Islet Activating Protein-Sensitive Guanosine Triphosphate-Binding Protein Regulates K⁺ -Channels Coupled with FMRFamide Receptors

A neuropeptide Phe-Met-Arg-Phe-NH₂ (FMRFamide) induces K⁺ -dependent outward current in Aplysia neurons. Intracellular application of islet activating protein (IAP) irreversibly and selectively blocked this outward current without affecting resting membrane conductance. An injection of GTPYS, a nonhydrolyzable analogue of guanosine triphosphate (GTP), caused very slow irreversible increase in K⁺ -conductance of the resting membrane. However, repetitive applications of FMRFamide significantly expedited the effect of GTPYS. These results strongly suggest that K⁺ -channel opening induced by FMRFamide is regulated by an IAP-sensitive GTP-binding protein.