Marchelletta RR, Jacobs DT, Schechter JE, Cheney RE, Hamm-Alvarez SF. The class V myosin motor, myosin 5c, localizes to mature secretory vesicles and facilitates exocytosis in lacrimal acini. Am J Physiol Cell Physiol 295: C13-C28, 2008. First published April 23, 2008 doi:10.1152/ajpcell.00330.2007.-We investigated the role of the actin-based myosin motor, myosin 5c (Myo5c) in vesicle transport in exocrine secretion. Lacrimal gland acinar cells (LGAC) are the major source for the regulated secretion of proteins from the lacrimal gland into the tear film. Confocal fluorescence and immunogold electron microscopy revealed that Myo5c was associated with secretory vesicles in primary rabbit LGAC. Upon stimulation of secretion with the muscarinic agonist, carbachol, Myo5c was also detected in association with actin-coated fusion intermediates. Adenovirus-mediated expression of green fluorescent protein (GFP) fused to the tail domain of Myo5c (Ad-GFP-Myo5c-tail) showed that this protein was localized to secretory vesicles. Furthermore, its expression induced a significant (P Յ 0.05) decrease in carbachol-stimulated release of two secretory vesicle content markers, secretory component and syncollin-GFP. Adenovirus-mediated expression of GFP appended to the full-length Myo5c (Ad-GFP-Myo5c-full) was used in parallel with adenovirus-mediated expression of GFP-Myo5c-tail in LGAC to compare various parameters of secretory vesicles labeled with either GFP-labeled protein in resting and stimulated LGAC. These studies revealed that the carbachol-stimulated increase in secretory vesicle diameter associated with compound fusion of secretory vesicles that was also exhibited by vesicles labeled with GFP-Myo5c-full was impaired in vesicles labeled with GFP-Myo5c-tail. A significant decrease in GFP labeling of actin-coated fusion intermediates was also seen in carbachol-stimulated LGAC transduced with GFPMyo5c-tail relative to LGAC transduced with GFP-Myo5c-full. These results suggest that Myo5c participates in apical exocytosis of secretory vesicles.actin; lacrimal gland; tear film THE LACRIMAL GLAND (LG) is the principal source of proteins released into the tear film. These proteins play critical roles in protection of the ocular surface from pathogens and also provide nutrients and growth factors essential for maintenance of the cornea (3, 50). The lacrimal gland acinar cells (LGAC) constitute ϳ85% of the LG and are largely responsible for this regulated secretion of tear proteins including secretory immunoglobulin A (sIgA), secretory component (SC), lysosomal hydrolases, and growth factors, among others (50). The development of decreased LG output occurs in individuals with syndromes ranging in severity from mild dry eye to the autoimmune disorder,