Ssp2 Binding Activates the Smk1 Mitogen-Activated Protein Kinase

ABSTRACT

Smk1 is a meiosis-specific mitogen-activated protein kinase (MAPK) in Saccharomyces cerevisiae that couples spore morphogenesis to the completion of chromosome segregation. Similar to other MAPKs, Smk1 is controlled by phosphorylation of a threonine (T) and a tyrosine (Y) in its activation loop. However, it is not activated by a dual-specificity MAPK kinase. Instead, T207 in Smk1's activation loop is phosphorylated by the cyclin-dependent kinase (CDK)-activating kinase (Cak1), and Y209 is autophosphorylated in an intramolecular reaction that requires the meiosis-specific protein Ssp2. In this study, we show that Smk1 is catalytically inert unless it is bound by Ssp2. While Ssp2 binding activates Smk1 by a mechanism that is independent of activation loop phosphorylation, binding also triggers autophosphorylation of Y209 in Smk1, which, along with Cak1-mediated phosphorylation of T207, further activates the kinase. Autophosphorylation of Smk1 on Y209 also appears to modify the specificity of the MAPK by suppressing Y kinase and enhancing S/T kinase activity. We also found that the phosphoconsensus motif preference of Ssp2/Smk1 is more extensive than that of other characterized MAPKs. This study therefore defines a novel mechanism of MAPK activation requiring binding of an activator and also shows that MAPKs can be diversified to recognize unique phosphorylation motifs.

KEYWORDS:

• MAPK
• autophosphorylation
• meiosis
• yeast
• mitogen-activated protein kinases

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Supplemental material for this article may be found at https://doi.org/10.1128/MCB.00607-16.

ACKNOWLEDGMENTS

We thank Gino Cingolani and Ya-Ming Hou for helpful scientific discussions. We thank Hsin-Yao Tang and the Wistar Institute Proteomics Facility (Philadelphia, PA) for the phosphoproteomic analyses.

This work was supported by a research grant from the NIH (R01GM105947) to B.E.T. and by research grants from the NIH (R01GM120090) and the NSF (MCB-1516348) to E.W.