Limited heterogeneity of neurosecretory proteins produced by the bag cells of Aplysia

The heterogeneity of the low-molecular-weight (≤11,000 daltons) protein products synthesized by the bag cells of Aplysia in the presence of labeled amino acids was assessed by sequential sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing. The bag cells synthesize two major low-molecular-weight products: an acidic (p I 4.9) peptide with an apparent molecular weight of 4500 daltons, and a basic (p I 9.6) peptide that migrates at an apparent molecular weight of 1500 daltons in our SDS-PAGE system. In addition, a p I 8.2 peptide comigrates with the major basic product on SDS-PAGE, but comprises less than 20% of the labeled material in this region of the gels. The major basic product must be egg laying hormone, a basic (p I 9.3–9.7) peptide that is a known product of these cells. Since egg laying hormone has a molecular weight of 4385 daltons as calculated from its amino acid composition, it appears to migrate anomalously on SDS-PAGE. The apparent molecular-weight heterogeneity of bag cell products previously reported by other investigators can thus be attributed to the anomalous behavior of egg laying hormone in some SDS-PAGE systems.