Fertilization results in increased tyrosine phosphorylation of egg proteins

The sea urchin egg contains one or more tyrosine-specific protein kinase(s) which are active during the response of the egg to sperm fusion. Fertilization results in an 8-fold increase in the relative incorporation of [32P]orthophosphate into phosphotyrosine as compared to phosphoserine and phosphothreonine. Under defined in vitro conditions, plasma membranes from fertilized eggs incorporated 5-10-fold more phosphate into tyrosine than plasma membranes from unfertilized eggs. Analysis of the phosphorylated plasma membrane proteins by polyacrylamide gel electrophoresis demonstrated that at least four proteins were more actively phosphorylated in plasma membranes from fertilized eggs. Of these, a closely spaced doublet of approximately 120 kDa was found to contain phosphotyrosine. The properties of the egg tyrosine-specific kinase were studied using an artificial peptide substrate. The enzyme is membrane-bound and is enriched 8-fold in the egg plasma membrane. Enzyme activity in egg homogenates and plasma membranes increased 2- and 4-fold, respectively, as early as 20 min, post-insemination. These results suggest that the fertilization-dependent increase in tyrosine-specific protein kinase activity may play a role in the onset of embryonic development.