The crystal structure of MICU2 provides insight into Ca2+ binding and MICU1-MICU2 heterodimer formation

Astaxanthin Protects against Alcoholic Liver Injury via Regulating Mitochondrial Redox Balance and Calcium Homeostasis

Increasing evidence points to the critical role of calcium overload triggered by mitochondrial dysfunction in the development of alcoholic liver disease (ALD). As an important organelle for aerobic respiration with a double-layered membrane, mitochondria are pivotal targets of alcohol metabolism-mediated lipid peroxidation, wherein mitochondria-specific phospholipid cardiolipin oxidation to 4-hydroxynonenal (4-HNE) ultimately leads to mitochondrial integrity and function impairment. Therefore, it is absolutely essential to identify effective nutritional intervention targeting mitochondrial redox function for an alternative therapy of ALD, in order to compensate for the difficulty in achieving alcohol withdrawal due to addiction. In this study, we confirmed the significant advantages of astaxanthin (AX) against alcohol toxicity among various carotenoids via cell experiments and identified the potential in mitochondrion morphogenesis and calcium signaling pathway by bioinformatics analysis. The ALD model of Sprague-Dawley (SD) rats was also generated to investigate the effectiveness of AX on alcohol-induced liver injury, and the underlying mechanisms were further explored. AX intervention attenuated alcohol-induced oxidative stress and lipid peroxidation as well as mitochondrial dysfunction characterized by degenerative morphology changes and collapsed membrane potential. Also, AX reduced the production of 4-HNE by activating the Nrf2-ARE signaling pathway, which is closely associated with the redox balance of mitochondria. In addition, relieved mitochondrial Ca2+ accumulation caused by AX was observed both in vivo and in vitro. Furthermore, we revealed the structure-activity relationship of AX and mitochondrial membrane channel proteins MCU and VDAC1, implying potential acting targets. Altogether, our data indicated a new mechanism of AX intervention which protects against alcohol-induced liver injury through restoring redox balance and Ca2+ homeostasis in mitochondria, as well as provided novel insights into the development of AX as a therapeutic option for the management of ALD.

Mitochondrial Calcium Overload Plays a Causal Role in Oxidative Stress in the Failing Heart

Heart failure is a serious global health challenge, affecting more than 6.2 million people in the United States and is projected to reach over 8 million by 2030. Independent of etiology, failing hearts share common features, including defective calcium (Ca2+) handling, mitochondrial Ca2+ overload, and oxidative stress. In cardiomyocytes, Ca2+ not only regulates excitation-contraction coupling, but also mitochondrial metabolism and oxidative stress signaling, thereby controlling the function and actual destiny of the cell. Understanding the mechanisms of mitochondrial Ca2+ uptake and the molecular pathways involved in the regulation of increased mitochondrial Ca2+ influx is an ongoing challenge in order to identify novel therapeutic targets to alleviate the burden of heart failure. In this review, we discuss the mechanisms underlying altered mitochondrial Ca2+ handling in heart failure and the potential therapeutic strategies.

The mitochondrial calcium uniporter complex–A play in five acts

Mitochondrial Ca²⁺ (mitCa²⁺) uptake controls both intraorganellar and cytosolic functions. Within the organelle, [Ca²⁺] increases regulate the activity of tricarboxylic acid (TCA) cycle enzymes, thus sustaining oxidative metabolism and ATP production. Reactive oxygen species (ROS) are also generated as side products of oxygen consumption. At the same time, mitochondria act as buffers of cytosolic Ca²⁺ (cytCa²⁺) increases, thus regulating Ca²⁺-dependent cellular processes. In pathological conditions, mitCa²⁺ overload triggers the opening of the mitochondrial permeability transition pore (mPTP) and the release of apoptotic cofactors. MitCa²⁺ uptake occurs in response of local [Ca²⁺] increases in sites of proximity between the endoplasmic reticulum (ER) and the mitochondria and is mediated by the mitochondrial Ca²⁺ uniporter (MCU), a highly selective channel of the inner mitochondrial membrane (IMM). Both channel and regulatory subunits form the MCU complex (MCUC). Cryogenic electron microscopy (Cryo-EM) and crystal structures revealed the correct assembly of MCUC and the function of critical residues for the regulation of Ca²⁺ conductance.

