Identification of an abl-related protein tyrosine kinase in the cortex of the sea urchin egg: possible role at fertilization.

Fertilization results in the tyrosine phosphorylation of several egg proteins within minutes of sperm-egg binding, although the identity of the kinase(s) involved and the mechanism of regulation is not known. In the present study, we have used site-directed antibodies based on the predicted amino acid sequence of a sea urchin egg transcript that shares significant homology with members of the ABL family of protein tyrosine kinases. These antibodies identified a 220-kDa protein kinase, highly enriched in the egg cortex, where it is tightly associated with detergent-insoluble cytoskeletal elements. The enzyme is capable of phosphorylating synthetic peptide substrates which were used to characterize the kinase activity in an immune-complex assay. Measurement of the protein tyrosine kinase activity immunoprecipitated at different times after fertilization revealed that the level of kinase activity is transiently elevated during the first few minutes postinsemination. Western blot analysis indicated that the amount of the 220-kDa protein did not increase significantly during this period, so the increased kinase activity probably results from activation of the enzyme. These in vitro studies indicate that the 220-kDa abl-related kinase is one of the protein kinases activated during fertilization and suggest that it may play a role in the egg activation process.