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 GTP gamma S, 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 GTP gamma S. These results strongly suggest that K+-channel opening induced by FMRFamide is regulated by an IAP-sensitive GTP-binding protein.