DIRECT MEMBRANE RETRIEVAL INTO LARGE VESICLES AFTER EXOCYTOSIS IN SEA-URCHIN EGGS

At fertilization in sea urchin eggs, elevated cytosolic Ca2+ leads to the exocytosis of 15,000-18,000 1.3-mu m-diam cortical secretory granules to form the fertilization envelope. Cortical granule exocytosis more than doubles the surface area of the egg. It is thought that much of the added membrane is retrieved by subsequent endocytosis. We have investigated how this is achieved by activating eggs in the presence of aqueous- and lipid-phase fluorescent dyes. We find rapid endocytosis of membrane into 1.5-mu m-diam vesicles starting immediately after cortical granule exocytosis and persisting over the following 15 min. The magnitude of this membrane retrieval can compensate for the changes in the plasma membrane of the egg caused by exocytosis. This membrane retrieval is not stimulated by PMA treatment which activates the endocytosis of clathrin-coated vesicles. When eggs are treated with short wavelength ultraviolet light, cortical granule exocytosis still occurs. but granule cores fail to disperse. After egg activation, large vesicles containing semi-intact cortical granule protein cores are observed. These data together with experiments using sequential pulses of fluid-phase markers support the hypothesis that the bulk of membrane retrieval immediately after cortical granule exocytosis is achieved through direct retrieval into large endocytotic structures.