A Common Ankyrin-G-Based Mechanism Retains KCNQ and NaVChannels at Electrically Active Domains of the Axon

Abstract

KCNQ (K_V7) potassium channels underlie subthreshold M-currents that stabilize the neuronal resting potential and prevent repetitive firing of action potentials. Here, antibodies against four different KCNQ2 and KCNQ3 polypeptide epitopes show these subunits concentrated at the axonal initial segment (AIS) and node of Ranvier. AIS concentration of KCNQ2 and KCNQ3, like that of voltage-gated sodium (Na_V) channels, is abolished in ankyrin-G knock-out mice. A short motif, common to KCNQ2 and KCNQ3, mediates bothin vivoankyrin-G interaction and retention of the subunits at the AIS. This KCNQ2/KCNQ3 motif is nearly identical to the sequence on Na_Vα subunits that serves these functions. All identified Na_Vand KCNQ genes of worms, insects, and molluscs lack the ankyrin-G binding motif. In contrast, vertebrate orthologs of Na_Vα subunits, KCNQ2, and KCNQ3 (including from bony fish, birds, and mammals) all possess the motif. Thus, concerted ankyrin-G interaction with KCNQ and Na_Vchannels appears to have arisen through convergent molecular evolution, after the division between invertebrate and vertebrate lineages, but before the appearance of the last common jawed vertebrate ancestor. This includes the historical period when myelin also evolved.