Yeast KEX2 Endopeptidase Correctly Cleaves a Neuroendocrine Prohormone in Mammalian Cells

Mammalian cell lines (BSC-40, NG108-15, and GH$_{4}$C$_{1}$) that cannot process the murine neuroendocrine peptide precursor prepro-opiomelanocortin (mPOMC) when its synthesis is directed by a vaccinia virus vector were coinfected with a second recombinant vaccinia virus carrying the yeast KEX2 gene, which encodes an endopeptidase that cleaves at pairs of basic amino acid residues. mPOMC was cleaved intracellularly to a set of product peptides normally found in vivo, including mature $\gamma $-lipotropin and $\beta $-endorphin$_{1-31}$. In GH$_{4}$C$_{1}$ cells (a rat pituitary line), product peptides were incorporated into stored secretory granules. These results suggest that the inability of any particular cell line to process a prohormone precursor is due to the absence of a suitable endogenous processing enzyme.