Activation of the Smk1 mitogen-activated protein kinase by developmentally regulated autophosphorylation.

Smk1 is a meiosis-specific mitogen-activated protein kinase (MAPK) in Saccharomyces cerevisiae that controls spore morphogenesis. Similar to other MAPKs, it is controlled by dual phosphorylation of its T-X-Y activation motif. However, Smk1 is not phosphorylated by a prototypical MAPK kinase. Here, we show that the T residue in Smk1's activation motif is phosphorylated by the cyclin-dependent kinase (CDK)-activating kinase, Cak1. The Y residue is autophosphorylated in an independent intramolecular reaction that requires the meiosis-specific protein Ssp2. Although both SMK1 and SSP2 are expressed as middle-meiosis-specific genes, Smk1 protein starts to accumulate before Ssp2. Thus, Smk1 exists in a low-activity (pT) form early in sporulation and a high-activity (pT/pY) form later in the program. Ssp2 must be present when Smk1 is being produced to activate the autophosphorylation reaction, suggesting that Ssp2 acts through a transitional intermediate form of Smk1. These findings provide a mechanistic explanation for how Smk1 activity thresholds are generated. They demonstrate that intramolecular autophosphorylation of MAPKs can be regulated and suggest new mechanisms for coupling MAPK outputs to developmental programs.