Non-canonical MAPK signaling in yeast

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Project Summary/Abstract: Mitogen activated protein kinases (MAPKs) are key signaling enzymes that are activated by phosphorylation of a TXY motif in their activation loops. While dual-specificity MAPK kinases activate many MAPKs (canonical signaling), atypical kinases and autophosphorylation can also activate MAPKs (non-canonical signaling). A great deal is known about the role of canonical signaling pathways in health and disease. Much less is known about non-canonical signaling. Smk1 is a meiosis- specific MAPK in the yeast S. cerevisiae that controls gamete formation (sporulation) through a non-canonical signaling pathway that generates different forms of the MAPK at different stages of meiosis. First, the CDK activating kinase, Cak1, phosphorylates Smk1's activating T-residue as the meiotic divisions are taking place. This monophosphorylated form of Smk1 is distributed throughout the cell. Later in the program (at anaphase II), a meiosis-specific protein, Ssp2, binds to Smk1 and activates the cis-autophosphorylation of its Y-residue. This reaction occurs at specific membraneous structures that cellularize the haploids generated in this specialized cell division. Thus, Ssp2 delivers a pulse of the doubly phosphorylated MAPK to the “right place” at the “right time” in this developmental program. This spatiotemporal specificity is achieved by modularity of the Ssp2 protein; it is localized by a targeting domain (TD) and it activates cis-autophosphorylation of the MAPK via a kinase activating domain (KAD). The Ssp2KAD is itself composed of segments that bind Smk1 and a regulatory RNA recognition motif (RRM). This proposal is to use the Ssp2KAD/Smk1 reaction as a model to study autoactivation of MAPKs in the context of a developmental system. The specific aims of this proposal are: 1- Elucidate molecular mechanisms of non-canonical MAPK signaling using a bacterial reconstitution system and purified proteins, 2-Test a competition-based switch-site model for Smk1 activation, and 3- Identify the RNA(s) recognized by the RNA recognition motif in Ssp2.