Dynamics of Kv1 Channel Transport in Axons

Concerted actions of various ion channels that are precisely targeted along axons are crucial for action potential initiation and propagation, and neurotransmitter release. However, the dynamics of channel protein transport in axons remain unknown. Here, using time-lapse imaging, we found fluorescently tagged Kv1.2 voltage-gated K(+) channels (YFP-Kv1.2) moved bi-directionally in discrete puncta along hippocampal axons. Expressing Kv beta 2, a Kv1 accessory subunit, markedly increased the velocity, the travel distance, and the percentage of moving time of these puncta in both anterograde and retrograde directions. Suppressing the Kv beta 2-associated protein, plus-end binding protein EB1 or kinesin II/KIF3A, by siRNA, significantly decreased the velocity of YFP-Kv1.2 moving puncta in both directions. Kv beta 2 mutants with disrupted either Kv1.2-Kv beta 2 binding or Kv beta 2-EB1 binding failed to increase the velocity of YFP-Kv1.2 puncta, confirming a central role of Kv beta 2. Furthermore, fluorescently tagged Kv1.2 and Kv beta 2 co-moved along axons. Surprisingly, when co-moving with Kv1.2 and Kv beta 2, EB1 appeared to travel markedly faster than its plus-end tracking. Finally, using fission yeast S. pombe expressing YFP-fusion proteins as reference standards to calibrate our microscope, we estimated the numbers of YFP-Kv1.2 tetramers in axonal puncta. Taken together, our results suggest that proper amounts of Kv1 channels and their associated proteins are required for efficient transport of Kv1 channel proteins along axons.