Negative Gating Modulation by (R)-N-(Benzimidazol-2-yl)-1234-tetrahydro-1-naphthylamine (NS8593) Depends on Residues in the Inner Pore Vestibule: Pharmacological Evidence of Deep-Pore Gating of KCa2 Channels

摘要

Acting as a negative gating modulator, (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphthylamine (NS8593) shifts the apparent Ca²⁺-dependence of the small-conductance Ca²⁺-activated K⁺ channels KCa2.1–2.3 to higher Ca²⁺ concentrations. Similar to the positive KCa channel-gating modulators 1-ethyl-2-benzimidazolinone (1-EBIO) and cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methylpyrimidin-4-yl]-amine (CyPPA), the binding site for NS8593 has been assumed to be located in the C-terminal region, in which these channels interact with their Ca²⁺ sensor calmodulin. However, by using a progressive chimeric approach, we were able to localize the site-of-action of NS8593 to the KCa2 pore. For example, when we transferred the C terminus from the NS8593-insensitive intermediate-conductance KCa3.1 channel to KCa2.3, the chimeric channel remained as sensitive to NS8593 as wild-type KCa2.3. In contrast, when we transferred the KCa2.3 pore to KCa3.1, the channel became sensitive to NS8593. Using site-directed mutagenesis, we subsequently identified two specific residues in the inner vestibule of KCa2.3 (Ser507 and Ala532) that determined the effect of NS8593. Mutation of these residues to the corresponding residues in KCa3.1 (Thr250 and Val275) made KCa2.3 insensitive to NS8593, whereas introduction of serine and alanine into KCa3.1 was sufficient to render this channel highly sensitive to NS8593. It is noteworthy that the same two residue positions have been found previously to mediate sensitivity of KCa3.1 to clotrimazole and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34). The location of Ser507 in the pore-loop near the selectivity filter and Ala532 in an adjacent position in S6 are within the region predicted to contain the KCa2 channel gate. Hence, we propose that NS8593-mediated gating modulation occurs via interaction with gating structures at a position deep within the inner pore vestibule.