Presence of immunoreactive α-bag cell peptide[1–8] in bag cell neurons of Aplysia suggests novel carboxypeptidase processing of neuropeptides

Abstract α-bag cell peptide (αBCP) is a putative neurotransmitter released from bag cell neurons of the marine mollusc Aplysia . αBCP is present in bag cell extracts and releasate from bag cells in two neuroactive forms: αBCP[1–9] and αBCP[1–8]. αBCP[1–8] is 30 times as potent as [1–9] in inhibiting target neurons, suggesting that both forms of the peptide serve as neurotransmitters. However, biochemical and molecular genetic data suggest that only αBCP[1–9] is originally cleaved directly from a larger precursor protein and that generation of αBCP[1–8] would require an unusual C-terminal leucine cleavage of αBCP[1–9]. To further ascertain which forms of αBCP are normally present in bag cells, we generated highly specific antisera to each peptide. We found intense immunostaining for both peptides in bag cell somata and nerve terminals. Moreover, both forms were stable in bag cell extract for at least 1 hr, which suggests that proteolysis in the extracts had been effectively inhibited. These results suggest that both αBCP[1–8] and [1–9] are normally present in bag cell somata and terminals and that a small amount of αBCP[1–9] is processed to αBCP[1–8] in vesicles before release. The results support the interpretation that the activity of an intravesicular carboxypeptidase generates αBCP[1–8] and thereby regulates the amount of inhibitory activity released during a bag cell discharge.