MAP kinases phosphorylate many of their substrates using a two-step mechanism: MAPKs first attach themselves to docking sites/domains in substrates that are distal to the target phosphorylation site(s), and then phosphorylate those sites. We recently exploited this observation to develop a computational tool (D-finder) that searches genome databases for MAPK-docking sites [1]. Among the novel substrates predicted by D-finder were the human Gli1 and Gli3 transcription factors. We then showed that both ERK and JNK-family MAPKs bind to Gli1/3 via the predicted docking site, and phosphorylate multiple nearby target residues. These phosphosites lie near the binding site for SUFU, and important negative regulator of Gli proteins. In further biochemical experiments, we found that phosphorylation of Gli1 by MAPKs weakens Gli1-SUFU binding. This work is the first to provide evidence that MAPKs bind to and directly phosphorylate Gli proteins; this may be relevant to documented crosstalk between the Ras/MAPK and Hedgehog pathways seen in several cancers. I will also describe our recent progress in identifying other novel MAPK substrates, and in determining why certain docking interaction are highly specific for particular MAPKs while others are not. Finally (time permitting), I will discuss our combined theory/experiment approach to investigating MAPK signaling specificity at the network level.
[1] Whisenant TC… & Bardwell L (2010) PLoS Comput Biol. 6 pii: e1000908.