Structural Mechanism of EMRE-Dependent Gating of the Human Mitochondrial Calcium Uniporter

Abstract

Mitochondrial calcium uptake is crucial to the regulation of eukaryotic Ca 2+ homeostasis and is mediated by the mitochondrial calcium uniporter (MCU). While MCU alone can transport Ca 2+ in primitive eukaryotes, metazoans require an essential single membrane-spanning auxiliary component called EMRE to form functional channels; however, the molecular mechanism of EMRE regulation remains elusive. Here, we present the cryo-EM structure of the human MCU-EMRE complex, which defines the interactions between MCU and EMRE as well as pinpoints the juxtamembrane loop of MCU and extended linker of EMRE as the crucial elements in the EMRE-dependent gating mechanism among metazoan MCUs. The structure also features the dimerization of two MCU-EMRE complexes along an interface at the N-terminal domain (NTD) of human MCU that is a hotspot for post-translational modifications. Thus, the human MCU-EMRE complex, which constitutes the minimal channel components among metazoans, provides a framework for future mechanistic studies on MCU.

Discovery of EMRE in fungi resolves the true evolutionary history of the mitochondrial calcium uniporter

“…Here, we perform the most comprehensive phylogenomic analysis of the mt-Ca 2+ uptake system and trace its evolution across 1,156 fully-sequenced eukaryotes. In contrast to earlier assumptions 9–11 we find compelling evidence that previously identified animal and fungal MCUs, the targets of several structural and functional efforts 11–16 , represent two distinct paralogous subfamilies originating from an ancestral duplication. We further uncover a complete “animal-like” uniporter complex within chytrid fungi, including bona-fide orthologs of MCU, MICU1, and EMRE.…”

“…Instead, EMRE was found as a specific interactor of MCU in human cells, required for both conductivity and binding of the channel to MICU1 8 . Although MCU and MICU1 showed correlated evolutionary histories across 138 sequenced eukaryotic organisms, EMRE apparently lacked any homolog outside the metazoan lineage and it was therefore suggested to be an animal-specific innovation 9–11 . While those observations pointed to an ancient eukaryotic origin of mt-Ca 2+ uptake, they also implied a very different composition and regulation of the uniporter in different clades.…”

“…Similarly in ( b ) EMRE’s sequence diversity across opisthokonts is shown for the β-hairpin, the TM, and CAD domain. Residues found important for the interaction between MCU and EMRE in 11 and fully conserved positions are indicated with gray and red arrows, respectively. c,d,e,f Representative traces and quantification of mt-Ca 2+ transients in yeast cells expressing either human and animal-like S. punctatus and A. macrogynus MCU with human EMRE ( c ), species-specific EMRE with human MCU ( d ), animal-like S. punctatus and A. macrogynus MCU with their respective wild-type or truncated (-t) EMREs ( e ) or with human EMRE fused to the fungal extra C-terminal domain ( f ) upon glucose-induced calcium (GIC) stimulation in presence of 1 mM CaCl 2 .…”

Structural characterization of the N-terminal domain of theDictyostelium discoideummitochondrial calcium uniporter

“…Such polar interactions are also observed in the crystal structures of the HsMCU-NTD. Apart from the interfaces within one MCU channel, extensive dimerization interactions at the NTD were observed in the cryo-EM structure of MCU-EMRE complex that multiple sets of salt bridges and hydrogen bonds forms the dimerization interface between two MCU tetramers (Wang et al, 2019). In DdMCU-NTD crystal structures, three pairs of parallel polar interactions are also identified, including N59/D60-N93, K66-D74, E72-N76, which play an important role in the formation and stabilization of the DdMCU-NTD oligomers.…”

“…Remarkably, the NTDs of these homologues also share high structural similarity despite of the low sequence homology between NTDs. Recently, Wang et al reported the cryo-EM structure of the human MCU-EMRE complex, providing the structural features for the EMRE regulation on the conformation of HsMCU (Wang et al, 2019). It is interesting to note that the NTD adopts a unique side-by-side configuration to mediate the dimerization of two HsMCU tetramers, which indicates that the NTD may play an important role in modulating the function of MCU, although the removal of the NTD does not affect mitochondrial Ca 2+ uptake (Lee et al, 2015).…”