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High resolution single-particle cryo-EM of a 200 kDa membrane ion channel

Friday, September 18, 2015 — Poster Session IV

12:00 p.m. – 1:30 p.m.
FAES Terrace
NCI
STRUCTBIO-9

* FARE Award Winner

Authors

  • D Matthies
  • O Dalmas
  • MJ Borgnia
  • A Merk
  • P Dominik
  • P Rao
  • B Reddy
  • S Islam
  • A Bartesaghi
  • E Perozo
  • S Subramaniam

Abstract

Magnesium ions are the most abundant divalent intracellular cations and are essential for life, as they play key roles in signaling, nucleic acid action and metabolism. Mg2+-deficiency is associated with diseases affecting the heart, muscle, bone, immune and nervous system, so it is very important to fully understand this ion uptake system. The ~200 kDa pentameric membrane channel CorA is the major Mg2+ uptake system in bacteria and a homolog of the eukaryotic mitochondrial Mrs2 proteins which it can complement. CorA contributes to Mg2+ homeostasis through a negative feedback loop, where Mg2+ binding at the subunit interface leads to channel closure and low Mg2+ concentrations stabilize the open conformation. Electron paramagnetic resonance (EPR) spectroscopic studies of purified CorA revealed large quaternary conformational changes associated with magnesium binding/unbinding. Using single-particle cryo-EM, we have determined the structure for the "closed" magnesium-bound state of CorA at a resolution of 3.8 Å. At this resolution, we were able to trace the entire polypeptide backbone including many bulky residues and build a structural model of the 200 kDa protein complex. Side-chain densities as well as densities for several magnesium ions per subunit can be clearly discerned. We are actively pursuing efforts to determine the extent and nature of conformational changes induced by Mg2+ binding. Initial observations suggest that channel opening is likely to involve a significant change in quaternary structure with loss of five-fold symmetry.

Category: Structural Biology