Purification and Primary Structure of the Neuropeptide Egg-laying Hormone of Aplysia Californica

Egg-laying hormone (ELH), a neuropeptide synthesized by the bag cell neurons, induces egg laying and its correlated behavior in Aplysia californica. In the present study, ELH has been purified to homogeneity and its primary structure has been determined. We find this molecule to have 36 amino acid residues with a M(r) of 4385 and a calculated isoelectric point of 9.7. Direct microsequence analysis revealed a single amino acid sequence that is in agreement with the amino acid composition determined after acid hydrolysis of ELH: H-Ile-Ser-Ile-Asn-Gln-Asp-Leu-Lys-Ala-Ile-Thr-Asp-Met-Leu-Leu-Thr-Glu-Gln- Ile-Arg-Glu-Arg-Gln-Arg-Tyr-Leu-Ala-Asp-Leu-Arg-Gln-Arg-Leu-Leu-Glu-Lys-OH. Enzyme data indicate that the COOH-terminal lysine may be modified but its exact nature remains to be determined. There is no similarity between the amino acid sequence of ELH and that of presently known vertebrate neuropeptides. The two-step purification procedure, starting with a homogenate of bag cell clusters, consisted of cation exchange chromatography on SP C25 (Sephadex) followed by gel filtration on Bio-Gel P-6. Our purification results in a 100-fold enrichment of ELH from bag cell homogenates and a 36% recovery of purified radiolabeled marker ELH. Analysis of purified ELH radiolabeled with [(35)S]methionine or [(3)H]leucine on isoelectric focusing gels and on 8 M urea/sodium dodecyl sulfate gels showed only a single peak containing 90% of the radiolabel. Radiolabeled ELH migrated with a pI of 9.0-9.2 and an apparent M(r) of 3500-5700. ELH retained egg-laying bioactivity when eluted from this segment of the gel. We find that 2.5 nmol of pure ELH consistently induces egg laying at 20 degrees C.