An unexpected effect of risperidone reveals a nonlinear relationship between cytosolic Ca2+ and mitochondrial Ca2+ uptake

Mitochondria actively contribute to cellular Ca²⁺ homeostasis. The molecular mechanisms of mitochondrial Ca²⁺ uptake and release are well characterized and are attributed to the multi-protein assembly of the mitochondrial Ca²⁺ uniporter complex (MCUC) and the mitochondrial sodium-calcium exchanger (NCLX), respectively. Hence, Ca²⁺ transfer from the endoplasmic reticulum (ER) and store-operated Ca²⁺ entry (SOCE) into the mitochondrial matrix has been quantitatively visualized on the subcellular level using targeted fluorescent biosensors. However, a correlation between the amplitude of cytosolic Ca²⁺ elevation with that in the mitochondrial matrix has not been investigated in detail so far. In the present study, we combined the Ca²⁺-mobilizing agonist histamine with the H₁-receptor antagonist risperidone to establish a well-tunable experimental approach allowing the correlation between low, slow, high, and fast cytosolic and mitochondrial Ca²⁺ signals in response to inositol 1,4,5-trisphosphate (IP₃)-triggered ER Ca²⁺ release. Our present data confirm a defined threshold in cytosolic Ca²⁺, which is necessary for the activation of mitochondrial Ca²⁺ uptake. Moreover, our data support the hypothesis of different modes of mitochondrial Ca²⁺ uptake depending on the source of the ion (i.e., ER vs SOCE).

PLK1-mediated phosphorylation of PPIL2 regulates HR via CtIP

Homologous recombination (HR) is an error-free DNA double-strand break (DSB) repair pathway, which safeguards genome integrity and cell viability. Human C-terminal binding protein (CtBP)—interacting protein (CtIP) is a central regulator of the pathway which initiates the DNA end resection in HR. Ubiquitination modification of CtIP is known in some cases to control DNA resection and promote HR. However, it remains unclear how cells restrain CtIP activity in unstressed cells. We show that the ubiquitin E3 ligase PPIL2 is recruited to DNA damage sites through interactions with an HR-related protein ZNF830, implying PPIL2’s involvement in DNA repair. We found that PPIL2 interacts with and ubiquitinates CtIP at the K426 site, representing a hereunto unknown ubiquitination site. Ubiquitination of CtIP by PPIL2 suppresses HR and DNA resection. This inhibition of PPIL2 is also modulated by phosphorylation at multiple sites by PLK1, which reduces PPIL2 ubiquitination of CtIP. Our findings reveal new regulatory complexity in CtIP ubiquitination in DSB repair. We propose that the PPIL2-dependent CtIP ubiquitination prevents CtIP from interacting with DNA, thereby inhibiting HR.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

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

The mitochondrial uniporter is a Ca²⁺-selective ion-conducting channel in the inner mitochondrial membrane that is involved in various cellular processes. The components of this uniporter, including the pore-forming membrane subunit MCU and the modulatory subunits MCUb, EMRE, MICU1, and MICU2, have been identified in recent years. Previously, extensive studies revealed various aspects of uniporter activities and proposed multiple regulatory models of mitochondrial Ca²⁺ uptake. Recently, the individual auxiliary components of the uniporter and its holocomplex have been structurally characterized, providing the first insight into the component structures and their spatial relationship within the context of the uniporter. Here, we review recent uniporter structural studies in an attempt to establish an architectural framework, elucidating the mechanism that governs mitochondrial Ca²⁺ uptake and regulation, and to address some apparent controversies. This information could facilitate further characterization of mitochondrial Ca²⁺ permeation and a better understanding of uniporter-related disease conditions.

