Primary structure and tissue distribution of anglerfish carboxypeptidase H

Most peptide hormones are synthesized as part of larger precursor proteins which must be processed after translation to generate bioactive peptides. This usually involves cleavage of the precursor by an endopeptidase at sites marked by basic amino acids, followed by removal of N- or C-terminal basic residues by the action of an aminopeptidase or carboxypeptidase. These processing events have been observed in a variety of species, from yeast to mammals. As part of an effort to characterize prohormone processing enzymes in the anglerfish, Lophius americanus, we have cloned and sequenced a cDNA for the fish prohormone processing carboxypeptidase H (CPH). Polyadenylated RNA from anglerfish (AF) islet organs was used to construct a cDNA library in phage λgt11. The library was screened with a probe derived from the cDNA for rat CPH. A 2400 base pair AF cDNA clone was isolated. This cDNA encodes a polypeptide which is similar in size and composition to mammalian CPH. The sequence data indicate that the AF CPH precursor is a 454 amino acid polypeptide. The derived amino acid sequence of the putative fish CPH is 81% homologous to the rat and bovine CPH enzymes. Significantly, all of the amino acid residues thought to be important for metal ion and substrate binding, glycosylation, and catalytic activity of mammalian CPH are conserved in the fish enzyme. Northern hybridization using RNA from AF tissues indicates that a 2.5 kb fish CPH mRNA is expressed in brain, pituitary and islet organs, but not in other tissues which do not secrete peptide hormones.