The Egg-Laying Hormone Family: Precursors, Products, and Functions

¹ The Marine Biomedical Institute and Department of Anatomy and Neurosciences, The University of Texas Medical Branch, Galveston, Texas 77550

The marine mollusc Applysia produces an egg-laying hormone (ELH), which induces ovulation and acts on central neurons to effect egg-laying behavior. ELH is synthesized in the neuroendocrine bag cells; it is encoded by the ELH gene, one of a small family of genes, each of which is expressed in a tissue-specific manner. We review what is known about post-translational processing of the ELH precursor, and report the isolation and chemical characterization of -bag-cell peptide, the seventh peptide product of the ELH precursor to be identified to date. Amino acid compositional and sequence analyses demonstrated that the primary structure of the 19-residue peptide is: NH₂-Ser-Val-Leu-Thr-Pro-Ser-Leu-Ser-Ser-Leu-Gly-Glu-Ser-Leu-Glu-Ser-Gly-Ile-Ser-COOH. Several other ELH-related genes are expressed in the atrial gland, an exocrine organ secreting into the oviduct of Aplysia. We review post-translational processing of these ELH-related precursors, and compare the events to those in the neuroendocrine bag cells. Finally, we compare the sequences of six ELH-related peptides from Aplysia with one ELH-related peptide (caudodorsal cell hormone) from Lymnaea to gain insight into the structure-activity relations of ELH at the ovotestis receptor.

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