Ca2+/Calmodulin Disrupts AKAP79/150 Interactions with KCNQ (M-Type) K+ Channels

M-type channels are localized to neuronal, cardiovascular, and epithelial tissues, where they play critical roles in control of excitability and K+ transport, and are regulated by numerous receptors via G(q/11)-mediated signals. One pathway shown for KCNQ2 and muscarinic receptors uses PKC, recruited to the channels by A-kinase anchoring protein (AKAP) 79/150. As M-type channels can be variously composed of KCNQ1-5 subunits, and M current is known to be regulated by Ca2+/calmodulin (CaM) and PIP2, we probed the generality of AKAP79/150 actions among KCNQ1-5 channels, and the influence of Ca2+/CaM and PIP2 on AKAP79/150 actions. We first examined which KCNQ subunits are targeted by AKAP79 in Chinese hamster ovary (CHO) cells heterologously expressing KCNQ1-5 subunits and AKAP79, using fluorescence resonance energy transfer (FRET) under total internal reflection fluorescence (TIRF) microscopy, and patch-clamp analysis. Donor-dequenching FRET between CFP-tagged KCNQ1-5 and YFP-tagged AKAP79 revealed association of KCNQ2-5, but not KCNQ1, with AKAP79. In parallel with these results, CHO cells stably expressing M-1 receptors studied under perforated patch-clamp showed cotransfection of AKAP79 to "sensitize" KCNQ2/3 heteromers and KCNQ2-5, but not KCNQ1, homomers to muscarinic inhibition, manifested by shifts in the dose-response relations to lower concentrations. The effect on KCNQ4 was abolished by the T553A mutation of the putative PKC phosphorylation site. We then probed the role of CaM and PIP2 in these AKAP79 actions. TIRF/FRET experiments revealed cotransfection of wild-type, but not dominant-negative (DN), CaM that cannot bind Ca2+, to disrupt the interaction of YFP-tagged AKAP79(1-153) with CFP-tagged KCNQ2-5. Tonic depletion of PIP2 by cotransfection of a PIP2 phosphatase had no effect, and sudden depletion of PIP2 did not delocalize GFP-tagged AKAP79 from the membrane. Finally, patch-clamp experiments showed cotransfection of wild-type, but not DN, CaM to prevent the AKAP79-mediated sensitization of KCNQ2/3 heteromers to muscarinic inhibition. Thus, AKAP79 acts on KCNQ2-5, but not KCNQ1-containing channels, with effects disrupted by calcified CaM, but not by PIP2 depletion.