NMR structure of the "ball-and-chain" domain of KCNMB2, the ß2-subunit of large conductance Ca2+- and voltage-activated potassium channels

Abstract

The auxiliary ß-subunit KCNMB2 (ß2) endows the non-inactivating large conductance Ca2+- and voltage-dependent potassium (BK) channel with fast inactivation. This process is mediated by the N terminus of KCNMB2 and closely resembles the "ball-and-chain"-type inactivation observed in voltage-gated potassium channels. Here we investigated the solution structure and function of the KCNMB2 N terminus (amino acids 1-45, BKß2N) using NMR spectroscopy and patch clamp recordings. BKß2N completely inactivated BK channels when applied to the cytoplasmic side; its interaction with the BK α-subunit is characterized by a particularly slow dissociation rate and an affinity in the upper nano-molar range. The BKß2N structure comprises two domains connected by a flexible linker: the pore-blocking "ball domain" (formed by residues 1-17) and the "chain domain" (between residues 20-45) linking it to the membrane segment of KCNMB2. The ball domain is made up of a flexible N terminus anchored at a well ordered loop-helix motif. The chain domain consists of a 4-turn helix with an unfolded linker at its C terminus. These structural properties explain the functional characteristics of BKß2N-mediated inactivation. © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.