title: Cyclin-specific docking motifs promote phosphorylation of yeast signaling proteins by G1/S Cdk complexes.
abstract: BACKGROUND
  The eukaryotic cell cycle begins with a burst of cyclin-dependent kinase (Cdk) phosphorylation. In budding yeast, several Cdk substrates are preferentially phosphorylated at the G1/S transition rather than later in the cell cycle when Cdk activity levels are high. These early Cdk substrates include signaling proteins in the pheromone response pathway. Two such proteins, Ste5 and Ste20, are phosphorylated only when Cdk is associated with the G1/S cyclins Cln1 and Cln2 and not G1, S, or M cyclins. The basis of this cyclin specificity is unknown.
  
  
  RESULTS
  Here we show that Ste5 and Ste20 have recognition sequences, or "docking" sites, for the G1/S cyclins. These docking sites, which are distinct from Clb5/cyclin A-binding "RXL" motifs, bind preferentially to Cln2. They strongly enhance Cln2-driven phosphorylation of each substrate in vivo and function largely independent of position and distance to the Cdk sites. We exploited this functional independence to rewire a Cdk regulatory circuit in a way that changes the target of Cdk inhibition in the pheromone response pathway. Furthermore, we uncover functionally active Cln2 docking motifs in several other Cdk substrates. The docking motifs drive cyclin-specific phosphorylation, and the cyclin preference can be switched by using a distinct motif.
  
  
  CONCLUSIONS
  Our findings indicate that some Cdk substrates are intrinsically capable of being phosphorylated by several different cyclin-Cdk forms, but they are inefficiently phosphorylated in vivo without a cyclin-specific docking site. Docking interactions may play a prevalent but previously unappreciated role in driving phosphorylation of select Cdk substrates preferentially at the G1/S transition.
authors.affiliation: Department of Microbiology and Physiological Systems, University of Massachusetts Medical School, Worcester, MA 01605, USA.
authors.forename: Samyabrata
authors.initials: S
authors.lastname: Bhaduri
authors.forename: Peter M
authors.initials: PM
authors.lastname: Pryciak
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D019925:Cyclin A / Q000378:metabolism; D019926:Cyclin B / Q000378:metabolism; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016193:G1 Phase; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D020930:MAP Kinase Kinase Kinases; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D016196:S Phase; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C076558:CLB5 protein, S cerevisiae; D019925:Cyclin A; D019926:Cyclin B; D047908:Intracellular Signaling Peptides and Proteins; C080321:STE5 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D017346:Protein Serine-Threonine Kinases; D018844:Cyclin-Dependent Kinases; D020930:MAP Kinase Kinase Kinases; C078754:STE20 protein, S cerevisiae