Regulation of Kv4.3 Closed-State Inactivation and Recovery by Extracellular Potassium and Intracellular KChIP2b

Abstract

Mechanisms underlying Kv4 channel inactivation and recovery are presently unclear, although there is general consensus that the basic characteristics of these processes are not consistent with Shaker (Kv1) N- and P/C-type mechanisms. Kv4 channels also differ from Shaker in that they can undergo significant inactivation from pre-activated closed-states (closed-state inactivation, CSI), and that inactivation and recovery kinetics can be regulated by intracellular KChIP2 isoforms. To gain insight into the mechanisms regulating Kv4.3 CSI and recovery, we have analyzed the effects of increasing [K+]o from 2 mM to 98mM in the absence and in the presence of KChIP2b, the major KChIP2 isoform expressed in the mammalian ventricle. In the absence of KChIP2b, high [K+]o promoted Kv4.3 inactivated closed-states and significantly slowed the kinetics of recovery from both macroscopic and closed-state inactivation. Coexpression of KChIP2b in 2 mM [K+]o promoted non-inactivated closed-states and accelerated the kinetics of recovery from both macroscopic and CSI. In high [K+]o, KChIP2b eliminated or significantly reduced the slowing effects on recovery. Attenuation of CSI by the S4 charge-deletion mutant R302A, which produced significant stabilization of non-inactivated closed-states, effectively eliminated the opposing effects of high [K+]o and KChiP2b on macroscopic recovery kinetics, confirming that these results were due to alterations of CSI. Elevated [K+]o therefore slows Kv4.3 recovery by stabilizing inactivated closed-states, while KChIP2b accelerates recovery by destabilizing inactivated closed-states. Our results challenge underlying assumptions of presently popular Kv4 gating models and suggest that Kv4.3 possesses novel allosteric mechanisms, which are absent in Shaker, for coupling interactions between intracellular KChIP2b binding motifs and extracellular K+-sensitive regulatory sites.