Distinct contributions of different structural regions to the current kinetics of the Cav3.3 T-type Ca2+ channel

Electrophysiological characterization of T-type Ca2+ channel isoforms (Cav3.1, Cav3.2, and Cav3.3) has shown that all of the isoforms are low voltage-activated around resting membrane potential, but their current kinetics are distinctly different, with the activation and inactivation kinetics of the Cav3.1 and Cav3.2 channels being much faster than those of the Cav3.3 channel. We previously reported that multiple structural regions of the Cav3.3 T-type channel participate in determining its current kinetics. Here we have evaluated the relative contributions of individual cytoplasmic and trans-membrane regions to the current kinetics of the channel, by systematically replacing individual regions of Cav3.3 with the corresponding regions of Cav3.1. Introduction of the Cav3.1 III–IV loop into the Cav3.3 backbone accelerated both the activation and inactivation kinetics more prominently than any other intracellular loop or tail. Among the trans-membrane domains, introduction of the domain I of Cav3.1 into Cav3.3 accelerated both the activation and inactivation kinetics most effectively. These findings suggest that the current kinetics of the Cav3.3 channel are differentially controlled by several structural regions, among which the III–IV loop and domain I are the most prominent in governing both activation and inactivation kinetics.