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 t rans 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 behave s as a peripheral membrane protein localized to the lumen of the zymogen gr anule but the cDNA for this protein predicts it is a type I membrane protei n with a short, 16-amino-acid, cytosolic tail (C-Tail). Using domain-specif ic antibodies, we demonstrate that Muclin is derived from a precursor, pro- Muclin, which is cleared to produce Muclin and an similar to 80-kDa membran e glycoprotein (p80), Incubation of pulse-labeled cells at less than or equ al to 22 degreesC to block exit from the trans Golgi network also blocks cl eavage of pro-Muclin but not sulfation, a trans Golgi network event, sugges ting that cleavage occurs in a post-Golgi compartment, After cleavage the t no products of pro-Muclin diverge with Muclin remaining in the regulated se cretory pathway and p80 trafficking to the apical plasma membrane, presumab ly via the constitutive-like pathway. When transfected into exocrine AR42J cells, Muclin labeling is perinuclear and in large sub-plasma membrane punc ta, Transiently transfected AR42J cells have greater immunolabeling for amy lase than nontransfected cells, suggesting a role for Muclin in cargo accum ulation in the regulated secretory pathway A construct,vith the C-Tail dele ted targets to small: diffusely-distributed puncta and without the large su b-plasma membrane structures. Thus, the C-Tail is required for proper Mucli n targeting. When transfected into neuroendocrine AtT-20 cells Muclin is no t colocalized nifh ACTH in cell processes, and it appears to be costitutive ly trafficked to the plasma membrane, suggesting that Muclin has exocrine-s pecific information, me present a working model for pro-Muclin as a Golgi c argo receptor for exocrine secretory granule formation at the trans Golgi n etwork.