Dissimilar associations of two secretory peptides with a neurosecretory granule-enriched fraction from the bag cells.

The bag cell neurons of Aplysia californica synthesize and secrete several neuropeptides. To gain more detailed information about their posttranslational routing and transport, we have undertaken isolation of the neurosecretory granules (NSG). Extracts of radiolabeled cells were subjected to discontinuous, isosmotic density-gradient centrifugation. Radiolabeled peptides likely to be contained in NSG were found to relocate from the starting zone and to be associated with particulate structures. Assay of enzyme markers for lysosomes and endoplasmic reticulum disclosed gradient distributions that differed from that shown by the peptides. Hence, it is probable that the position of peak concentrations of particulate peptides represents the location of NSG. Of particular interest is the further observation that the known secretory peptides ELH and AP do not evidence strict covariance across the gradient. This deviation from covariance is consistent with hypotheses that the peptides are in different associations with the NSG cores or that more than one type of neurosecretory granule is produced in the bag cells.