A MUTATION IN THE PORE REGION OF HERG K-DEPENDENCE OF INACTIVATION( CHANNELS EXPRESSED IN XENOPUS OOCYTES REDUCES RECTIFICATION BY SHIFTINGTHE VOLTAGE)

1. The effects of a mutation in the human ether-a-go-go-related gene ( HERG) (Ser631 to Ala, S631A) on the voltage- and extracellular [K+] de pendence of inactivation were studied in Xenopus oocytes using two mic roelectrode and single channel voltage-clamp techniques. 2. The voltag e required for half-inactivation of S631A HERG was 102 mV more positiv e than for wild-type (WT)-HERG, resulting in reduced rectification of the steady-state current-voltage relationship. In contrast, the voltag e dependence of channel activation was not altered by the S631A mutati on. These findings indicate that inactivation of HERG channels is not linked to activation. 3. Rectification of whole-cell S631A HERG curren t was caused by a voltage-dependent reduction in open probability and inward rectification of the current-voltage relationship of single cha nnels. 4. Elevation of extracellular [K+] from 2 to 20 mar shifted the half-point for inactivation by +20 mV for WT-HERG, and +25 mV for S63 1A HERG. Thus, elevated [K+](0) and the S631A. mutation affect HERG in activation by different mechanisms. 5. The S631A mutation altered the ion translocation rate of HERG channels. The single channel conductanc e (gamma) of S631A HERG was 20 pS between -40 and -100 mV, and 6.0 pS between +40 and +100 mV (120 mM extracellular K+). This compares to a gamma of 12.1 and 5.1 pS for WT-HERG channels under the same condition s.