NMR structure of the ball-and-chain domain of KCNMB2, the β2-subunit of large conductance Ca2+ and voltage-activated potassium channels

The auxiliary beta -subunit KCNMB2 (beta (2)) endows the noninactivating 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, BK beta N-2) using NMR spectroscopy and patch clamp recordings. BK beta N-2 completely inactivated BK channels when applied to the cytoplasmic side; its interaction with the BK alpha -subunit is characterized by a particularly slow dissociation rate and an affinity in the upper nanomolar range. The BK beta N-2 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 BK2N-mediated inactivation.