The Ste 5 p scaffold

All eukaryotic cells use mitogen-activated protein kinase (MAPK) cascades as central cores of complex signal transduction pathways that respond to a wide variety of external stimuli and regulate numerous cellular responses (Gustin et al., 1998; Cobb et al., 1999; Schaeffer and Weber, 1999; Widmann et al., 1999). A MAPK cascade is a module of three to four serially activated protein kinases named after the last kinase of the series (Fig. 1). Eukaryotic cells typically contain multiple MAPK cascades, which contain unique or shared protein kinases. Surprisingly, the same MAPK cascade can perform different functions in response to different stimuli or different levels of the same stimulus (Marshall, 1995; Elion, 2000; Pan et al., 2000). This is particularly apparent in Saccharomyces cerevisiae , in which four of the six MAPK cascades use subsets of the same protein kinases (Fig. 1). All of the yeast pathways exhibit a high degree of specificity, with different effector proteins activated depending on the stimulus, even in instances where the pathways share kinases. This conclusion is largely based on phenotypic analysis of yeast strains harboring null and point mutations in the different kinases (Gustin et al., 1998; Madhani and Fink, 1998). A major question is what ensures the fidelity of signaling, particularly in instances in which the same protein kinase functions in more than one pathway. Work over the years suggests that signaling specificity is the result of many levels of control, which include: preferred kinase-kinase interactions within the MAPK cascade (Bardwell et al., 1996; Xia et al., 1998); preferred MAPK-substrate interactions (Kallunki et al., 1996; Jacobs et al., 1999); the use of pathway-specific MAPKs (Madhani and Fink, 1998); cross-regulation between MAPK modules (Hall et al., 1996; O’Rourke and Herskowitz, 1998; Davenport et al., 1999); controlled localization of the MAPKs (Schaeffer and Weber, 1999; Cobb, 1999; Cobb and Goldsmith, 1999); and control of the strength of activation (Marshall, 1995). Another crucial level of control arises from the formation of discrete complexes involving the cytoplasmic kinases within a given MAPK cascade. These multienzyme complexes form through interactions with scaffold proteins that bind to multiple components of a single MAPK cascade. The scaffold proteins are thought to facilitate interactions between the associated kinases and can also link them to r ceptors, G proteins or other signaling components localized on the submembrane side of the plasma membrane (Elion, 1995; Elion, 1998; Whitmarsh and Davis, 1998; Schaeffer and Weber, 1999; Burack and Shaw, 2000). MAPK cascade scaffolds are widely divergent, which suggests that they have evolved independently for the individual requirements of subsets of kinases and upstream activating events (Elion, 1998). The compartmentalization of signal transduction pathway components using scaffold or adapter proteins is a frequently encountered form of regulation (Pawson and Scott, 1997; Burack and Shaw, 2000; Miller and Lefkowitz, 2001). In the case of MAPK cascades, scaffold proteins, in theory, provide a wealth of opportunities for protein kinase regulation through localization of upstream signaling components, allosteric control of the associated kinases, control of MAPK ubstrate availability and feedback attenuating mechanisms. The first MAPK scaffold to be described was the yeast Ste5 protein (i.e. Ste5p) (Kranz et al., 1994; Choi et al., 1994; Marcus et al., 1994; Printen and Sprague, 1994), which associates with G protein, MAPKKK, MAPKK and MAPK of the mating pathway (Fig. 1). Since its discovery, a number of analogs of Ste5p have been described in yeast (Fig. 2) and 3967