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Volume 271, Number 26, Issue of June 28, 1996 pp. 15729-15735
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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A Weakly Inward Rectifying Potassium Channel of the Salmon Brain
GLUTAMATE 179 IN THE SECOND TRANSMEMBRANE DOMAIN IS INSUFFICIENT FOR STRONG RECTIFICATION

(Received for publication, December 26, 1995, and in revised form, February 23, 1996)

Yoshihiro Kubo , Tomoyuki Miyashita and Kaoru Kubokawa ^¶

From the  Department of Neurophysiology, Tokyo Metropolitan Institute for Neuroscience, Musashidai 2-6, Fuchu, Tokyo 183, Japan and the ^¶ Department of Molecular Biology, Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano, Tokyo 164, Japan

A cDNA encoding for a weakly inward rectifying K⁺ channel (sWIRK: salmon weakly inward rectifying K⁺ channel) was isolated from the masu salmon brain by expression cloning. The sWIRK channel exhibited the highest similarity with members of the ROMK1 subfamily, BIR10/K_AB-2 (70% amino acid identity) and ROMK1 (46%). An ATP binding motif which is characteristic of this subfamily was also conserved. The sWIRK RNA was detected in the brain, but not in the heart, kidney, skeletal muscle, liver, testis, and ovary. In the brain, the expression was observed in the ependymoglial cells on the surface of the ventricles as well as in the small perineuronal glia-like cells in the midbrain and the medulla. When compared with the strong inward rectifier IRK1 channel, the sWIRK channel showed a much weaker inward rectification property, and the activation kinetics upon hyperpolarization was slower and less voltage-dependent. The slope conductance of the single channel inward current was 37 pS (140 mM K⁺_o), and outward current channel events were also observed. The weak rectification of sWIRK is significant in that it has a negatively charged residue (glutamate) in the M2 region which is reported to cause strong inward rectification. By introducing a point mutation to remove this negative charge (glutamine), the sWIRK E179Q mutant channel lost its inward rectification property completely, and the single channel property (45 pS; 140 mM K⁺_o) was ohmic up to highly depolarized potential, even in the presence of the physiological cytoplasmic blockers such as Mg²⁺ and polyamines.

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