title: Robust network structure of the Sln1-Ypd1-Ssk1 three-component phospho-relay prevents unintended activation of the HOG MAPK pathway in Saccharomyces cerevisiae.
abstract: BACKGROUND
  The yeast Saccharomyces cerevisiae relies on the high-osmolarity glycerol (HOG) signaling pathway to respond to increases in external osmolarity. The HOG pathway is rapidly activated under conditions of elevated osmolarity and regulates transcriptional and metabolic changes within the cell. Under normal growth conditions, however, a three-component phospho-relay consisting of the histidine kinase Sln1, the transfer protein Ypd1, and the response regulator Ssk1 represses HOG pathway activity by phosphorylation of Ssk1. This inhibition of the HOG pathway is essential for cellular fitness in normal osmolarity. Nevertheless, the extent to and mechanisms by which inhibition is robust to fluctuations in the concentrations of the phospho-relay components has received little attention.
  
  
  RESULTS
  We established that the Sln1-Ypd1-Ssk1 phospho-relay is robust-it is able to maintain inhibition of the HOG pathway even after significant changes in the levels of its three components. We then developed a biochemically realistic mathematical model of the phospho-relay, which suggested that robustness is due to buffering by a large excess pool of Ypd1. We confirmed experimentally that depletion of the Ypd1 pool results in inappropriate activation of the HOG pathway.
  
  
  CONCLUSIONS
  We identified buffering by an intermediate component in excess as a novel mechanism through which a phospho-relay can achieve robustness. This buffering requires multiple components and is therefore unavailable to two-component systems, suggesting one important advantage of multi-component relays.
authors.affiliation: Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ, USA. jdexter@fas.harvard.edu.|Department of Chemistry, Princeton University, Princeton, NJ, USA. jdexter@fas.harvard.edu.|Department of Systems Biology, Harvard Medical School, Boston, MA, USA. jdexter@fas.harvard.edu.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Dexter
authors.affiliation: Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ, USA. pingxu@princeton.edu.
authors.forename: Ping
authors.initials: P
authors.lastname: Xu
authors.affiliation: Department of Systems Biology, Harvard Medical School, Boston, MA, USA. jeremy@hms.harvard.edu.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Gunawardena
authors.affiliation: Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ, USA. mmcclean@wisc.edu.|Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, WI, USA. mmcclean@wisc.edu.
authors.forename: Megan N
authors.initials: MN
authors.lastname: McClean
mesh_terms: D005990:Glycerol / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System; D008954:Models, Biological; D008958:Models, Molecular; D009994:Osmolar Concentration; D010766:Phosphorylation; D011494:Protein Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000166:cytology* / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; C087199:SSK1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D011494:Protein Kinases; C102035:YPD1 protein, S cerevisiae; C083749:SLN1 protein, S cerevisiae; D005990:Glycerol