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Background  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.    Results  Here we report detailed insights into the ion- and water-binding sites inside the pore of a TRIC channel from  Sulfolobus solfataricus  ( Ss TRIC). Like the mammalian TRIC channels,  Ss TRIC 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  Ss TRIC, 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.    Conclusions  The asymmetric filter-like structure at the pore center of  Ss TRIC 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.    Electronic supplementary material  The online version of this article (doi:10.1186/s12915-017-0372-8) contains supplementary material, which is available to authorized users.

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