THE DICTYOSTELIUM DUAL-SPECIFICITY KINASE SPLA IS ESSENTIAL FOR SPOREDIFFERENTIATION

We have studied the structure and function of the Dictyostelium kinase splA. A truncated form of the splA protein exhibited primarily tyrosi ne kinase activity in vitro; however, it also autophosphorylated on se rine and threonine residues, The kinase domain of splA exhibits approx imately 38% identity to the CTR1 kinase of Arabidopsis, which is a mem ber of the Raf family, Outside its kinase domain, splA shares homology with the byr2 kinase of S. pombe. By aligning the sequences of splA, byr2 and STE11, a homologue of byr2 in S, cerevisiae, we have identifi ed a conserved motif that is also found in members of the Eph family o f growth factor receptor tyrosine kinases, SplA is expressed throughou t development with a peak during the mound stage of morphogenesis. Str ains in which the splA gene had been disrupted completed fruiting body formation; however, spore cells spontaneously lysed before completing their differentiation. Northern analysis revealed the expression of t he prespore marker cotB and the prestalk markers ecmA and ecmB in the mutant strain during development. The spore differentiation marker spi A was detected in the mutant spores both by northern and immunoblottin g, but these cells failed to assemble spore coats, Immunoblot analysis of the developmental pattern of tyrosine phosphorylation revealed a p rotein that was phosphorylated in mutants but was not phosphorylated i n the wild-type cells, SplA is a novel dual specificity kinase that re gulates the differentiation of spore cells.