A molecular link between activation and inactivation of sodium channels.

Abstract

A pair of tyrosine residues, located on the cytoplasmic linker be- tween the third and fourth domains of human heart sodium channels, plays a crit- ical role in the kinetics and voltage dependence of inactivation. Substitution of these residues by glutamine (y1494ya495/QQ), but not phenylalanine, nearly elimi- nates the voltage dependence of the inactivation time constant measured from the decay of macroscopic current after a depolarization. The voltage dependence of steady state inactivation and recovery from inactivation is also decreased in YY/ QQ channels. A characteristic feature of the coupling between activation and in- activation in sodium channels is a delay in development of inactivation after a de- polarization. Such a delay is seen in wild-type but is abbreviated in YY/QQ chan- nels at -30 mV. The macroscopic kinetics of activation are faster and less voltage dependent in the mutant at voltages more negative than -20 inV. Deactivation ki- netics, by contrast, are not significantly different between mutant and wild-type channels at voltages more negative than -70 inV. Single-channel measurements show that the latencies for a channel to open after a depolarization are shorter and less voltage dependent in YY/QQ than in wild-type channels; however the peak open probability is not significantly affected in YY/QQ channels. These data demonstrate that rate constants involved in both activation and inactivation are altered in YY/QQ channels. These tyrosines are required for a normal coupling between activation voltage sensors and the inactivation gate. This coupling in- sures that the macroscopic inactivation rate is slow at negative voltages and accel- erated at more positive voltages. Disruption of the coupling in YY/QQ alters the microscopic rates of both activation and inactivation.