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 binds to the neuroendocrine-specific protein 7B2 in the endoplasmic reticulum (ER) and is activated late in the regulated secretory pathway of neuroendocrine cells. Several roles, sometimes contradictory, have been suggested for 7B2 with regard to PC2 cellular fate. Thus, 7B2 was proposed to act as a PC2 chaperone in the ER, or to facilitate 7B2 transport from the ER to the trans-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 in PC2-expressing endocrine cells using antisense strategies. We have previously shown that the endocrine rMTC 6-23 cell line expresses PC2 and that the 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 protein was coordinately induced with PC2 and proNT/NN by dexamethasone. Stable transfection of rMTC 6-23 cells with 7B2 antisense cDNA led to a marked reduction (>90%) in 7B2 levels. ProPC2 was expressed to normal levels and cleaved to yield a PC2 form that was constitutively released, was not stored within secretory granules and was unable to process proNT/NN. We conclude that 7B2 is essential for the sorting and activation of PC2 into the regulated secretory pathway of endocrine cells.