Refinement of the Binding Site and Mode of Action of the Anticonvulsant Retigabine on KCNQ K+ Channels

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Abstract
The discovery of retigabine has provided access to alternative anticonvulsant compounds with a novel mode of action. Acting as potassium channel opener, retigabine exclusively activates neuronal KCNQ-type K+ channels, mainly by shifting the voltage-dependence of channel activation to hyperpolarizing potentials. So far, only parts of the retigabine-binding site have been described, including Trp-265 and Gly-340 (according to KCNQ3 numbering) within transmembrane segments S5 and S6, respectively. Using a refined chimeric strategy, we additionally identified a Leu-314 within the pore region of KCNQ3 as crucial for the retigabine effect. Both Trp-265 and Leu-314 are likely to interact with the retigabine molecule, representing the upper and lower margins of the putative binding site. Guided by a structural model of KCNQ3, which was constructed based on the Kv1.2 crystal structure, further residues affecting retigabine-binding could be proposed and were experimentally verified as mediators for the action of the compound. These results strongly suggest that, besides Trp-265 and Leu-314, it is highly likely that another S5 residue, Leu-272, which is conserved in all KCNQ subunits, contributes to the binding site in KCNQ3. More importantly, Leu-338, extending from S6 of the neighboring subunit is also apparently involved in lining the hydrophobic binding pocket for the drug. This pocket, which is formed at the interface of two adjacent subunits, may be present only in the open state of the channel, consistent with the idea that retigabine stabilizes an open-channel conformation.