Structural insights into the Ca2+-dependent gating of the human mitochondrial calcium uniporter

Abstract

Mitochondrial Ca2+ uptake is mediated by an inner mitochondrial membrane protein called the mitochondrial calcium uniporter. In humans, the uniporter functions as a holocomplex consisting of MCU, EMRE, MICU1 and MICU2, among which MCU and EMRE form a subcomplex and function as the conductive channel while MICU1 and MICU2 are EF-hand proteins that regulate the channel activity in a Ca2+ dependent manner. Here we present the EM structures of the human mitochondrial calcium uniporter holocomplex (uniplex) in the presence and absence of Ca2+, revealing distinct Ca2+ dependent assembly of the uniplex. Our structural observations suggest that Ca2+ changes the dimerization interaction between MICU1 and MICU2, which in turn determines how the MICU1-MICU2 subcomplex interacts with the MCU-EMRE channel and, consequently, changes the distribution of the uniplex assemblies between the blocked and unblocked states.

Structural characterization of the mitochondrial Ca2+ uniporter provides insights into Ca2+ uptake and regulation

“…Similar to the C-helix of MICU1, the rigid helix of MICU2 plays a vital role in maintaining the normal threshold under resting conditions (Kamer et al, 2019). To date, a backto-back configuration has been observed in the tetrameric MICU1-MICU2 complex and is the most likely preferred configuration (as further elaborated in the following) (Fan et al, 2020;Wang et al, 2020c;Zhuo et al, 2020). However, the possibility of the other packing configuration cannot be excluded.…”

“…However, extensive subsequent experimental evidence demonstrated that MICU1 and MICU2 could form a stable heterodimer in the absence of disulfide in vitro (Kamer et al, 2017;Li et al, 2016;Wu et al, 2019). Recently, MICU1-MICU2 heterodimer structures were solved by X-ray crystallography, revealing a face-to-face antiparallel assembly (Park et al, 2020;Wu et al, 2020 (Fan et al, 2020;Wang et al, 2020bWang et al, , 2020c. The superimposed structures of Ca 2+ -free and Ca 2+ -bound MICU1-MICU2 heterodimers demonstrate that Ca 2+ -bound structures are more compact than the apo form (Figure 2G).…”

Mitochondrial Dysfunction and Heart Disease: Critical Appraisal of an Overlooked Association

“…The outer mitochondrial membrane shows high permeability to calcium, so that cytosol and intermembrane space calcium concentrations are virtually equal. Calcium enters the matrix via the mitochondrial calcium uniporter (MCU) [ 26 , 27 ] complex on the inner mitochondrial membrane, which shows adaptive response patterns to low-versus-high calcium concentrations [ 28 ]; during diastole, the MCU does not import calcium due to low cytosol concentrations and subsequent blockage by MCU regulatory proteins. Instead, during systole, rising calcium concentrations trigger a conformational change of said proteins, and calcium import is allowed.…”

From the Identification to the Dissection of the Physiological Role of the Mitochondrial Calcium Uniporter: An Ongoing Story

“…The acidic residues present in this stretch (in particular E257, D261, E264) are critical for the Ca 2+ transport since, if substituted with uncharged residues, MCU mutants failed to rescue the mitochondrial Ca 2+ uptake in MCU-silenced cells [ 41 , 42 ]. However, the definitive description of MCU 3D structure had to wait till the very last years, when cryo-EM and X-ray diffraction analyses finally allowed the resolution of full-length MCU structure [ 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. These studies coherently confirmed that purified MCU from different sources (fungi and metazoan) arranges in a tetramer, confuting previous assumptions on a putative pentameric MCU architecture [ 52 ].…”

“…Indeed, MEMMS appears as a 480 kDa integral unit where EMRE coordinates the matrix gate of the MCU channel and MICU proteins interact with the C -terminus of EMRE in the IMS thus enhancing Ca 2+ influx through the MCU pore in high [Ca 2+ ] conditions [ 54 ] ( Figure 3 ). Finally, the distinct Ca 2+ -dependent assembly conformations of the beetle and human MCU holocomplexes with human MICUs have also been detailed [ 46 , 47 ]. In the presence of Ca 2+ , the multiprotein complex shows a two-fold symmetry and consists of two V-shaped MCU-EMRE tetrameric subcomplexes and two MICU1-MICU2 heterodimers that bridge the tops of the subcomplexes ( Figure 1 ).…”