osmostress activates the Hog1 mitogen-activated protein kinase (MAPK), which regulates diverse osmoadaptive responses. Hkr1 is a large, highly glycosylated, single-path transmembrane protein that is a putative osmosensor in one of the Hog1 upstream pathways termed the HKR1 subbranch. The extracellular region of Hkr1 contains both a positive and a negative regulatory domain. However, the function of the cytoplasmic domain of Hkr1 (Hkr1-cyto) is unknown. Here, using a mass spectrometric method, we identified a protein, termed Ahk1 (Associated with Hkr1), that binds to Hkr1-cyto. Deletion of the AHK1 gene (in the absence of other Hog1 upstream branches) only partially inhibited osmostress-induced Hog1 activation. In contrast, Hog1 could not be activated by constitutively active mutants of the Hog1 pathway signaling molecules Opy2 or Ste50 in ahk1Δ cells, whereas robust Hog1 activation occurred in AHK1(+) cells. In addition to Hkr1-cyto binding, Ahk1 also bound to other signaling molecules in the HKR1 subbranch, including Sho1, Ste11, and Pbs2. Although osmotic stimulation of Hkr1 does not activate the Kss1 MAPK, deletion of AHK1 allowed Hkr1 to activate Kss1 by cross talk. Thus, Ahk1 is a scaffold protein in the HKR1 subbranch and prevents incorrect signal flow from Hkr1 to Kss1.
authors.affiliation: Division of Molecular Cell Signaling, Institute of Medical Science, University of Tokyo, Tokyo, Japan Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan.
authors.forename: Akiko
authors.initials: A
authors.lastname: Nishimura
authors.affiliation: Division of Molecular Cell Signaling, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
authors.forename: Katsuyoshi
authors.initials: K
authors.lastname: Yamamoto
authors.affiliation: Medical Proteomics Laboratory, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Oyama
authors.affiliation: Medical Proteomics Laboratory, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Kozuka-Hata
authors.affiliation: Division of Molecular Cell Signaling, Institute of Medical Science, University of Tokyo, Tokyo, Japan Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan h-saito@ims.u-tokyo.ac.jp tategone@ims.u-tokyo.ac.jp.
authors.forename: Haruo
authors.initials: H
authors.lastname: Saito
authors.affiliation: Division of Molecular Cell Signaling, Institute of Medical Science, University of Tokyo, Tokyo, Japan Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan h-saito@ims.u-tokyo.ac.jp tategone@ims.u-tokyo.ac.jp.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Tatebayashi
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D034521:Nuclear Matrix-Associated Proteins / Q000378:metabolism*; D064587:Osmoregulation; D009997:Osmotic Pressure; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: C000609629:Ahk1 protein, S cerevisiae; D002352:Carrier Proteins; C512735:HKR1 protein, S cerevisiae; D008565:Membrane Proteins; D034521:Nuclear Matrix-Associated Proteins; C459787:SHO1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C541984:KSS1 protein, S cerevisiae; D020928:Mitogen-Activated Protein Kinases; D020930:MAP Kinase Kinase Kinases; C067018:Ste11 protein, S cerevisiae; D020929:Mitogen-Activated Protein Kinase Kinases; C089356:PBS2 protein, S cerevisiae