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. I will be presenting the cryo-electron microscopy reconstructions of MCU channels from zebrafish and Cyphellophora europaea at 8.5 angstrom and 3.2 angstrom resolutions, respectively. The small molecular weight of MCU , just approximately 110 kDa for the ordered region of the channel assembly, posed challenges for structure determination by cryo-EM, and it is one of the smallest membrane proteins to have the structure determined to atomic resolution by cryo-EM. 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.