δ-Bag cell peptide from the egg-laying hormone precursor of Aplysia. Processing, primary structure, and biological activity

The bag cells of the marine mollusk Aplysia express a gene encoding a 271-residue egg-laying hormone (ELH) precursor that is processed into at least nine peptide products. Four of the peptides have been identified in bag cell releasates and are known to act as nonsynaptic neurotransmitters in the abdominal ganglion. The isolation, primary structure, and proposed biological activity of a fifth peptide product [δ-bag cell peptide (δ-BCP)] from the ELH precursor are described. δ-BCP was established to be a 39-residue peptide: NH₂-Asp-Gln-Asp-Glu-Gly-Asn-Phe-Arg-Arg-Phe-Pro-Thr-Asn-Ala-Val-Ser-Met- Ser-Ala-Asp-Glu-Asn-Ser-Pro-Phe-Asp-Leu-Ser-Asn-Glu-Asp-Gly-Ala-Val-Tyr- Gln-Arg-Asp-Leu-COOH. This sequence corresponds to residues 81-119 of the ELH prohormone and shares sequence identity with atrial gland peptides A and B. Significantly, synthetic δ-BCP stimulated Ca²⁺ uptake into mitochondria of secretory cells in the albumin gland in vitro, suggesting that the peptide regulates the cellular release of perivitelline fluid by the gland. Similar results were obtained with purified peptide A and a shorter version of δ-BCP [δ-BCP-(14-33)]. These results indicate that δ-BCP belongs to a family of structurally related peptides with similar pharmacological activities that center at a conserved region of sequence corresponding to δ-BCP-(14-33).