| Repolarization of the cardiac action potential is mediated by the K + delayed rectifier current, IK, which is composed of two distinct currents, IKr and IKs. IKr is conducted by channels encoded by the human ether-a-go-go related gene (hERG/KCNH2), and IKs by KCNQ1 channels. Long QT syndrome (LQTS) can arise from inherited mutations in hERG and KCNQ1, or by block of cardiac IKr by a wide range of drugs largely intended for other therapeutic targets. To identify novel proteins involved in hERG/IKr channel regulation, we screened a human heart library using a yeast two-hybrid assay and discovered a direct interaction between a C-terminal hERG protein fragment and the protein Trio associated repeat on actin (Tara). Tara is ubiquitously expressed and has been shown to directly bind Trio and F-actin, and to stabilize F-actin. Using co-immunoprecipitation, we confirmed association of hERG and Tara in a heterologous system and in canine ventricle. Two-electrode voltage clamp recordings in Xenopus oocytes revealed that Tara suppressed currents expressed from WT hERG by approximately 39% but had no significant effect on C-truncated hERG lacking the Tara binding region. This decrease in current was attributed to a decrease in hERG protein expression. Recent studies suggest endogenous hERG and KCNQ1 associate in heterologous systems and heart. Co-immunoprecipitation studies confirmed association of hERG and KCNQ1 in HEK-293 cells, and in canine and human hearts. Screening of the human heart library revealed a direct association of the C-terminus of Tara with the C-terminus of KCNQ1, and overexpression of Tara increased the amount of KCNQ1 that co-immunoprecipitated with hERG approximately 3 fold.;Increasing evidence suggests endogenous hERG channels are comprised of proteins encoded by alternate transcripts of the same gene, hERG 1a and 1b. Quantitative western blots of lysates from healthy human male and female donors showed both hERG 1a and 1b isoforms are robustly expressed and that isoform expression does not differ between the sexes. |
