Coupling of voltage-dependent potassium channel inactivation and oxidoreductase active site of Kvβ subunits

The accessory beta subunits of voltage-dependent potassium (Kv) channels form tetramers arranged with 4-fold rotational symmetry like the membrane-integral and pore-forming alpha subunits (Gulbis, J. M., Mann, S., and MacKinnon, R. (1999) Cell. 90, 943-952). The crystal structure of the Kv beta2 subunit shows that Kv beta subunits are oxidoreductase enzymes containing an active site composed of conserved catalytic residues, a nicotinamide (NADPH)-cofactor, and a substrate binding site. Also, Kv beta subunits with an N-terminal inactivating do main like Kv beta1.1 (Rettig, J., Heinemann, S. H., Wunder, F., Lorra, C., Parcej, D. N., Dolly, O., and Pongs, O. (1994) Nature 369, 289-294) and Kv beta3.1 (Heinemann, S. H., Rettig, J., Graack, H. R,, and Pongs, O. (1996) J. Physiol. (Lond.) 493, 625-633) confer rapid N-type inactivation to otherwise non-inactivating channels. Here we show by a combination of structural modeling and electrophysiological characterization of structure-based mutations that changes in Kv beta oxidoreductase activity may markedly influence the gating mode of Kv channels. Amino acid substitutions of the putative catalytic residues in the Kv beta1.1 oxidoreductase active site attenuate the inactivating activity of Kv beta1.1 in Xenopus oocytes. Conversely, mutating the substrate binding domain and/or the cofactor binding domain rescues the failure of Kv beta3.1 to confer rapid inactivation to Kv1.5 channels in Xenopus oocytes. We propose that Kv beta oxidoreductase activity couples Kv channel inactivation to cellular redox regulation.