Kvβ subunit oxidoreductase activity and Kv1 potassium channel trafficking

Voltage-gated Kv1 potassium channels consist of poreforming alpha subunits and cytoplasmic Kvbeta subunits. The latter play diverse roles in modulating the gating, stability, and trafficking of Kv1 channels. The crystallographic structure of the Kvbeta2 subunit revealed surprising structural homology with aldo-keto reductases, including a triosephosphate isomerase barrel structure, conservation of key catalytic residues, and a bound NADP+ cofactor (Gulbis, J. M., Mann, S., and MacKinnon, R. (1999) Cell 90, 943-952). Each Kv1-associated Kvbeta subunit (Kvbeta1.1, Kvbeta1.2, Kvbeta2, and Kvbeta2) shares striking amino acid conservation in key catalytic and cofactor binding residues. Here, by a combination of structural modeling and biochemical and cell biological analyses of structure-based mutations, we investigate the potential role for putative Kvbeta subunit enzymatic activity in the trafficking of Kv1 channels. We found that all Kvbeta subunits promote cell surface expression of coexpressed Kv1.2 alpha subunits in transfected COS-1 cells. Kvbeta1.1 and Kvbeta2 point mutants lacking a key catalytic tyrosine residue found in the active site of all aldo-keto reductases have wild-type trafficking characteristics. However, mutations in residues within the NADP(+) binding pocket eliminated effects on Kv1.2 trafficking. In cultured hippocampal neurons, Kvbeta2 subunit coexpression led to axonal targeting of Kv1.2, recapitulating the Kv1.2 localization observed in many brain neurons. Similar to the trafficking results in COS-1 cells, mutations within the cofactor binding pocket reduced axonal targeting of Kv1.2, whereas those in the catalytic tyrosine did not. Together, these data suggest that NADP(+) binding and/or the integrity of the binding pocket structure, but not catalytic activity, of Kvbeta subunits is required for intracellular trafficking of Kv1 channel complexes in mammalian cells and for axonal targeting in neurons.