Ion and inhibitor binding of the double-ring ion selectivity filter of the mitochondrial calcium uniporter

Proc Natl Acad Sci U S A. 2017 Apr 4;114(14):E2846-E2851. doi: 10.1073/pnas.1620316114. Epub 2017 Mar 21.


The calcium (Ca2+) uniporter of mitochondria is a holocomplex consisting of the Ca2+-conducting channel, known as mitochondrial calcium uniporter (MCU), and several accessory and regulatory components. A previous electrophysiology study found that the uniporter has high Ca2+ selectivity and conductance and this depends critically on the conserved amino acid sequence motif, DXXE (Asp-X-X-Glu) of MCU. A recent NMR structure of the MCU channel from Caenorhabditis elegans revealed that the DXXE forms two parallel carboxylate rings at the channel entrance that seem to serve as the ion selectivity filter, although direct ion interaction of this structural motif has not been addressed. Here, we use a paramagnetic probe, manganese (Mn2+), to investigate ion and inhibitor binding of this putative selectivity filter. Our paramagnetic NMR data show that mutants with a single carboxylate ring, NXXE (Asn-X-X-Glu) and DXXQ (Asp-X-X-Gln), each can bind Mn2+ specifically, whereas in the WT the two rings bind Mn2+ cooperatively, resulting in ∼1,000-fold higher apparent affinity. Ca2+ can specifically displace the bound Mn2+ at the DXXE site in the channel. Furthermore, titrating the sample with the known channel inhibitor ruthenium 360 (Ru360) can displace Mn2+ binding from the solvent-accessible Asp site but not the inner Glu site. The NMR titration data, together with structural analysis of the DXXE motif and molecular dynamics simulation, indicate that the double carboxylate rings at the apex of the MCU pore constitute the ion selectivity filter and that Ru360 directly blocks ion entry into the filter by binding to the outer carboxylate ring.

Keywords: MCU; NMR; Ru360 binding; calcium channel; selectivity filter.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Binding Sites
  • Caenorhabditis elegans Proteins / antagonists & inhibitors
  • Caenorhabditis elegans Proteins / chemistry*
  • Caenorhabditis elegans Proteins / genetics
  • Caenorhabditis elegans Proteins / metabolism*
  • Calcium / metabolism*
  • Magnetic Resonance Spectroscopy
  • Manganese / metabolism*
  • Mitochondrial Membrane Transport Proteins / antagonists & inhibitors
  • Mitochondrial Membrane Transport Proteins / chemistry*
  • Mitochondrial Membrane Transport Proteins / genetics
  • Mitochondrial Membrane Transport Proteins / metabolism*
  • Molecular Dynamics Simulation
  • Mutation
  • Ruthenium Compounds / metabolism
  • Ruthenium Compounds / pharmacology


  • Caenorhabditis elegans Proteins
  • Mcu-1 protein, C elegans
  • Mitochondrial Membrane Transport Proteins
  • Ru 360
  • Ruthenium Compounds
  • Manganese
  • Calcium