Patching plasma membrane disruptions with cytoplasmic membrane

Vesicle-vesicle fusion initiated in cell cytoplasm by high Ca2+ can rapidly erect large membrane boundaries. These might be used as a 'patch' for rese aling plasma membrane disruptions. Three central predictions of this 'patch ' hypothesis are here established in sea urchin eggs. First, we show that s urface markers for plasma membrane protein and lipid are initially absent o ver disruption sites after resealing is complete. Second, we demonstrate th at resealing capacity is strongly dependent upon local availability of fusi on competent cytoplasmic organelles, specifically the reserve or yolk granu le. Lastly, we demonstrate that the reserve granule is capable of rapid (t( 1/2) <1 second), Ca2+-regulated (high threshold) fusion capable of erecting large (>1000 mu m(2)), continuous membrane boundaries, Production of patch vesicles for resealing may proceed by an 'emergency' fusion mechanism dist inct from that utilized for the much slower, highly regulated, cytosol-requ iring organelle-organelle fusion events typical of constitutive membrane tr afficking pathways.