Protein 7B2 is essential for the targeting and activation of PC2 into the regulated secretory pathway of rMTC 6-23 cells

Among the prohormone convertases, PC2 is unique in that it specifically bin ds to the neuroendocrine-specific protein 7B2 in the endoplasmic reticulum (ER) and is activated late in the regulated secretory pathway of neuroendoc rine cells. Several roles, sometimes contradictory, have been suggested for 7B2 with regard to PC2 cellular fate. Thus, 7B2 was proposed to act as a P C2 chaperone in the ER, or to facilitate 7B2 transport from the ER to the t rans-Golgi network and to be necessary for proPC2 activation, or to inhibit PC2 enzymatic activity until the latter reaches the secretory granules. To gain insight into the function of 7B2, we sought to block its expression i n PC2-expressing endocrine cells using antisense strategies. We have previo usly shown that the endocrine rMTC 6-23 cell line expresses PC2 and that th e enzyme is responsible for the processing of pro-neurotensin/neuromedin N (proNT/NN). Here, we show that rMTC 6-23 cells express 7B2 and that the pro tein was coordinately induced with PC2 and proNT/NN by dexamethasone. Stabl e transfection of rMTC 6-23 cells with 7B2 antisense cDNA led to a marked r eduction (>90%) in 7B2 levels. ProPC2 was expressed to normal levels and cl eaved to yield a PC2 form that was constitutively released, was not stored within secretory granules and was unable to process proNT/NN. We conclude t hat 7B2 is essential for the sorting and activation of PC2 into the regulat ed secretory pathway of endocrine cells. (C) 1999 Academic Press.