Ion- and water-binding sites inside an occluded hourglass pore of a trimeric intracellular cation (TRIC) channel

Ion- and water-binding sites inside an occluded hourglass pore of a trimeric intracellular cation (TRIC) channel

BMC Biology, April 2017

28431535

Xiaomin Ou, Jianli Guo, Longfei Wang, Hanting Yang, Xiuying Liu, Jianyuan Sun, Zhenfeng Liu

Trimeric intracellular cation (TRIC) channels are crucial for Ca(2+) handling in eukaryotes and are involved in K(+) uptake in prokaryotes. Recent studies on the representative members of eukaryotic and prokaryotic TRIC channels demonstrated that they form homotrimeric units with the ion-conducting pores contained within each individual monomer. Here we report detailed insights into the ion- and water-binding sites inside the pore of a TRIC channel from Sulfolobus solfataricus (SsTRIC). Like the mammalian TRIC channels, SsTRIC is permeable to both K(+) and Na(+) with a slight preference for K(+), and is nearly impermeable to Ca(2+), Mg(2+), or Cl(-). In the 2.2-Å resolution K(+)-bound structure of SsTRIC, ion/water densities have been well resolved inside the pore. 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. The asymmetric filter-like structure at the pore center of SsTRIC might serve as the basis for the channel to bind and select monovalent cations. A Velcro-like plug-pore interacting model has been proposed and suggests a unified framework accounting for the gating mechanisms of prokaryotic and eukaryotic TRIC channels.