II. Aplysia brasiliana neurons R3-R14: Primary structure of the myoactive histidine-rich basic peptide and its prohormone

Neurons R3-R14 of the marine mollusc Aplysia are model neuroendocrine cells thought to regulate cardiovascular activity in vivo. The cells express a gene encoding three peptides-peptides I, II and the histidine-rich basic peptide (HRBP)-each of which has been chemically characterized in Aplysia californica. In the studies presented here, HRBP and its prohormone (proHRBP) were purified from A. brasiliana abdominal ganglion extracts by reversed-phase high-performance liquid chromatography and characterized by amino acid compositional and sequence analyses. ProHRBP was an 85-residue peptide whose sequence was: NH₂-Glu-Glu-Val-Phe-Asp-Asp-Thr-Asp- Val-Gly-Asp-Glu-Leu-Thr-Asn-Ala-Leu-Glu-Ser-Val-Leu-Thr-Asp-Leu-Lys-Asp-Lys-Arg-Asp-Ala-Glu-Glu-Pro-Ser-Ala-Phe-Met- Thr-Arg-Leu-Arg-Arg-Gln-Val-Ala-Gln-Met-His-Ile-Trp-Arg-Ala-Val-Asn-His-Asp-Arg-His-His-Ser-Thr-Gly-Ser-Gly-Arg-His- Ser-Arg-Phe-Leu-Thr-Arg-Asn-Arg-Tyr-Arg-Tyr-Gly-Gly-Gly-His-Leu-Ser-Asp-Ala-COOH. It differed from A. californica pro-HRBP at seven of the 85 positions. Compositional and sequence analyses demonstrated that A. brasiliana HRBP was a 43-residue peptide corresponding to residues 43 through 85 of proHRBP, and that a significant proportion of the isolated peptide possessed a blocked NH₂ terminus. Although this sequence differed from that of A. californica HRBP at five of 43 residues, the two peptides were approximately equipotent in inducing contractions of A. californica crop muscle in vitro, suggesting that the substituted residues may not be critical for biological activity.