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The uniporter contains multiple subunits regulating various cellular processes

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Significant structural progresses have been made for the holo-complex of uniporter

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The holo-complex structures have inspired to propose several regulatory models

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

The notion of mitochondria being involved in the decoding and shaping of intracellular Ca²⁺ signals has been circulating since the end of the 19th century. Despite that, the molecular identity of the channel that mediates Ca²⁺ ion transport into mitochondria remained elusive for several years. Only in the last decade, the genes and pathways responsible for the mitochondrial uptake of Ca²⁺ began to be cloned and characterized. The gene coding for the pore-forming unit of the mitochondrial channel was discovered exactly 10 years ago, and its product was called mitochondrial Ca²⁺ uniporter or MCU. Before that, only one of its regulators, the mitochondria Ca²⁺ uptake regulator 1, MICU1, has been described in 2010. However, in the following years, the scientific interest in mitochondrial Ca²⁺ signaling regulation and physiological role has increased. This shortly led to the identification of many of its components, to the description of their 3D structure, and the characterization of the uniporter contribution to tissue physiology and pathology. In this review, we will summarize the most relevant achievements in the history of mitochondrial Ca²⁺ studies, presenting a chronological overview of the most relevant and landmarking discoveries. Finally, we will explore the impact of mitochondrial Ca²⁺ signaling in the context of muscle physiology, highlighting the recent advances in understanding the role of the MCU complex in the control of muscle trophism and metabolism.

The Parkinson's disease-associated gene ITPKB protects against α-synuclein aggregation by regulating ER-to-mitochondria calcium release

Parkinson’s disease (PD) is the second most prevalent neurodegenerative disease of aging, affecting approximately 10 million patients worldwide with no approved therapies to modify progression of disease. Further understanding of the cellular mechanisms contributing to the development of PD is necessary to discover therapies. Here, we characterize the role of a recently identified GWAS hit for sporadic PD, ITPKB, in the aggregation of α-synuclein, the primary pathological feature of disease. These results identify inhibition of inositol-1,4,5,-triphosphate (IP₃)-mediated ER-to-mitochondria calcium release as a potential therapeutic approach for reducing neuropathology in PD.

Inositol-1,4,5-triphosphate (IP₃) kinase B (ITPKB) is a ubiquitously expressed lipid kinase that inactivates IP₃, a secondary messenger that stimulates calcium release from the endoplasmic reticulum (ER). Genome-wide association studies have identified common variants in the ITPKB gene locus associated with reduced risk of sporadic Parkinson’s disease (PD). Here, we investigate whether ITPKB activity or expression level impacts PD phenotypes in cellular and animal models. In primary neurons, knockdown or pharmacological inhibition of ITPKB increased levels of phosphorylated, insoluble α-synuclein pathology following treatment with α-synuclein preformed fibrils (PFFs). Conversely, ITPKB overexpression reduced PFF-induced α-synuclein aggregation. We also demonstrate that ITPKB inhibition or knockdown increases intracellular calcium levels in neurons, leading to an accumulation of calcium in mitochondria that increases respiration and inhibits the initiation of autophagy, suggesting that ITPKB regulates α-synuclein pathology by inhibiting ER-to-mitochondria calcium transport. Furthermore, the effects of ITPKB on mitochondrial calcium and respiration were prevented by pretreatment with pharmacological inhibitors of the mitochondrial calcium uniporter complex, which was also sufficient to reduce α-synuclein pathology in PFF-treated neurons. Taken together, these results identify ITPKB as a negative regulator of α-synuclein aggregation and highlight modulation of ER-to-mitochondria calcium flux as a therapeutic strategy for the treatment of sporadic PD.

Molecular machinery regulating mitochondrial calcium levels: The nuts and bolts of mitochondrial calcium dynamics

