ENDOPROTEOLYTIC PROCESSING OF PROOPIOMELANOCORTIN AND PROHORMONE CONVERTASE-1 AND CONVERTASE-2 IN NEUROENDOCRINE CELLS OVEREXPRESSING PROHORMONE CONVERTASE-1 OR CONVERTASE-2

AtT-20 mouse corticotrope tumor cell lines overexpressing the prohormo ne convertases PC1 or PC2 were established and used to examine prohorm one and pro hormone convertase biosynthetic processing. On a molar bas is, wild-type AtT20 cells synthesize about 20% as much PC1 as the endo genous prohormone, proopiomelanocortin (POMC). Kinetic, oligosaccharid e, and temperature blockade analyses established that proPC1 is conver ted to PC1 in the endoplasmic reticulum at a rate independent of the l evel of PC1 or PC2 expression. In contrast, proPC2 is converted to PC2 primarily in a post-trans-Golgi compartment. PC1 is further shortened from its COOH-terminal end in a post-trans-Golgi compartment in a ste p that is accelerated at higher levels of PC1 expression, but unaltere d by PC2 overexpression. The initial steps in POMC processing are spee ded up by overexpression of PC1, and overexpression of PC1 leads to mo re extensive cleavage of POMC to smaller products. However, even when the rate of PC1 synthesis exceeds that for POMC by 2-fold, PC1 does no t cleave the Lys Lys or Arg-Lys bonds cleaved upon overexpression of P C2.