Ingredients for Neuronal Coincidence Detection:
a Calcium/Protein Kinase/Ion Channel Souffle

A central theme common to current models of neural plasticity is that protein phosphorylation/ dephosphorylation of one or more synaptic proteins leads to a use-dependent alteration in the electrical properties of neurons. Although the identity of the molecular species mediating such plasticity have not been identified, K_Ca channels represent a likely target due to their predominantly presynaptic location, and their ability to act as feedback regulators of the voltage-activated Ca²⁺ channels involved in NT release. These properties endow presynaptic K_Ca channels with the ability to regulate the duration of the presynaptic action potential, and hence, indirectly modulate the presynaptic Ca²⁺ concentration. Such regulation modulates the amount of neurotransmitter released from presynaptic terminals.

K_Ca channels are also expressed in cell bodies, and in postsynaptic terminals, locations known to contain a number of other Ca²⁺-permeable channels such as some types of NMDA and AMPA receptors. If K_Ca channels are positioned close to such Ca²⁺-permeable channels in the membrane then they will be activated by the entry of Ca²⁺. The resulting hyperpolarization of the surrounding membrane will result in the direct feedback inhibition of voltage-sensitive channels, and counteract depolarizations induced by Na⁺ influx across these glutamate receptors.

One molecular mechanism ensuring the exact placement of K_Ca channels with respect to other ion channels and channel modulators is through the formation of protein complexes. A number of proteins have been identified to be in close proximity to K_Ca channels. These include some subtypes of Ca²⁺ channels, and protein kinases/ phosphatases. Such findings raise the possibility that ion channels contain specific binding sites for other proteins such as ion channels, protein kinases and protein phosphatases.

To determine whether K_Ca channels can form such modulatory protein complexes, and to characterize such binding partners we used K_Ca channel fragments to screen a human brain yeast two-hybrid library. Of 171 clones identified as potentially interacting with K_Ca channels, 23 correspond to the a -subunit of calcium calmodulin kinase II (CaMKII-a ). A CaMKII/K_Ca channel interaction was further probed using biochemical assays with GST-fusion protein constructs containing C-terminal hslo fragments, and either native or recombinant CaMKII or CaMKII fragments. Functional effects of CaMKII/K_Ca protein complexes were assayed by expressing hslo in Xenopus oocytes, and recording channel activity from inside-out macropatches. We conclude that CaMKII forms a protein complex with hslo Ca²⁺-activated K⁺ channels and can modulate the activity of this ion channel.

 

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