Unstructured to structured transition of an intrinsically disordered protein peptide in coupling Ca²⁺-sensing and SK channel activation

Most proteins, such as ion channels, form well-organized 3D structures to carry out their specific functions. A typical voltagegated potassium channel subunit has six transmembrane segments (S1-S6) to form the voltage-sensing domain and the pore domain. Conformational changes of these domains result in opening of the channel pore. Intrinsically disordered (ID) proteins/peptides are considered equally important for the protein functions. However, it is difficult to explore the structural features underlying the functions of ID proteins/peptides by conventionalmethods, such as X-ray crystallography, because of the flexibility of their secondary structures. Unlike voltage-gated potassium channels, families of small- and intermediate-conductance Ca ²⁺-activated potassium(SK/IK) channels with important roles in regulating membrane excitability are activated exclusively by Ca ²⁺-bound calmodulin (CaM). Upon binding of Ca²⁺ to CaM, a 2 × 2 structure forms between CaM and the CaMbinding domain. A channel fragment that connects S6 and the CaMbinding domain is not visible in the protein crystal structure, suggesting that this fragment is an ID fragment. Here we show that the conformation of the ID fragment in SK channels becomes readily identifiable in the presence of NS309, the most potent compound that potentiates the channel activities. This well-defined conformation of the ID fragment, stabilized by NS309, increases the channel open probability at a given Ca²⁺ concentration. Our results demonstrate that the ID fragment, itself a target for drugs modulating SK channel activities, plays a unique role in coupling Ca²⁺ sensing by CaM and mechanical opening of SK channels.