pH regulation of an egg cortex tyrosine kinase

Fertilization of the echinoderm egg is known to result in the phosphorylation, on tyrosine, of a high-molecular-weight cortical protein (HMWCP) localized in the egg cortex. Studies using various parthenogenic agents indicate that this phosphorylation event occurs in response to the alkaline shift in cytoplasmic pH 1 which normally occurs 1 to 2 min after fertilization. In the present study, the purified egg cell surface complex was used as an in vitro system to determine whether a small alkaline shift in pH, such as occurs upon fertilization, could stimulate the activity of theegg cortex-associated tyrosine kinase toward endogenous protein substrates. The results demonstrated thatthe cell surface complex is highly enriched in a tyrosine kinase activity which accounts for the majority of the protein kinase activity in this preparation. The activity of this tyrosine kinase toward the HMWCP and other cortical proteins was highly dependent on pH over the range pH 6.8 to 7.3. This indicates that the fertilization-associatedchange in cytoplasmic pH would be sufficient to trigger increased tyrosine phosphorylation of the high-molecular-weight cortical protein in vivo. The regulation of tyrosine phosphorylation by small changes in pH represents a novel control mechanism in which a tyrosine protein kinase may act as a pH-sensitive transducer.