Actin coating of secretory granules during regulated exocytosis correlateswith the release of rab3D

The present study describes a novel phenomenon in pancreatic acinar cells u ndergoing regulated exocytosis, When acinar cell preparations were challeng ed with the secretagogue carbamylcholine, a subpopulation of zymogen granul es became coated with filamentous actin, These zymogen granules were always in proximity of the acinar cell apical membrane (the site of exocytosis) b ut did not appear to have fused yet. They were distinct from regular zymoge n granules not only because of their association with filamentous actin, bu t also because the majority of them lacked the zymogen granule marker rab3D , a small GTPase implicated in regulated exocytosis, The apparent loss of r ab3D, presumed to result from the release of rab3D from the granule membran es, could be prevented by agents that modulate the actomyosin system as wel l as by CTP[gamma S], These data suggest that zymogen granules engaging in exocytosis become coated with actin before fusion and that this actin coati ng is tightly coupled to the release of rab3D, We propose that rab3D is inv olved in the regulation of actin polymerization around secretory granules a nd that actin coating might facilitate the movement of granules across the subapical actin network and toward their fusion site.