NMR structure of the "ball-and-chain" domain of KCNMB2, the beta(2)-subunit of large conductance Ca2+ and voltage-activated potassium channels

The auxiliary beta -subunit KCNMB2 (beta (2)) endows the noninactivating la rge conductance Ca2+ and voltage-dependent potassium (BK) channel with fast inactivation. This process is mediated by the N terminus of KCNMB2 and clo sely resembles the "ball-and-chain"-type inactivation observed in voltage-g ated potassium channels. Here we investigated the solution structure and fu nction of the KCNMB2 N terminus (amino acids 1-45, BK beta N-2) using NMR s pectroscopy 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 co mprises 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 ma de 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 characteris tics of BK2N-mediated inactivation.