Proteolytic processing of the Aplysia egg-laying hormone and R3-14 neuropeptide precursors

A number of animal behaviors are influenced by the actions of neuropeptides that arise from the processing of complex protein precursors. In this report we investigate the proteolytic processing of neuropeptide precursors expressed in the Aplysia californica bag cells, which govern egg-laying, and neurons R3-14, which mediate aspects of cardiac output. Peptides were purified by fractionation on 2 high-pressure liquid chromatography systems followed by determination of amino acid compositions. Most of these compositions are indicative of processing products derived from the egg-laying hormone (ELH) and R3-14 precursors by cleavage at basic residues. We characterized 9 peptides that arise from the ELH precursor by cleavage of the signal sequence, as well as 7 out of 8 dibasic residues and at least 1 single Arg residue. The peptides range in size from 5 to about 60 amino acids. The R3-14 neuropeptide precursor is cleaved at 2 internal dibasic residues in addition to the signal sequence, resulting in 3 peptides. Shortened forms of several peptides probably result from amino- and carboxy-terminal peptidase action. It is likely that the complex mixtures of neuropeptides arising from these single protein precursors are co-secreted.