The subunit composition of the G protein-gated inwardly rectifying K+ (KG) channel protein extracted from mouse forebrain membranes was examined. A polyclonal antibody (anti-GIRK1C1) was prepared against the carboxyl terminal region of GIRK1, the major subunit of the KG channel. The anti-GIRK1C1 IgG detected a single protein at approximately 65 kDa in SDS-PAGe of brain membranes. This IgG immunoprecipitated a macroprotein complex composed of GIRK1 and several associated proteins whose molecular weights ranging from 50 to 62 kDa on SDS-polyacrylamide gel. Upon size fractionation by both sucrose density gradient centrifugation and gel filtration, the solubilized KG channel proteins migrated into a single peak, which suggests that the component subunits form a single macromolecule. On the basis of the values of size fractionation, the molecular mass of the KG channel macromolecule could be estimated at approximately 231,000. These results suggest that the KG channel is most likely a tetrameric macroprotein composed of GIRK1 and co-immunoprecipitated proteins in the forebrain.