Characterization of heteromultimeric G protein-coupled inwardly rectifying potassium channels of the tunicate tadpole with a unique pore property.
Two cDNAs that encode the G protein-coupled inwardly rectifying K(+) channel (GIRK, Kir3) of tunicate tadpoles (tunicate G protein-coupled inwardly rectifying K(+) channel-A and -B; TuGIRK-A and -B) have been isolated. The deduced amino acid sequences showed approximately 60% identity with the mammalian Kir3 family. Detected by whole mount in situ hybridization, both TuGIRK-A and -B were expressed similarly in the neural cells of the head and neck region from the tail bud stage to the young tadpole stage. By co-injecting cRNAs of TuGIRK-A and G protein beta(1)/gamma(2) subunits (Gbetagamma) in Xenopus oocytes, an inwardly rectifying K(+) current was expressed. In contrast, coinjection of TuGIRK-B with Gbetagamma did not express any current. When both TuGIRK-A and -B were coexpressed together with Gbetagamma, an inwardly rectifying K(+) current was also detected. The properties of this current clearly differed from those of TuGIRK-A current, since it displayed a characteristic decline of the macroscopic conductance at strongly hyperpolarized potentials. TuGIRK-A/B current also differed from TuGIRK-A current in terms of the lower sensitivity to the Ba(2+) block, the higher sensitivity to the Cs(+) block, and the smaller single channel conductance. Taken together, we concluded that TuGIRK-A and -B form functional heteromultimeric G protein-coupled inwardly rectifying K(+) channels in the neural cells of the tunicate tadpole. By introducing a mutation of Lys(161) to Thr in TuGIRK-B, TuGIRK-A/B channels acquired a higher sensitivity to the Ba(2+) block and a slightly lower sensitivity to the Cs(+) block, and the decrease in the macroscopic conductance at hyperpolarized potentials was no longer observed. Thus, the differences in the electrophysiological properties between TuGIRK-A and TuGIRK-A/B channels were shown to be, at least partly, due to the presence of Lys(161) at the external mouth of the pore of the TuGIRK-B subunit.
Department of Physiology, Tokyo Medical and Dental University, Graduate School and Faculty of Medicine, Bunkyo, Tokyo 113-8519, Japan.
Y
Y
Murata
H
H
Okado
Y
Y
Kubo
D000595:Amino Acid Sequence; D000818:Animals; D018076:DNA, Complementary / Q000032:analysis / Q000235:genetics; D004594:Electrophysiology; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D007814:Larva; D008969:Molecular Sequence Data; D015221:Potassium Channels / Q000032:analysis / Q000502:physiology*; D024661:Potassium Channels, Inwardly Rectifying
D018076:DNA, Complementary; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying