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J. Biol. Chem., Vol. 276, Issue 21, 18529-18539, May 25, 2001

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Characterization of Heteromultimeric G Protein-coupled Inwardly Rectifying Potassium Channels of the Tunicate Tadpole with a Unique Pore Property^*

Yoshimichi Murata, Haruo Okado^§, and Yoshihiro Kubo^¶

From the  Department of Physiology, Tokyo Medical and Dental University, Graduate School and Faculty of Medicine, Bunkyo, Tokyo 113-8519, Japan and the Departments of ^§ Neurobiology and ^¶ Neurophysiology, Tokyo Metropolitan Institute for Neuroscience, Fuchu, Tokyo 183-8526, Japan

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 ~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 ₁/₂ subunits (G) in Xenopus oocytes, an inwardly rectifying K⁺ current was expressed. In contrast, coinjection of TuGIRK-B with G did not express any current. When both TuGIRK-A and -B were coexpressed together with G, 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 Ba²⁺ 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 Lys¹⁶¹ to Thr in TuGIRK-B, TuGIRK-A/B channels acquired a higher sensitivity to the Ba²⁺ 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 Lys¹⁶¹ at the external mouth of the pore of the TuGIRK-B subunit.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) AB050440 (TuGIRK-A) and AB050441 (TuGIRK-B).

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