Self/non-self recognition in the ciliate Euplotes raikovi: characterization of Er-MAPK1, a downstream component of the autocrine signal transduction pathway. REPORT of the IXth scientific meeting of the Italian Association of Developmental and Comparative Immunobiology (IADCI), Varese

In the ciliate Euplotes raikovi, cell type-specific, water-borne signal proteins (pheromones) control self/non-self recognition phenomena by binding their target cell-surface receptors and activating downstream signal transduction pathways. Immunorecognition analyses of E. raikovi cell extracts revealed that at least three distinct protein kinases are activated (phosphorylated) in functional association with the autocrine pheromone-receptor loop that promotes the vegetative (mitogenic) cell growth. One of these kinases, designated as Er-MAPK1 (from Mitogen-Activated Protein Kinases), was structurally characterized by molecular cloning of the relevant gene. The Er-MAPK1 N-terminal half of 300 amino acids bears unmistakable structural homology with the “intestinal cell kinase” and the “male-germ cell associated kinase”, that are involved in the regulation of proliferation and differentiation of specialized animal cells. It contains all the basic structural features that are required for a MAPK catalytic activity, in particular the dual phosphorylation site represented by the Thr-Asp-Tyr motif in the activation loop. In contrast, the Er-MAPK1 C-terminal half of 331 amino acids appears to be structurally unique. It is particularly rich in glycine residues and potential sites of regulatory activities, and shows sequence motifs that clearly predicts a nuclear localization of Er-MAPK1.