Role of actin in regulated exocytosis and compensatory membrane retrieval:Insights from an old acquaintance

This review summarizes new insights into the role of the actin cytoskeleton in exocytosis and compensatory membrane retrieval from mammalian regulated secretory cells. Data from our lab and others now indicate that the actin cytoskeleton is involved in exocytosis both as a negative regulator of memb rane fusion under resting conditions and as a facilitator of movement of se cretory granules to their site of fusion with the apical plasmalemma, Coati ng of docked secretory granules with actin filaments correlates with the di ssociation of secretory-granule-associated rab3D, pointing out a novel role for rab proteins in modulating the actin cytoskeleton during regulated exo cytosis. Compensatory membrane retrieval following regulated exocytosis is also critically dependent on the actin cytoskeleton both in initiating the formation of clathrin-coated retrieval vesicles and subsequent trafficking back into the cell. We propose that, insertion of secretory granule membran e into the plasmalemma initiates a trigger for membrane retrieval, possibly by exposing sites where proteins involved in compensatory membrane retriev al are assembled. The results summarized in this review were derived primar ily from investigations on the pancreatic acinar cell, an old friend who is providing modern wisdom not attainable in other simpler systems. (C) 1999 Academic Press.