Evidence for common structural determinants of activation and inactivation in T-type Ca2+ channels

Abstract

One of the most distinctive features of T-type Ca(2+) channels is their fast inactivation. Recent structure-function studies indicate that the rate of macroscopic inactivation of these channels is influenced by several structural components, including intracellular linkers, transmembrane segments, and pore loops. The macroscopic inactivation of T-type channels is partially coupled to activation. It is therefore possible that changes in the rate of macroscopic inactivation after alteration in the structure of these channels might actually result from changes in activation kinetics. In this study, we use kinetic simulations to illustrate how the alteration of the rate of channel activation may lead to changes in the rate of macroscopic inactivation. By examining data pooled from several structure-function studies we demonstrate that gating modifications induced by alteration in the channel structure unveils a correlation between the time constants of macroscopic inactivation and activation. This analysis underscores the relevance of considering the inactivation-activation coupling when analyzing the structural determinants of T-type channel inactivation. Furthermore, we demonstrate that slow-inactivating mutants, with modifications in the IIIS6 segment and the proximal C terminus, display significant alterations in the voltage dependencies of activation and deactivation with respect to the wild type channel Ca(V)3.1. Our results indicate that common structures, most likely the S6 transmembrane segments, are involved in the conformational changes occurring during both channel activation and inactivation.

Characterization of the Gating Brake in the I-II Loop of Cav3.2 T-type Ca2+ Channels

“…Any disruption of this structure leads to channels that open at more negative potentials, inactivate at more negative potentials, and show faster kinetics. As noted previously, inactivation of T-channels is coupled to activation (25), and this may explain the concomitant shifts in steady state activation and inactivation observed in this study. Our results indicate that the normal function of this region is as a brake that prevents channel opening after small depolarizations.…”

“…Previous measurements of T-channel gating currents indicate that 80% of the channels open after only 20% of total charge movement (8,26). This suggests that T-channels open after minimal movement of their voltage sensors, and modeling studies suggest that this might explain why T-channel kinetics are so voltage-dependent (8,25,27). We suggest that the brake region stabilizes the closed state, adopting the role of one of the S4-S5 linkers to keep the pore closed at resting membrane potentials.…”

Amazing T-type calcium channels: updating functional properties in health and disease

“…This was a decisive step to allow the unraveling of structural determinants for permeation and kinetic T-type channel properties (41,44). Even more exciting, the functional role of T-type Ca 2+ channels could now be studied systematically, the distribution and expression pattern of the three T-type isoforms in several tissues could be unraveled, and, importantly, opened the possibility to use antisense and knock-out strategies to unveil the physiological and pathological role of these channel (for some reviews from the early period after cloning see 15,33,35,36,42,43). The molecular cloning of the T-type Ca 2+ channels is now approximately 15 years old and has been so far the focus of the new developments in this exciting field.…”

Characterization of the gating brake in the I-II loop of CaV3 T-type calcium channels

“…31,32 This suggests that T-channels open after minimal movement of their voltage sensors and modeling studies suggest this might explain why T-channel kinetics are so voltagedependent. [32][33][34] We propose two mechanisms by which the brake region stabilizes the closed state. These models were developed by superimposing the gating brake on the crystal structure of the K V 1.2-K V 2.1 paddle chimera channel.…”