A novel protein kinase from the ciliate Euplotes raikovi with close structural identity to the mammalian intestinal and male-germ cell kinases: characterization and functional implications in the autocrine pheromone signaling loop

In the free-living ciliate Euplotes raikovi, we identified (and designated as Er-MAPK1) a protein kinase of 631 amino acids, that appears to be constantly phosphorylated in cells which are in growth stage and interact in autocrine fashion with their water-soluble signal pheromones. Er-MAPK1 is specified by a gene that requires a+1 translational frame-shift to be expressed. Its amino-terminal region represents a canonical catalytic domain and carries an activation loop distinctive of the mitogen-activated protein kinases, with the Thr-Asp-Tyr motif deputed to be site of double phosphorylation. In contrast, the carboxy-terminal region appears to be structurally unique. It shows a strongly basic amino acid composition, is very rich in glycine repetitions, and contains a bipartite signal for translocation of Er-MAPK1 into the nucleus.