Evidence for the expression of three genes encoding homologous atrial gland peptides that cause egg laying in Aplysia

Three peptide complexes which can induce egg laying in Aplysia were isolated from the atrial gland of the marine mollusc Aplysia californica and chemically characterized. Amino acid sequence analyses established the covalent structures, including disulfide assignments, of all three dimeric complexes. Each complex consisted of an identical 18-residue peptide (A-AP) which was disulfide-bonded to a 36-residue peptide that was homologous to bag cell egg-laying hormone (EHL). The primary structure of A-AP was determined to be: NH₂-Asp-Ser-Asp-Val-Ser-Leu-Phe-Asn-Gly-Asp-Leu-Leu-Pro-Asn -Gly-Arg-Cys-Ser-COOH. The primary structure of one of the three ELH-regulated peptides (A-ELH) was determined to be NH₂-Ile-Ser-Ile-Asn-Gln-Asp-Leu-Lys-Ala-Ile-Thr-Asp-Met-Leu -Leu-Thr-Glu-Gln-Ile-Gln-Ala-Arg-Arg-Arg-Cys-Leu-Asp-Ala-Leu -Arg-Gln-Arg-Leu-Leu-Asp-Leu-COOH. The two ELH-related peptides, [Ala²⁷]A-ELH and [Gln²³, Ala²⁷]A-ELH, differed from A-ELH at 1 and 2 residues, respectively. Both [Ala²⁷]A-ELH and [Gln²³, Ala²⁷]A-ELH were novel peptide sequences representing products of as yet uncharacterized genes within the ELH family. These structural studies provide the first direct chemical evidence that (a) an 18-residue peptide (A-AP) derived from a polypeptide precursor encoded by the A gene, as predicted from nucleotide sequence analysis, occurs in the atrial gland; (b) the Cys¹⁷ residue of A-AP is disulfide-bonded to Cys²⁵ of A-ELH; and (c) A-AP also occurs disulfide-bonded to two additional, previously undescribed ELH-related peptides, [Ala²⁷]A-ELH and [Gln²³, Ala²⁷]A-ELH.