Complex subunit assembly of neuronal voltage-gated K+ channels - Basis for high-affinity toxin interactions and pharmacology

Neurons require specific patterns of K+ channel subunit expression as well as the precise coassembly of channel subunits into heterotetrameric structures for proper integration and transmission of electrical signals, In vivo subunit coassembly was investigated by studying the pharmacological profile, distribution, and subunit composition of voltage-gated Shaker family K+ (K(v)1) channels in rat cerebellum that are labeled by I-125-margatoxin (I-125-MgTX; K-d, 0.08 pm), High-resolution receptor autoradiography showed spatial receptor expression mainly in basket cell terminals (52% of all cerebellar sites) and the molecular layer (39% of sites), Sequence directed antibodies indicated overlapping expression of K(v)1.1 and K(v)1.2 in basket cell terminals, whereas the molecular layer expressed K(v)1.1, K(v)1.2, K(v)1.3, and K(v)1.6 proteins, Immunoprecipitation experiments revealed that all I-125-MgTX receptors contain at least one K(v)1.2 subunit and that 83% of these receptors are heterotetramers of K(v)1.1 and K(v)1.2 subunits, Moreover, 33% of these K(v)1.1/K(v)1.2-containing receptors possess either an additional K(v)1.3 or K(v)1.6 subunit, Only a minority of the I-125-MgTX receptors (<20%) seem to be homotetrameric K(v)1.2 channels, Heterologous coexpression of K(v)1.1 and K(v)1.2 subunits in COS-1 cells leads to the formation of a complex that combines the pharmacological profile of both parent subunits, reconstituting the native MgTX receptor phenotype, Subunit assembly provides the structural basis for toxin binding pharmacology and can lead to the association of as many as three distinct channel subunits to form functional K+ channels in vivo.