Abstract
The human ether-à-go-go related gene product (HERG) channel is essential for electrical activity of heart cells, and block of this channel by many drugs leads to lethal arrhythmias. Tyr652 and Phe656 of the sixth transmembrane helix are candidates for the drug binding site. In the tetrameric HERG channel, a drug with asymmetric structure should interact unevenly with multiple residues from different subunits. To elucidate the topology of the drug-binding site, we constructed tandem dimers of HERG channels and the aromatic Tyr652 and Phe656 residues were replaced by alanine singly or doubly. Eight types of HERG channels, including homotetrameric mutants, having different numbers and arrangements of aromatic residues at the blocking site, were studied. Effects of cisapride on channels expressed in Xenopus laevis oocytes were examined electrophysiologically. The inhibition constants (Ki) were increased significantly as the diagonal Tyr652 were deleted, whereas those for the diagonal Phe656-deleted mutant were not changed. These results suggest that Tyr652 residues from adjacent subunits contributed to the binding. Two types of double mutants of tandem dimers showed significantly distinct affinities, suggesting that the coexistence of Tyr652 and Phe656 on a subunit in diagonal position is crucial to having a high affinity. Thermodynamic double-mutant cycle analyses revealed interactions between Tyr652 and Phe656 upon binding. The kinetics and voltage-dependence of blocking suggested transitions of the binding site from low to high affinity. These approaches using a set of mutant HERG channels gave a dynamic picture of the spatial arrangements of residues that contribute to the drug-channel interaction.
Footnotes
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This work is supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan.
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ABBREVIATIONS: HERG, human ether-à-go-go related gene product; V½, the half activation voltage; Ki, the blocking inhibition constant.
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The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
- Received September 25, 2007.
- Accepted March 7, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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