The amino side of the C-terminus determines fast inactivation of the T-type calcium channel α1G

1. We analysed the kinetic properties of the fast inactivating T-type calcium channel alpha (1G) in HEK 293 cells transfected with different alpha (1G) chimeras, containing the N-terminus, III-IV linker or various C-terminal regions of the slowly inactivating L-type alpha (1C). 2. A highly negatively charged region of 23 amino acids at the amino side of the intracellular carboxy terminus of alpha (1G) was found to be critical for fast inactivation. 3. The N-terminus of alpha (1G) does not seem to be necessary for inactivation of the T-type calcium channel because replacement of the alpha (1G) N-terminus with the alpha (1C) N-terminus did not influence channel kinetics at all. 4. Replacing the III-IV linker of alpha (1G) with that of alpha (1C) 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. 5. 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 alpha (1C) to alpha (1G) does not confer Ca2+-dependent inactivation on alpha (1G), suggesting that other sequences besides the C-terminus are needed for Ca2+-dependent inactivation of alpha (1C).