Sea urchin egg preparations as systems for the study of calcium-triggered exocytosis

This paper reviews recent work in our laboratory on the mechanism of calciu m-triggered exocytosis. Upon echinoderm egg fertilization, cortical secreto ry vesicle exocytosis is massive and synchronous. By combining physiologica l and molecular analyses with a variety of purified membrane isolates conta ining secretory vesicles that fuse to the plasma membrane or each other, we have characterized the final steps of this calcium-triggered exocytosis. O ur kinetic analysis led to a functional definition of the fusion complex wh ose activation by calcium follows Poisson statistics. The properties of thi s complex are compared with the properties of the heterotrimeric SNARE prot ein complex that is present in the cortical vesicle system. Our data do not support the hypothesis that this particular heterotrimeric complex is by i tself the biological fusogen.