Non-canonical MAPK signaling in yeast

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Project Summary/Abstract: Mitogen activated protein kinases (MAPKs) are a family of conserved signaling enzymes that are dysregulated in numerous human diseases. Many MAPKs are activated in canonical signaling pathways by MAPK kinases. This mechanism of activation has been well-studied. Other MAPKs are activated in non- canonical signaling pathways by binding proteins that trigger autophosphorylation of the MAPK. Relatively little is known about this mechanism of activation and how it is regulated. The yeast Saccharomyces cerevisiae is a powerful model system in which to study MAPK signaling. Smk1 is a meiosis-specific MAPK in yeast that is activated in a non-canonical pathway by a binding protein, Ssp2, as the meiotic divisions are being completed. Smk1 then controls the post-meiotic program of gamete (spore) formation by phosphorylating regulatory substrates. The anaphase promoting complex (APC) E3 ubiquitin-ligase is a key regulator of chromosome segregation. The APC also plays a role in coupling the differentiation of animal cells to the G1/G0 phase of the cell-cycle. The APC is required for Ssp2 to activate Smk1 yet the mechanism linking the APC to MAPK activation was until recently unknown. In preliminary data, Isc10 has been identified as an inhibitory protein that links the APC to Smk1 activation. A working model for this pathway posits that Isc10 forms a complex with Smk1 and Ssp2 in the cytoplasm of meiotic cells that is poised for activation. In this model, the poised ternary complex is imported into nuclei, where the nuclear resident APC, complexed with a meiosis-specific targeting subunit, Ama1, triggers ubiquitylation of Isc10 after anaphase of meiosis II. This allows Ssp2 to activate the intramolecular autophosphorylation of Smk1, thereby activating the MAPK and coupling spore differentiation to the completion of nuclear segregation. To test and extend this model we will: 1- Elucidate how the inhibitor protein Isc10 controls Smk1 activation, 2- Decipher the spatiotemporal regulation that links MAPKs to the APC, 3- Determine how activated Smk1 controls post-meiotic processes. Insights from these studies will be broadly relevant to mechanisms that control MAPK signaling in the context of developmental programs and how the APC couples differentiation programs to the cell-cycle in higher eukaryotes.