Biophysics and structure-function relationship of T-type Ca2+ channels

T-type channels are distinguished among voltage-gated Ca2+ channels by their low voltage thresholds for activation and inactivation, fast inactivation and small single channel conductance in isotonic Ba2+. Detailed biophysical and pharmacological characterization of native T-type channels indicated that these channels represent a heterogeneous family. Cloning of three family members (CaV3.1-3.3) confirmed these observations and allowed the study of the structure-function relationship of these channels. T-type channels are likely heterotetrameric structures consisting of a single polypeptide of four homologous domains (I-IV), each one containing six transmembrane spans (S1-S6), and cytoplasmic N- and C-termini. Structure-function studies have revealed that fast macroscopic inactivation of CaV3.1 is modulated by specific residues in the proximal C-terminus and in the transmembrane domain IIIS6. The particular gating properties within the T-type channel subfamily are determined by several parts of the protein, whereas differences with respect to high-voltage-activated Ca2+ channels are mostly determined by domains I, II and III. Several gating properties are affected by alternative splicing, C-terminal truncations and mutations associated to idiopathic epilepsy. Intriguingly, the aspartate residues of the EEDD locus of the selectivity filter not only determine the permeation properties and the block by Cd2+ and protons, but also activation and deactivation. Mutagenesis has also revealed that the outermost arginines of the S4 segment of domain IV influence the activation of CaV3.2, though no specific voltage-sensing amino acid has yet been properly identified. The selective modulation of CaV3.2 by G-proteins, CaMKII and PKA is determined by the II-III linker and the high-affinity inhibition of CaV3.2 by Ni2+ relies on a histidine residue in the IS3-S4 linker. Certainly, more structure-function studies are needed for a better understanding of T-type channel physiology and the rational design of treatments against T-type channel-related pathologies.