We analysed the kinetic properties of the fast inactivating T-type calcium channel α1G in HEK 293 cells transfected with different α1G chimeras, containing the N-terminus, III-IV linker or various C-terminal regions of the slowly inactivating L-type α1C.A highly negatively charged region of 23 amino acids at the amino side of the intracellular carboxy terminus of α1G was found to be critical for fast inactivation.The N-terminus of α1G does not seem to be necessary for inactivation of the T-type calcium channel because replacement of the α1G N-terminus with the α1C N-terminus did not influence channel kinetics at all.Replacing the III-IV linker of α1G with that of α1C decreased the rate of inactivation at -20 mV from 15.8 ± 1.8 to 8.5 ± 1.1 ms, and shifted the potential for half-maximal inactivation from -69.6 ± 0.8 to -54.0 ± 1.7 mV. However, these parameters were not significantly different at other potentials.We suggest a putative ‘ball-and-chain’-like mechanism for inactivation in which the negative charges function as an acceptor domain for a ball, hypothetically located at a different intracellular part of the channel. In addition, transferring the IQ motif and EF hand of α1C to α1G does not confer Ca2+-dependent inactivation on α1G, suggesting that other sequences besides the C-terminus are needed for Ca2+-dependent inactivation of α1C.
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