A unique ion channel clustering domain on the axon initial segment of mammalian neurons.

The axon initial segment (AIS) plays a key role in initiation of action potentials and neuronal output. The plasma membrane of the AIS contains high densities of voltage-gated ion channels required for these electrical events, and much recent work has focused on defining the mechanisms for generating and maintaining this unique neuronal plasma membrane domain. The Kv2.1 voltage-gated potassium channel is abundantly present in large clusters on the soma and proximal dendrites of mammalian brain neurons. Kv2.1 is also a component of the ion channel repertoire at the AIS. Here we show that Kv2.1 clusters on the AIS of brain neurons across diverse mammalian species including humans define a noncanonical ion channel clustering domain deficient in Ankyrin-G. The sites of Kv2.1 clustering on the AIS are sites where cisternal organelles, specialized intracellular calcium release membranes, come into close apposition with the plasma membrane, and are also sites of clustering of γ-aminobutyric acid (GABA)ergic synapses. Using an antibody specific for a single Kv2.1 phosphorylation site, we find that the phosphorylation state differs between Kv2.1 clusters on the proximal and distal portions of the AIS. Together, these studies show that the sites of Kv2.1 clustering on the AIS represent specialized domains containing components of diverse neuronal signaling pathways that may contribute to local regulation of Kv2.1 function and AIS membrane excitability.