Exocytotic insertion of calcium channels constrains compensatory endocytosis to sites of exocytosis

Proteins inserted into the cell surface by exocytosis are thought to be ret rieved by compensatory endocytosis, suggesting that retrieval requires gran ule proteins. In sea urchin eggs, calcium influx through P-type calcium cha nnels is required for retrieval, and the large size of sea urchin secretory granules permits the direct observation of retrieval. Here we demonstrate that retrieval is limited to sites of prior exocytosis. We tested whether c hannel distribution can account for the localization of retrieval at exocyt otic sites. We find that P-channels reside on secretory granules before fer tilization, and are translocated to the egg surface by exocytosis. Our stud y provides strong evidence that the transitory insertion of P-type calcium channels in the surface membrane plays an obligatory role in the mechanism coupling exocytosis and compensatory endocytosis.