Activation of Xenopus eggs by RGD-containing peptides accompanied by intracellular Ca2+ release.

To investigate the mechanism of egg activation in Xenopus, unfertilized eggs were treated with various oligopeptides related to the RGD sequence-containing peptides. When dejellied eggs were treated with RGDS at concentrations higher than 250 microM, the eggs produced a positive-going activation potential concomitant with intracellular Ca2- release. The eggs underwent cortical granule breakdown, cortical contraction, and resumption of meiosis. The activation by RGDS was independent of the voltage of egg membranes. GRGD, GRGDTP, and RGPFPI caused activation in some eggs, but RGES, RFDS, GPRP amide, and GPA did not. Denuded eggs were activated by Sepharose beads bound to RGDS, indicating an interaction of the RGD sequence with receptor molecules on the egg plasma membrane. The activation by RGDS did not occur under a Ca(2+)- and Mg(2+)-free condition, but occurred in a Ca(2+)-free solution containing Mg2+, concomitant with an intracellular Ca2+ release. These results indicate that a receptor of the RGD sequence on egg plasma membrane causes egg activation through an intracellular signal transduction system. A sperm agonist including the RGD sequence may play an important role in egg activation through the egg membrane receptor during the normal fertilization process.