Voltage-Dependent Stochastic Gating Models of TRIC-B Channels

TRIC-A and TRIC-B are two, related, trimeric intracellular cation channels present in sarcoplasmic/endoplasmic reticulum (SR) and are thought to provide counter-current for SR Ca²⁺-release. TRIC-B knockout mice die immediately after birth demonstrating the importance of this isoform [Yazawa et al., 2007, Nature, 448, 78-82]. To study the distinct single-channel gating behaviour of TRIC-B, we incorporated skeletal muscle light SR from TRIC-A knockout mice into artificial membranes under voltage-clamp conditions in symmetrical 210 mM K-PIPES, pH 7.2. We developed Markov models of TRIC-B gating, with up to 4 distinct sub-conductance states (S₁-S₄), using both QuB [Qin F., 2004, Biophys J.,86(3), 1488-501] and our own software. Our models incorporate different connectivity schemes to account for the intrinsic variability in gating that was observed between different channels. Despite the variability, some obvious trends emerged. TRIC-B activity was higher at positive than at negative holding potentials. At positive potentials, the majority of channels exhibited long bursts of openings where predominant gating transitions were between the full open state and S₁, the largest sub-conductance state. Some channels, however, gated preferentially in sub-states S₃ and S₄, only visiting the full open state briefly. At negative potentials, channel activity consisted primarily of brief transitions between sub-conductance states. Closed lifetime distributions at positive potentials comprised of fast components (τ ≈ 1 ms), corresponding to brief transitions from the full open state, as well as slower components corresponding to inter-burst intervals. At negative potentials, inter-burst intervals were orders of magnitude longer demonstrating that the frequency of channel opening is heavily dependent on voltage. It will be important to develop comprehensive models of TRIC-B channel gating in order to fully understand the role of this important ion-channel in intracellular Ca²⁺-release.