1.A.62.3.1
Archaeal TRIC family homologue of 205 aas and 7 TMSs.  In animals, Ca²⁺ release from the sarcoplasmic reticulum (SR) or endoplasmic reticulum (ER) is crucial for muscle contraction, cell growth, apoptosis, learning and memory. The eukaryotic TRIC channels are cation channels balancing the SR and ER membrane potentials, and are implicated in Ca²⁺ signaling and homeostasis. Kasuya et al. 2016 presented crystal structures of two prokaryotic TRIC channels in the closed state and conducted structure-based functional analyses of these channels. Each trimer subunit consists of seven TMSs with two inverted 3 TMS repeats (Silverio and Saier 2011). The electrophysiological, biochemical and biophysical analyses revealed that TRIC channels possess an ion-conducting pore within each subunit, and that trimer formation contributes to the stability of the protein. The symmetrically related TMS2 and TMS5 helices are kinked at conserved glycine clusters, and these kinks are important for channel activity. The kinks in TMS2 and TMS5 generate lateral fenestrations at each subunit interface that are occupied by lipid molecules (Kasuya et al. 2016).  TRIC channels are involved in K⁺ uptake in prokaryotes, and have ion-conducting pores contained within each monomer. In a 2.2-Å resolution K⁺-bound structure, ion/water densities have been resolved inside the pore (PDB# 5H35) (Su et al. 2017). At the central region, a filter-like structure is shaped by the kinks on the second and fifth transmembrane helices and two nearby phenylalanine residues. Below the filter, the cytoplasmic vestibule is occluded by a plug-like motif attached to an array of pore-lining charged residues (Kasuya et al. 2016). The asymmetric filter-like structure at the pore center of SsTRIC may serve as a basis for the channel to bind and select monovalent cations, K⁺ and Na⁺ (Ou et al. 2017).