Anion currents in yeast K+ transporters (TRK) characterize a structural homologue of ligand-gated ion channels

Abstract

Patch clamp studies of the potassium-transport proteins TRK1,2 in Saccharomyces cerevisiae have revealed large chloride efflux currents: at clamp voltages negative to -100 mV, and intracellular chloride concentrations >10 mM (J. Membr. Biol. 198:177, 2004). Stationary-state current-voltage analysis led to an in-series two-barrier model for chloride activation: the lower barrier (α) being 10–13 kcal/mol located ∼30% into the membrane from the cytoplasmic surface; and the higher one (β) being 12–16 kcal/mol located at the outer surface. Measurements carried out with lyotrophic anions and osmoprotective solutes have now demonstrated the following new properties: (1) selectivity for highly permeant anions changes with extracellular pH; at pHo=5.5: I−≈Br−>Cl−>SCN−>NO3−, and at pHo 7.5: I−≈Br−>SCN−>NO3−>Cl−. (2) NO2− acts like “superchoride”, possibly enhancing the channel's intrinsic permeability to Cl−. (3) SCN− and NO3− block chloride permeability. (4) The order of selectivity for several slightly permeant anions (at pHo=5.5 only) is formate > gluconate > acetate ≫ phosphate−1. (5) All anion conductances are modulated (choked) by osmoprotective solutes. (6) The data and descriptive two-barrier model evoke a hypothetical structure (Biophys. J. 77:789, 1999) consisting of an intramembrane homotetramer of fungal TRK molecules, arrayed radially around a central cluster of four single helices (TM7) from each monomer. (7) That tetrameric cluster would resemble the hydrophobic core of (pentameric) ligand-gated ion channels, and would suggest voltage-modulated hydrophobic gating to underlie anion permeation.

A refined atomic scale model of the Saccharomyces cerevisiae K+-translocation protein Trk1p combined with experimental evidence confirms the role of selectivity filter glycines and other key residues

“…The first structural models [8,10] however, assumed a tetrameric structure to provide a favorable environment for the charged residues within M2 D . This notion was experimentally supported by the discovery that S. cerevisiae Trk1 can also mediate Cl − currents [51,52], which could most easily be explained by assuming the existence of a second "metapore" formed at the central interface of four symmetrically arranged monomers. On the other hand, the known crystal structures [14,15,45] show a dimeric composition.…”

Monovalent cation transporters at the plasma membrane in yeasts

“…Later on, it was shown that these transporters also mediate transport of other anions such as I − , Br − , SCN − , or NO3 − and that the process is suppressed by osmoprotective agents. It has been proposed that the Trk tetramer would resemble the hydrophobic core of ligand‐gated ion channels creating a low selectivity pathway regulated by membrane voltage (Kuroda, Bihler, Bashi, Slayman, & Rivetta, ; Pardo et al, ; Rivetta, Kuroda, & Slayman, ). A series of additional but more indirect functions of Trk have been proposed: The potassium transporters are involved in programmed cell death through the control of calcium homeostasis (Lauff & Santa‐Maria, ), in the regulation of intracellular pH or in the maintenance of membrane potential (Madrid, Gómez, Ramos, & Rodríguez‐Navarro, ; Navarrete et al, ).…”

Regulation of Cation Balance inSaccharomyces cerevisiae

“…The physiological relevance of this efflux activity is not well understood, but may allow cells to balance charges created by Pma1-mediated proton extrusion (Rivetta et al 2011).…”

“…2 ), an activity that is not coupled to K + transport (Kuroda et al 2004;Rivetta et al 2011). The physiological relevance of this efflux activity is not well understood, but may allow cells to balance charges created by Pma1-mediated proton extrusion (Rivetta et al 2011).…”