Subcellular sites of processing of precursors to neurosecretory peptides in the bag cells of Aplysia: Inferences from the effects of monensin, FCCP, and chloroquine

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Abstract
The effects of the sodium ionophore monensin were examined in the bag cells of Aplysia californica in order to identify the subcellular sites of processing of precursors to their neurosecretory products. Incubation of bag cells in media containing 10 μM monensin led to a marked disruption of the morphology of the Golgi apparatus without affecting that of other organelles. Exposure of bag cells to monensin led to a significant impairment of processing of the largest precursor and of an intermediate protein which gives rise to the immediate precursors to the final secreted products, the egg‐laying hormone (ELH) and the acidic peptide (AP). Furthermore, ELH and AP were never produced in the presence of monensin during the time course of these experiments. When axonal transport was allowed to proceed, the contents of bag‐cell terminals indicated that the intermediate protein is the first to be packaged in Golgi‐derived vesicles, and in monensin‐treated cells may be transported without being processed further. In contrast to these results, the protonophore FCCP‐impaired precursor and intermediate cleavage equally, indicating that monensin and FCCP have different effects on intracellular transport and precursor processing. These data are interpreted to indicate that the largest ELH‐AP precursor is normally processed within the Golgi apparatus, and that the disruption of this organelle induced by monensin produces the impairment seen in its processing. The impairment of cleavage of the intermediate species, and the blockade of production of AP and ELH, are probably the result of monensin‐induced impairment of production of proteolytically competent secretory granules by the Golgi apparatus.