Solid‐state NMR Investigation of the Structure of a Potassium Channel‐Toxin Complex

The active site of K+ channels, i.e. the conduction pore and the selectivity filter, that catalyses the transport of K+ ions across the plasma membrane, is highly conserved in structure. Similarly, scorpion toxins, that are potent inhibitors of the pore entrance way of K+ channels, exhibit conserved structures. Accordingly, mutagenesis and molecular modelling studies have proposed detailed structural models for the K+ channel/scorpion toxin binding interface assuming a lock‐and‐key type mechanism. Here, we demonstrate that high‐resolution solid‐state NMR spectroscopy is a sensitive method to analyse the structure of a membrane protein – inhibitor complex at atomic resolution. We were able to directly investigate interaction of a scorpion toxin (kaliotoxin, KTX) with a K+ channel (KcsA‐Kv1.3). In addition to predicted side‐chain side‐chain interactions between KTX and KcsA‐Kv1.3, our solid‐state NMR results showed unexpected structural rearrangements in the backbone structures of both KTX and most markedly, KcsA‐Kv1.3 pore. The structural rearrangements in the active site of KcsA‐Kv1.3 indicated an induced fit mechanism accommodating KTX in the pore entrance way of the K+ channel. We propose that structural flexibility in both K+ channel pore entrance way and in KTX backbone fold represents an important determinant for the high specificity of toxin − K+ channel interactions.