Ca
2؉/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the regulation of neuronal excitability in many systems. Recent studies suggest that local regulation of membrane potential can have important computational consequences for neuronal function. In Drosophila, CaMKII regulates the eag potassium channel, but if and how this regulation was spatially restricted was unknown. Calcium/calmodulin-dependent protein kinase II (CaMKII) 1 has been shown to regulate ion channels and neuronal excitability in both vertebrates (1-6) and invertebrates (4,5,(7)(8)(9)(10)(11). Regulation of excitability has been proposed as a mechanism by which neurons can globally modify their firing to keep spike rates in a scalable range in the presence of synapse-specific plasticity (12)(13)(14). Recently, it has become clear that regulation of excitability can also occur as a local phenomenon (15-19). Regional changes in excitability mediated by differing levels of A-type potassium currents were shown to be important for gating Hebbian plasticity in CA1 pyramidal cell dendrites and limiting propagation of action potentials (19). These effects have important implications for the computational abilities of neurons that integrate over many inputs. Both slow, expression level changes and fast, enzymatic modifications could underlie local phenomena, and in CA1, local regulation of excitability was associated with both regional differences in current density (19) and region-specific post-translational modifications of ion channels (18). Fast modulation offers the advantage of dynamic retooling of the computational capability of the neuron.For signal transduction pathways to effect fast local changes in excitability, the activity of effector enzymes has to be spatially restricted. The ability of protein kinases with many substrates to act with specificity to regulate cellular functions is increasingly being ascribed to interactions with scaffolding molecules that bring kinases and their substrates into close proximity. Signaling platforms range from small complexes containing only a few proteins to very complex cellular specializations like the postsynaptic density of the mammalian central nervous system (20). The advantages of scaffolding enzymes and substrates include enhancement of reaction rates because of elimination of diffusional barriers and increased local concentration of substrates and segregation of the enzyme to prevent reaction with other substrates. Localization of effector enzymes to sources of small molecule regulators can also establish very restricted signaling domains. Scaffolding may be particularly important for regulation of membrane-bound proteins in neurons; diffusion in the membrane bilayer is relatively slow, and the ability of soluble enzymes to access substrates can be influenced by the complex geometry of neurons.The regulation of firing in Drosophila motor axons involves the ether-à -go-go (eag) potassium channel. eag is the founding member of an evolutionarily conserved superfamily of voltageg...