Alteration of channel characteristics by exchange of pore-forming regions between two structurally related Ca²⁺ Channels

Several types of structurally homologous high voltage-gated Ca²⁺ channels (L-, P-and N-type) have been identified via biochemical, pharmacological and electrophysiological techniques. Among these channels, the cardiac L-type and the brain BI-2 Ca²⁺ channel display significantly different biophysical properties. The BI-2 channel exhibits more rapid 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 various chimeric channel α₁ subunit cRNAs. The results show that a chimeric channel in which the segment between S5-SS2 in repeat IV of the cardiac L-type Ca²⁺ channel, was replaced by the corresponding region of the BI-2 channel, exhibited macroscopic current activation and inactivation time-courses and single-channel conductance, characteristic of the BI-2 Ca²⁺ channel. The voltage-dependence of steady-state inactivation was not affected by the replacement. Chimeras, in which the SS2-S6 segment in repeat III or IV of the cardiac channel was replaced 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 and single-channel conductance and that the SS2-S6 segment may control voltage-dependent kinetics of the Ca²⁺ channel.