Cryo-EM structures of fungal and metazoan mitochondrial calcium uniporters

Abstract

The mitochondrial calcium uniporter (MCU) is a highly selective calcium channel and a major route of calcium entry into mitochondria. How the channel catalyses ion permeation and achieves ion selectivity are not well understood, partly because MCU is thought to have a distinct architecture in comparison to other cellular channels. Here we report cryo-electron microscopy reconstructions of MCU channels from zebrafish and Cyphellophora europaea at 8.5 Å and 3.2 Å resolutions, respectively. In contrast to a previous report of pentameric stoichiometry for MCU, both channels are tetramers. The atomic model of C. europaea MCU shows that a conserved WDXXEP signature sequence forms the selectivity filter, in which calcium ions are arranged in single file. Coiled-coil legs connect the pore to N-terminal domains in the mitochondrial matrix. In C. europaea MCU, the N-terminal domains assemble as a dimer of dimers; in zebrafish MCU, they form an asymmetric crescent. The structures define principles that underlie ion permeation and calcium selectivity in this unusual channel.

Progress in understanding mitochondrial calcium uniporter complex‐mediated calcium signalling: A potential target for cancer treatment

“…They also indicate potential sites of channel regulation by other molecules within the mitochondrial matrix, besides the above listed MCU accessory proteins located at the IMM (Baradaran et al, 2018;Fan et al, 2018;Yoo et al, 2018). By using an N-terminal domain (NTD) dimer-of-dimers assembly manner, which is similar to that of ionotropic glutamate receptors, MCU forms the channel.…”

“…This conserved sequence faces the intermembrane space (IMS), while both the N-and C-terminal regions of MCU protrude into the mitochondria matrix (Baughman et al, 2011). The Asp residues of these motifs form a mouth with a radius of~2.2 Å on the side of the porefacing IMS, while Glu residues form a second ring with a radius of <1 Å (Baradaran, Wang, Siliciano, & Long, 2018;Kirichok et al, 2004;Yoo et al, 2018). The high affinity of MCU binding to Ca 2+ (K D < 2 nM) relies on its WDXXEP motif located between TM1 and TM2 (X denotes hydrophobic amino acids, WDIMEP in humans).…”

“…The channel formed by MCU is sensitive to ruthenium red (RuR), RU360, and lanthanides (Cao, Wang, Cui, Su, & Chou, 2017;Crompton, Heid, Baschera, & Carafoli, 1979;De Stefani et al, 2011). The various effects of the mutant X residues on MCU activities are probably due to the first X residue that has no protein contacts in the structure, while the second X residue can interact with TM1 ( Baradaran et al, 2018). This motif forms the narrowest region of the pore and is the only region lined by acidic amino acids.…”

“…The high affinity of MCU binding to Ca 2+ (K D < 2 nM) relies on its WDXXEP motif located between TM1 and TM2 (X denotes hydrophobic amino acids, WDIMEP in humans). In HeLa cells, the overexpressed hMCU QIMQ acts as a dominant negative regulator, suggesting that the active MCU channel works as an oligomer (Baradaran et al, 2018;De Stefani et al, 2011;Yoo et al, 2018). The Asp residues of these motifs form a mouth with a radius of~2.2 Å on the side of the porefacing IMS, while Glu residues form a second ring with a radius of <1 Å (Baradaran, Wang, Siliciano, & Long, 2018;Kirichok et al, 2004;Yoo et al, 2018).…”

Redox Stability Controls the Cellular Uptake and Activity of Ruthenium‐Based Inhibitors of the Mitochondrial Calcium Uniporter (MCU)

“…[1] Uptake of these ions by the mitochondria is mediated by the highly selective and inwardly rectifying channel known as the mitochondrial calcium uniporter (MCU). [2] This protein resides in the inner mitochondrial membrane (IMM) and forms atetrameric,dimer of dimers type of assembly, [3][4][5][6] with both the N-and C-terminal domains located within the mitochondrial matrix. [7,8] Although mitochondrial Ca 2+ uptake through the MCU is needed for proper cellular function, dysregulation of this process through mitochondrial calcium overload gives rise to cell damage and death.…”