MOLECULAR-CLONING OF A PROTEIN-KINASE WHOSE PHOSPHORYLATION IS REGULATED BY GAMETIC ADHESION DURING CHLAMYDOMONAS FERTILIZATION

Fertilization in Chlamydomonas is initiated by adhesive interactions b etween gametes of opposite mating types through flagellar glycoprotein s called agglutinins. Interactions between these cell adhesion molecul es Signal for the activation of adenylyl cyclase through an interplay of protein kinases and ultimately result in formation of a diploid zyg ote. One of the early events during adhesion-induced signal transducti on is the rapid inactivation of a flagellar. protein kinase that phosp horylates a 48-kDa protein in the flagella. We report the biochemical and molecular characterization of the 48-kDa protein. Experiments usin g a bacterially expressed fusion protein show that the 48-kDa protein is capable of autophosphorylation on serine and tyrosine and phosphory lation of bovine beta-casein on serine, confirming that the 48-kDa pro tein itself has protein kinase activity. This protein kinase exhibits limited homology to members of the eukaryotic protein kinase superfami ly and may be an important element in a signaling pathway in fertiliza tion.