Processing of pro-Muclin and divergent trafficking of its products to zymogen granules and the apical plasma membrane of pancreatic acinar cells

Proteins are sorted and packaged into regulated secretory granules at the trans Golgi network but holy such granules form is poorly understood. We are studying Muclin, the major sulfated protein of the mouse pancreatic acinar cell, and what its role mag be in zymogen granule formation. Muclin behaves as a peripheral membrane protein localized to the lumen of the zymogen granule but the cDNA for this protein predicts it is a type I membrane protein with a short, 16-amino-acid, cytosolic tail (C-Tail). Using domain-specific antibodies, we demonstrate that Muclin is derived from a precursor, pro-Muclin, which is cleared to produce Muclin and an similar to 80-kDa membrane glycoprotein (p80), Incubation of pulse-labeled cells at less than or equal to 22 degreesC to block exit from the trans Golgi network also blocks cleavage of pro-Muclin but not sulfation, a trans Golgi network event, suggesting that cleavage occurs in a post-Golgi compartment, After cleavage the tno products of pro-Muclin diverge with Muclin remaining in the regulated secretory pathway and p80 trafficking to the apical plasma membrane, presumably via the constitutive-like pathway. When transfected into exocrine AR42J cells, Muclin labeling is perinuclear and in large sub-plasma membrane puncta, Transiently transfected AR42J cells have greater immunolabeling for amylase than nontransfected cells, suggesting a role for Muclin in cargo accumulation in the regulated secretory pathway A construct,vith the C-Tail deleted targets to small: diffusely-distributed puncta and without the large sub-plasma membrane structures. Thus, the C-Tail is required for proper Muclin targeting. When transfected into neuroendocrine AtT-20 cells Muclin is not colocalized nifh ACTH in cell processes, and it appears to be costitutively trafficked to the plasma membrane, suggesting that Muclin has exocrine-specific information, me present a working model for pro-Muclin as a Golgi cargo receptor for exocrine secretory granule formation at the trans Golgi network.