Biosynthesis of peptides in the atrial gland of Aplysia californica

1. The atrial gland of the marine mollusc Aplysia californica is a secretory organ in the wall of the large hermaphroditic duct of the reproductive tract. The gland contains at least three peptides capable of inducing behavioral egg laying when injected into another animal. The goals of the present study were to investigate the biosynthesis and biochemical properties of these atrial gland peptides. 2. When the atrial gland is incubated for 2 h in a medium containing a mixture of³H-amino acids, radiolabel is incorporated into a low molecular weight peptide peak (>10,000 Dalton) and a broad asymmetrical protein peak (16,000-40,000 Dalton). Egg-laying activity is recovered only from the low molecular weight peptide peak. 3. Sephadex G-50 gel filtration of radiolabelled atrial gland extracts reveals two major low molecular weight peaks: the first (peak D) is strongly labelled and has a small but variable optical density profile; the second (peak E) is weakly labelled but has a large optical density peak. Egg-laying activity is concentrated in peak D but small amounts are also present in peak E. 4. Isoelectric focusing of the material in peak D reveals three major products: a basic (pI 9.6) peak containing 31% of the incorporated label, a neutral (pI 6.6) peak containing 19% of the label and an acidic (pI 4.9) peak containing 18% of the radiolabel. A minor peak (pI 7.6) is also occasionally observed. The pI 9.6 peak, which probably contains more than one peptide, induces egg laying when injected into intact animals. 5. These experiments demonstrate that the atrial gland rapidly synthesizes both a heterogeneous group of high molecular weight proteins and a complex mixture of low molecular weight peptides. Protein biosynthesis in the atrial gland is, therefore, more complex than has previously been suggested.