Organization of kinases, phosphatases, and receptor signaling complexes.

761 The issue of specificity in cellular signaling has attracted the interest of many researchers for four decades. At its heart lies a rather simple question: How do the protein kinases and phosphatases that govern changes in the phosphorylation state of cellular proteins modify the correct substrate? This is a complex problem when one considers that over 2,000 protein kinases and 1,000 protein phosphatases are estimated to be present in the human genome (1). Although many effector-mediated signaling pathways employ a similar repertoire of protein kinases and phosphatases to implement their intracellular effects, somehow fidelity is retained to ensure that the appropriate intracellular responses occur. One hypothesis that has gained acceptance over the past decade is the idea that targeting of kinases and phosphatases close to their substrates is crucial to ensure tight regulation of the phosphorylation events. The basis for this postulate has developed from evidence that a molecular framework of adapter, anchoring , and scaffold proteins exists to maintain kinases and phosphatases in defined subcellular compartments (2). Evidence is now also accumulating in support of the clustering and organization of receptors and channels with intracellular signaling cascades. This Perspective will highlight the importance of protein domains in the compartmentalization of signaling enzymes close to their activators and targets, with a focus on kinase-mediated signal transduction. Scaffolding of MAP kinase cascades. A classic example of kinase anchoring is the organization of components involved in the mitogen-activated protein (MAP) kinase cascades (Fig. 1a). For example, in the budding yeast Sac-charomyces cerevisiae, the pheromone mating response is initiated through G protein–linked activation of the kinase Ste20. This leads to stimulation of a MAP kinase cascade in which Ste11 phosphorylates and activates Ste7, which in turn phosphorylates and activates the MAP kinase homologs Fus 3 or Kss1 (3). This signaling pathway can be tightly controlled because each of the component enzymes is physically associated with a scaffold protein called Ste5. Through this type of organization , many of the early components in the pheromone mating pathway are localized together. This probably facilitates the rapid transduction of signals through the complex and also may ensure that there is a segregation of this MAP kinase module from related signaling cascades in yeast. This view is supported by the identification of a second yeast scaffold protein called Pbs2 that coordinates components of the osmoregulatory pathway. The transmembrane osmosensor Sho1 is coupled to a MAP kinase …