Abstract
The mitochondrial calcium uniporter (MCU) is ahighly selective calcium channeland a major route of calcium entry into mitochondria. How the channel catalyses ion permeation and achieves ionselectivity are not well understood, partly becauseMCU is thought to have a distinct architecture in comparison toother cellular channels. Here we report cryo-electron microscopy reconstructions of MCU channels from zebrafish and Cyphellophora europaea at 8.5Å and 3.2Å resolutions, respectively. In contrast to aprevious report of pentameric stoichiometryfor MCU, both channels are tetramers. The atomic model of C. europaea MCU shows that a conserved WDXXEP signature sequence forms the selectivity filter, in whichcalcium ionsarearranged 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 ionpermeation and calciumselectivity in this unusual channel.
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