Molecular cloning of Aplysia neuronal cDNAs that encode carboxypeptidases related to mammalian prohormone processing enzymes

The bag cell neurons of Aplysia synthesize an egg-laying hormone (ELH) precursor that initially is cleaved into two fragments in the Golgi apparatus, and the fragments are differentially packaged in separate granule populations and further processed. Aplysia Afurin, Afurin2, prohormone convertase 1 (PC1), and PC2 are thought to be involved in the posttranslational processing of the ELH prohormone. In the present study, we have cloned Aplysia neuronal cDNAs that encode an enzyme most closely related to mammalian carboxypeptidase E (CPE), a peptide hormone processing enzyme that removes basic residues during prohormone processing. Northern blot analysis identified a single Aplysia CPE mRNA (~5.2 kb) in central nervous system tissue. The C-terminal region of Aplysia CPE contains amphiphilic α- helices that may serve as a hydrophobic membrane anchor. A novel neuronal Aplysia enzyme was also identified by the polymerase chain reaction that was most closely related to the carboxypeptidase D (CPD)-related duck protein gp180 and the Drosophila silver gene carboxypeptidases. Aplysia CPE and the CPD-related enzyme are candidate processing enzymes that may play a role in the processing of the ELH prohormone and other Aplysia prohormones.