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 conventional methods, 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 Ca 2+ 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 Ca 2+ concentration. Our results demonstrate that the ID fragment, itself a target for drugs modulating SK channel activities, plays a unique role in coupling Ca 2+ sensing by CaM and mechanical opening of SK channels.calcium | signaling | gating M ost proteins typically adopt one or more well-defined 3D structures, or domains, to carry out their specific functions. Voltage-gated potassium channels are such an example, with six transmembrane segments (S1-S6) forming the voltage-sensing domain and the pore domain (1, 2). Conformational changes of these domains result in opening of the channel pore. Unlike the well-structured proteins, there are proteins or fragments of proteins that lack well-defined 3D conformations (3-8). These proteins, termed "intrinsically disordered" (ID) proteins/peptides, typically are flexible in their secondary structures and often can adopt multiple conformations. Consequently, the structures of ID proteins are difficult to determine by conventional methods, such as X-ray crystallography, and it is difficult to explore the structural features responsible for functions by ID proteins. Nevertheless, emerging evidence shows that ID proteins/peptides are equally important for the protein functions.Small-and intermediate-conductance Ca 2+