Kir6.1: a possible subunit of ATP-sensitive K+ channels in mitochondria.

We have investigated the subcellular localization of the inwardly rectifying K+ channel subunit Kir6.1 (uKATP-1). Immunoblot analysis of the mitochondrial fractions prepared from rat skeletal muscle and liver detected a single band of Kir6.1 at 51 kDa, the intensity of which was stronger than that found in the total homogenate of each tissue. By immunofluorescence staining, the labelling for Kir6.1 was observed as a dispersed array of fine dots throughout all the tissues examined in the rat, including skeletal muscle, cardiac muscle, liver, and pancreas. Electron-microscopic examination revealed that the punctate staining distribution was due to a specific labelling of Kir6.1 in the mitochondria. Immuno-positive colloidal gold particles were scattered over the mitochondria, suggesting that Kir6.1 was located on the inner membrane. Although gold particles were not observed at plasma membrane, a 47 kDa protein was detected in the isolated plasma membrane vesicles by immunoblot analysis against Kir6.1. These results suggest that Kir6.1 might be a subunit of the ATP-sensitive K+ channel in the mitochondrion, as well as in the plasma membrane.