Ca2+-dependent actin coating of lamellar bodies after exocytotic fusion: a prerequisite for content release or kiss-and-run.

Type II pneumocytes secrete surfactant, a lipoprotein-like substance reducing the surface tension in the lung, by regulated exocytosis of secretory vesicles termed lamellar bodies (LBs). This secretory process is characterized by a protracted postfusion phase in which fusion pores open slowly and may act as mechanical barriers for release. Combining dark-field with fluorescence microscopy, we show in ss-actin green fluorescent protein-transfected pneumocytes that LB fusion with the plasma membrane is followed by actin coating of the fused LB. This is inhibited by cytoplasmic Ca(2+) chelation or the phospholipase D inhibitor C2 ceramide. Actin coating occurs by polymerization of actin monomers, as evidenced by staining with Alexa 568 phalloidin. After actin coating of the fused LB, it either shrinks while releasing surfactant ("kiss-coat-and-release"), remains in this fused state without further action ("kiss-coat-and-wait"), or is retrieved and pushed forward in the cell on top of an actin tail ("kiss-coat-and-run"). In the absence of actin coating, no release or run was observed. These data suggest that actin coating creates a force needed for either extrusion of vesicle contents or retrieval and intracellular propulsion.