Cyclic AMP-dependent protein kinase closes the serotonin-sensitive K+ channels of Aplysia sensory neurones in cell-free membrane patches.

Selected actions of neurotransmitters and hormones on ion channels in nerve and muscle cells are now thought to be mediated by cyclic AMP-dependent protein phosphorylation. Although the cyclic AMP-dependent protein kinase (cAMP-PK) affects the cellular properties of several neurones, its mode of action at the single-channel level has not been characterized. In addition, little is known about the identity or subcellular localization of the phosphoproteins that control channel activity and, in particular, whether the critical substrate proteins are cytoplasmic or membrane-associated. In Aplysia sensory neurones, serotonin produces a slow modulatory synaptic potential mediated by cAMP-PK that contributes to presynaptic facilitation and behavioural sensitization. Previously, we have found that serotonin acts on cell-attached membrane patches to produce prolonged all-or-none closures of a specific class of K+ channels (S channels) whose gating is weakly dependent on voltage and independent of intracellular calcium. We demonstrate here that in cell-free membrane patches from Aplysia sensory neurones, the purified catalytic subunit of cAMP-PK produces all-or-none closures of the S channel, simulating most (but not all) aspects of the action of serotonin on cell-attached patches. This result suggests that protein kinase acts on the internal surface of the membrane to phosphorylate either the channel itself or a membrane-associated protein that regulates channel activity.