ALTERATION OF CHANNEL CHARACTERISTICS BY EXCHANGE OF PORE-FORMING REGIONS BETWEEN 2 STRUCTURALLY RELATED CA2+ CHANNELS

Several types of structurally homologous high voltage-gated Ca2(+) cha nnels (L-, P- and N-type) have been identified via biochemical, pharma cological and electrophysiological techniques. Among these channels, t he cardiac L-type and the brain BI-2 Ca2(+) channel display significan tly different biophysical properties. The BI-2 channel exhibits more r apid voltage-dependent current activation and inactivation and smaller single-channel conductance compared to the L-type Ca2(+) channel. To examine the molecular basis for the functional differences between the two structurally related Ca2(+) channels, we measured macroscopic and single-channel currents from oocytes injected with wild-type and vari ous chimeric channel alpha(1) subunit cRNAs. The results show that a c himeric channel in which the segment between S5-SS2 in repeat IV of th e cardiac L-type Ca2(+) channel, was replaced by the corresponding reg ion of the BI-2 channel, exhibited macroscopic current activation and inactivation time-courses and single-channel conductance, characterist ic of the BI-2 Ca2(+) channel. The voltage-dependence of steady-state inactivation was not affected by the replacement. Chimeras, in which t he SS2-S6 segment in repeat III or IV of the cardiac channel was repla ced by the corresponding BI-2 sequence, exhibited altered macroscopic current kinetics without changes in single-channel conductance. These results suggest that part of the S5-SS2 segment plays a critical role in determining voltage-dependent current activation and inactivation a nd single-channel conductance and that the SS2-S6 segment may control voltage-dependent kinetics of the Ca2(+) channel.