Regulation and function of the Dictyostelium atypical MAP kinase Erk2

In eukaryotic cell signaling, MAPKs mediate cellular processes like cell growth, differentiation, and movement. In Dictyostelium discoideum, a social amoeba, there are only two MAPKs, Erk1 and Erk2. Erk1 is a typical MAPK responsible for developmental kinetics and aggregate size while Erk2 is an atypical MAPK required for chemotaxis, multicellular development, and the translocation of GtaC, a key transcription factor in developmental gene expression. Little is known about the function and regulation of atypical MAPKs in other organisms but in D. discoideum, Erk2 is activated within 30 seconds after stimulation by a chemoattractant. Erk2 function is required for a burst of Erk1 activation in a secondary response to chemoattractants. To test regions in the Erk2 protein that may be important for regulation and specificity, chimeric proteins have been made with Erk1. Through this study we have discovered that the C-Terminal end of Erk2 is likely important the down regulation of Erk2 activation. Chimeras with junctions near the middle of the MAPKs can partially rescue growth on bacterial lawns. Atypical MAPKs have a conserved C-terminal motif (CTM) not found in other MAPKs. While not essential, the CTM is required for most Erk2 functions except for growth on bacteria but the CTM motif does not confer Erk2 function to the Erk1 MAPK. The phosphorylation of a kinase dead Erk2 mutant (Erk2ᵏᵈ) is phosphorylated after cells have been stimulated by a chemoattractant suggesting that Erk2 does not autophosphorylate and the existence of an upstream protein kinase that regulates Erk2 activity. Current investigation of possible upstream protein kinases responsible for Erk2 activation (Pak (PakE, & PakG) and Fray (Fray1, & Fray2) proteins) are underway. PakE disruption mutants have shown a delay in development from the mound to slug stage. Preliminary data suggest that Fray1 might also be important for this same stage in development. These results indicate that PakE and Fray1 cannot be the sole upstream activators of Erk2, but this does not exclude these kinases from having a potential role upstream of Erk2 during multicellular stages of development.