Regulated secretion requires the developmental coupling of neuronal or hormonal stimuli to an exocytotic response, a multistep pathway whose appearance may be linked with cellular adhesion to the newly formed exocrine cell basement membrane. We screened for adhesion-associated coupling activity using lacrimal acinar cells and have identified “BM180”, a novel basement membrane protein enriched in guanidine HCl extracts of lacrimal and parotid exocrine secretory glands. BM180 resides primarily in a previously inexamined lower molecular-mass basement membrane peak (peak 2) that contains cell adhesion activity inhibitable with the anti-BM180 monoclonal antibody 3E12. Removal of peak 2 by gel filtration or preincubation of basement membrane with 3E12 decreased regulated peroxidase secretion by one-half without affecting constitutive secretion or the amount of cellular peroxidase available for release. Adding back peak 2 restored regulated secretion in a dose-dependent and 3E12-inhibitable manner and suggested a synergistic relationship between BM180 and laminin 1. BM180 has a mobility of 180 and 60 kDa in the absence or presence of dithiothreitol, respectively, and shows no immunological identity by competitive enzyme-linked immunosorbent assay with laminin 1, collagen IV, entactin, fibronectin, BM-40, perlecan, or vitronectin. We propose that BM180 is an important resident of certain glandular basement membranes where it interacts with the cell surface, thereby possibly signaling the appearance of a transducing element in the stimulus-secretion coupling pathway.
Cloning and complete amino acid sequences of human and murine basement membrane protein BM-40 (SPARC, osteonectin)
