The cardiac inward rectifying K+ channel, CIR, and the strongly inward rectifying K+ channel, IRK1, exhibited clearly different electrophysiological properties. CIR formed a heteromultimer with the G-protein coupled inward rectifying K+ channel, GIRK1, whereas IRK1 did not, and CTR homo- and heteromultimeric channels were activated by G-protein beta 1 gamma 2 subunits (G beta 1 gamma 2), whereas IRK1 channels were not. To identify the domains of CIR involved in the heteromultimer formation with GIRK1 and in the G beta 1 gamma 2 gating, we constructed chimeras of CIR and IRK1 and examined their electrophysiological properties. The channels were divided into three domains; the N-terminal cytoplasmic domain, the C-terminal cytoplasmic domain and the residual core domain. By the analysis, it was concluded that (i) the core region of CIR, but not the N and C cytoplasmic domains, is critical for the heteromultimer formation with GIRK1; and (ii) the N and C terminal cytoplasmic regions of CIR are sufficient for the G beta 1 gamma 2 gating. We also showed that the N terminal cytoplasmic region is involved in the determination of the rate of activation upon hyperpolarization.