Expression and Post-Translational Processing of Preprodynorphin in the Rat Anterior Pituitary Cell Line, GH4C1

Glu300 of rat carboxypeptidase E is essential for enzymatic activity but not substrate binding or routing to the regulated secretory pathway

Several recently discovered members of the carboxypeptidase E (CPE) gene family lack critical active site residues that are conserved in other family members. For example, three CPE-like proteins contain a Tyr in place of Glu300 (equivalent to Glu270 of carboxypeptidase A and B). To investigate the importance of this position, Glu300 of rat CPE was converted into Gln, Lys, or Tyr, and the proteins expressed in Sf9 cells using the baculovirus system. All three mutants were secreted from the cells, but the media showed no enzyme activity above background levels. Wild-type CPE and the Gln300 point mutant bound to a p-aminobenzoyl-Arg-Sepharose affinity resin, and this binding was competed by an active site-directed inhibitor, guanidinoethylmercaptosuccinic acid. The affinity purified mutant CPE protein showed no detectable enzyme activity (<0.004% of wild-type CPE) toward dansyl-Phe-Ala-Arg. Expression of the Gln300 and Lys300 mutant CPE proteins in the NIT3 mouse pancreatic beta-cell line showed that these mutants are routed into secretory vesicles and secreted via the regulated pathway. Taken together, these results indicate that Glu300 of CPE is essential for enzyme activity, but not required for substrate binding or for routing into the regulated secretory pathway.

Processing of prodynorphin in BRL-3A cells, a rat liver-derived cell line: implications for the specificity of neuropeptide-processing enzymes

Prodynorphin is post-translationally processed into dynorphin B-13 and other peptides by the action of endopeptidases that cleave at pairs of basic amino acids and at single basic residues, followed by a carboxypeptidase that removes the C-terminal basic residues. To evaluate the specificity of neuropeptide processing enzymes, rat prodynorphin was transfected into BRL-3A cells, a rat liver-derived cell line which produces insulin-like growth factor II, but does not normally express prodynorphin. The transfected prodynorphin was post-translationally processed at both monobasic and dibasic cleavage sites, with the formation of dynorphin B-13 and other peptides. This finding indicates that BRL-3A cells express prodynorphin-processing enzymes. These cells were found to secrete two enzyme activities previously implicated in the processing of dynorphin, a monobasic cleaving 'dynorphin converting enzyme' and 'carboxypeptidase E', based on inhibitor sensitivities and pH optima. The dynorphin converting enzyme secreted from BRL-3A cells elutes from an anion exchange column under the same conditions as the enzyme secreted from pituitary-derived cell lines (AtT-20, GH4C1). Northern blot analysis indicates that BRL-3A cells express carboxypeptidase E mRNA in addition to mRNA encoding furin, a prohormone-processing endopeptidase. The mRNAs for two other related endopeptidases, prohormone convertase 1 and 2, were not detected on Northern blots, suggesting that these enzymes are not required for the processing of prodynorphin. The expression of carboxypeptidase E, furin, and dynorphin converting enzyme in BRL-3A cells suggests that these peptide processing enzymes are not specific for neuropeptides, but are also present in cells which process peptide growth factors.