Chimeras of Kir6.1 and Kir6.2 reveal structural elements involved in spontaneous opening and unitary conductance of the ATP-sensitive K+ channels

The ATP-sensitive K+ (K-ATP) channels, e.g, in heart and pancreatic p-cells , open spontaneously in the absence of intracellular ATP (ATP(i)). Their un itary conductance is similar to 80 pS with 150 mM extracellular K+-. These features are shared by the K+ channels composed of various sulfonylurea rec eptors (SURs) and Kir6.2, whereas SUR/Kir6.1 channels have a smaller conduc tance (similar to 35 pS) and do not open spontaneously in the absence of AT P(i). To identify the structural elements in Kir6.0 subunits which determin e these properties, we analyzed the properties of functional K+ channels co mposed of SUR2A, the cardiac type SUR, and various chimeras of Kir6.1 and K ir6.2 heterologously expressed in HEK (human embryonic kidney) 293T cells. The analyses indicate that the extracellular linker domain between the two putative membrane-spanning regions is responsible for the difference in the single channel conductance between SUR2A/ Kir6.1 and SUR2A/Kir6.2 channels . The cytosolic N-terminal domain of Kir6.2 was mandatory for spontaneous c hannel opening in the absence of ATP(i), although a part of C-terminus was also involved. These results implicate specific regions of Kir6.0 in the sp ontaneous opening and the single channel conductance.