A PROTEASE PROCESSING SITE IS ESSENTIAL FOR PRORENIN SORTING TO THE REGULATED SECRETORY PATHWAY

Transfected mouse pituitary AtT-20 cells were used to examine the sort ing of human prorenin to dense core secretory granules and the regulat ed secretory pathway, These cells secrete prorenin constitutively and sort a portion of the prorenin to secretory granules, where it is conv erted to active renin by proteolytic processing. Pulse chase labeling of transfected AtT-20 cells demonstrated that regulated secretion of p rorenin was prevented by: 1) the mutagenic deletion of the prosegment, 2) the premature proteolytic removal of the prosegment by a Golgi-res ident processing protease, or 3) the mutation of the native cleavage s ite so as to prevent removal of the prosegment. In addition, expressio n of fusion proteins containing portions of the prorenin prosegment de monstrated that exposure of potential proteolytic cleavage sites was s ufficient to confer cleavage-dependent regulated secretion of the corr esponding protein. These data implicate the protease cleavage event in the regulated secretion of prorenin and are consistent with the invol vement of a subclass of processing proteases in the sorting of certain proteins to secretory granules in AtT-20 cells.