Alpha-bag cell peptide inhibits bag cell adenylate cyclase via a GTP-dependent mechanism

alpha-Bag cell peptide (alpha-BCP), one of several secreted peptides encoded in the precursor to the egg-laying hormone (proELH) of the neurosecretory bag cells of Aplysia, has been variously reported to have autoexcitatory or autoinhibitory effects on the cells which secrete it. Since we had found previously that alpha-BCP reduces stimulated cAMP levels in intact bag cells, an effect that would be consistent with electrophysiological inhibition, we investigated the direct effect of the peptide on adenylate cyclase in bag cell membrane preparations. alpha-Bag cell peptide did not affect basal adenylate cyclase activity, but reduced forskolin-stimulated activity by about 30%. The potency of the peptide in this assay was within the range reported for observable physiological effects: half-maximal inhibition was seen at approximately 100 nM peptide. Both basal and forskolin-stimulated enzyme activity were dependent on GTP, and the inhibitory effect of alpha-BCP was inversely dependent on the nucleotide. The non-hydrolyzable analogue, GTP-gamma-S, stimulated both basal and forskolin-stimulated enzyme activity and enhanced alpha-BCP's effect to the extent that the peptide completely inhibited forskolin's stimulation of the enzyme. The peptide's effect could be blocked by pretreatment with pertussis toxin. We conclude that alpha-BCP inhibits bag cell adenylate cyclase, an effect which is consistent with an autoinhibitory role in bag cell function. Moreover, this inhibition appears to be mediated by a GTP-binding protein.