An Ankyrin-G N-terminal Gate and Protein Kinase CK2 Dually Regulate Binding of Voltage-gated Sodium and KCNQ2/3 Potassium Channels

In many mammalian neurons, fidelity and robustness of action potential generation and conduction depends on the colocalization of voltage-gated sodium (Na-v) and KCNQ2/3 potassium channel conductance at the distal axon initial segment (AIS) and nodes of Ranvier in a ratio of similar to 40 to 1. Analogous anchor peptides within intracellular domains of vertebrate KCNQ2, KCNQ3, and Na-v channel alpha-subunits bind Ankyrin-G (AnkG), thereby mediating concentration of those channels at AISs and nodes of Ranvier. Here, we show that the channel anchors bind at overlapping but distinct sites near the AnkG N terminus. In pulldown assays, the rank order of AnkG binding strength is Na(v)1.2 >> KCNQ3 > KCNQ2. Phosphorylation of KCNQ2 and KCNQ3 anchor domains by protein kinase CK2 (CK2) augments binding, as previously shown for Na(v)1.2. An AnkG fragment comprising ankyrin repeats 1 through 7 (R1-7) binds phosphorylated Na-v or KCNQ anchors robustly. However, mutational analysis of R1-7 reveals differences in binding mechanisms. A smaller fragment, R1-6, exhibits much-diminished KCNQ3 binding but binds Na(v)1.2 well. Two lysine residues at the tip of repeat 2-3 beta-hairpin (residues 105-106) are critical for Na(v)1.2 but not KCNQ3 channel binding. Another dibasic motif (residues Arg-47, Arg-50) in the repeat 1 front alpha-helix is crucial for KCNQ2/3 but not Na(v)1.2 binding. AnkG's alternatively spliced N terminus selectively gates access to those sites, blocking KCNQ but not Na-v channel binding. These findings suggest that the 40:1 Na-v:KCNQ channel conductance ratio at the distal AIS and nodes arises from the relative strength of binding to AnkG.