CHARACTERIZATION OF PROOPIOMELANOCORTIN PROCESSING IN HETEROLOGOUS NEURONAL CELLS THAT EXPRESS PC2 MESSENGER-RNA

We have investigated processing of monkey pro-opiomelanocortin (POMC) following transfection into heterologous neuronal Neuro 2A (N2A) cells . In several separately transfected stable cell lines (termed N2A/POMC 2-like; n = 4), POMC was processed to betaE only, by direct cleavage f rom the precursor. Thus, these cell lines did not produce betaE in the orderly manner observed in the pituitary, that is, via the intermedia te peptide betaLPH. Analysis of one representative N2A/POMC2 cell line revealed that the extent of processing to betaE appeared to be negati vely correlated with precursor expression level, suggesting that the p rocessing enzyme(s) in these cells was present in limiting amounts. No rthern analysis of PC1 and PC2, two recently cloned processing enzymes , showed that N2A/POMC2 cells expressed low levels of PC2 mRNA, but no detectable PC1 mRNA. These data suggest that (1) the order of process ing observed in the pituitary is not exclusively determined by tertiar y folding of the precursor, but rather by the complement of processing enzymes in a particular cell, and (2) if PC2 is responsible for POMC processing in N2A/POMC2 cells, this enzyme, expressed in limiting amou nts, appeared to show selectivity for the betaE amino terminal process ing site.