Proteolytic processing in the biogenesis of the neurosecretory egg-laying hormone in Aplysia. 1. Precursors, intermediates, and products.

The neurosecretory bag cells of the mollusk, Aplysia, produce a peptide egg-laying hormone, ELH, via a multistep proteolytic processing sequence analogous to those which have been demonstrated for secretory peptides in other systems. The goals of the present study were to identify the major members of this processing sequence by sequential sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing of bag cell proteins synthesized in the presence of labeled precursors and to elucidate the precursor-product relationships between these proteins in pulse-chase experiments. Eight major members of the processing sequence were identified. The ultimate precursor in a 29,000-dalton, pI = 7.7, protein which gives rise to a pI-7.2 protein with an apparent Mr of 6000 as well as heterogeneous species of Mr 16,000-20,000. The latter protein or proteins is/are processed to apparent end products of 13,000-14,500 daltons, while the pI = 7.2 species yields precursors to the final secretory products. These include a pI = 7.5 peptide which is cleaved to ELH (Mr 4385, pI greater than 9) and a Mr 4500, pI = 4.1 species which yields the other secretory product, AP (Mr 4500, pI = 4.9). Therefore, it appears that a single precursor is processed to yield three products, two of which are known to be secreted, and that each product is generated via at least one intermediate form.