Mitochondria play vital role in regulating the cellular energetics and metabolism. Further, it is a signaling hub for cell survival and apoptotic pathways. One of the key determinants that calibrate both cellular energetics and survival functions is mitochondrial calcium (Ca2+) dynamics. Mitochondrial Ca2+ regulates three Ca2+-sensitive dehydrogenase enzymes involved in tricarboxylic acid cycle (TCA) cycle thereby directly controlling ATP synthesis. On the other hand, excessive Ca2+ concentration within the mitochondrial matrix elevates mitochondrial reactive oxygen species (mROS) levels and causes mitochondrial membrane depolarization. This leads to opening of the mitochondrial permeability transition pore (mPTP) and release of cytochrome c into cytosol eventually triggering apoptosis. Therefore, it is critical for cell to maintain mitochondrial Ca2+ concentration. Since cells can neither synthesize nor metabolize Ca2+, it is the dynamic interplay of Ca2+ handling proteins involved in mitochondrial Ca2+ influx and efflux that take the center stage. In this review we would discuss the key molecular machinery regulating mitochondrial Ca2+ concentration. We would focus on the channel complex involved in bringing Ca2+ into mitochondrial matrix i.e. Mitochondrial Ca2+ Uniporter (MCU) and its key regulators Mitochondrial Ca2+ Uptake proteins (MICU1, 2 and 3), MCU regulatory subunit b (MCUb), Essential MCU Regulator (EMRE) and Mitochondrial Ca2+ Uniporter Regulator 1 (MCUR1). Further, we would deliberate on major mitochondrial Ca2+ efflux proteins i.e. Mitochondrial Na+/Ca2+/Li+ exchanger (NCLX) and Leucine zipper EF hand-containing transmembrane1 (Letm1). Moreover, we would highlight the physiological functions of these proteins and discuss their relevance in human pathophysiology. Finally, we would highlight key outstanding questions in the field.

The molecular complexity of the Mitochondrial Calcium Uniporter

The role of mitochondria in regulating cellular Ca²⁺ homeostasis is crucial for the understanding of different cellular functions in physiological and pathological conditions. Nevertheless, the study of this aspect was severely limited by the lack of the molecular identity of the proteins responsible for mitochondrial Ca²⁺ uptake. In 2011, the discovery of the gene encoding for the Mitochondrial Calcium Uniporter (MCU), the selective channel responsible for mitochondrial Ca²⁺ uptake, gave rise to an explosion of studies aimed to characterize the composition, the regulation of the channel and its pathophysiological roles. Here, we summarize the recent discoveries on the molecular structure and composition of the MCU complex by providing new insights into the mechanisms that regulate MCU channel activity.

The Physiological and Pathological Roles of Mitochondrial Calcium Uptake in Heart

Calcium ion (Ca²⁺) plays a critical role in the cardiac mitochondria function. Ca²⁺ entering the mitochondria is necessary for ATP production and the contractile activity of cardiomyocytes. However, excessive Ca²⁺ in the mitochondria results in mitochondrial dysfunction and cell death. Mitochondria maintain Ca²⁺ homeostasis in normal cardiomyocytes through a comprehensive regulatory mechanism by controlling the uptake and release of Ca²⁺ in response to the cellular demand. Understanding the mechanism of modulating mitochondrial Ca²⁺ homeostasis in the cardiomyocyte could bring new insights into the pathogenesis of cardiac disease and help developing the strategy to prevent the heart from damage at an early stage. In this review, we summarized the latest findings in the studies on the cardiac mitochondrial Ca²⁺ homeostasis, focusing on the regulation of mitochondrial calcium uptake, which acts as a double-edged sword in the cardiac function. Specifically, we discussed the dual roles of mitochondrial Ca²⁺ in mitochondrial activity and the impact on cardiac function, the molecular basis and regulatory mechanisms, and the potential future research interest.

The Mitochondrial Ca2+ Uptake and the Fine-Tuning of Aerobic Metabolism

Recently, the role of mitochondrial activity in high-energy demand organs and in the orchestration of whole-body metabolism has received renewed attention. In mitochondria, pyruvate oxidation, ensured by efficient mitochondrial pyruvate entry and matrix dehydrogenases activity, generates acetyl CoA that enters the TCA cycle. TCA cycle activity, in turn, provides reducing equivalents and electrons that feed the electron transport chain eventually producing ATP. Mitochondrial Ca²⁺ uptake plays an essential role in the control of aerobic metabolism. Mitochondrial Ca²⁺ accumulation stimulates aerobic metabolism by inducing the activity of three TCA cycle dehydrogenases. In detail, matrix Ca²⁺ indirectly modulates pyruvate dehydrogenase via pyruvate dehydrogenase phosphatase 1, and directly activates isocitrate and α-ketoglutarate dehydrogenases. Here, we will discuss the contribution of mitochondrial Ca²⁺ uptake to the metabolic homeostasis of organs involved in systemic metabolism, including liver, skeletal muscle, and adipose tissue. We will also tackle the role of mitochondrial Ca²⁺ uptake in the heart, a high-energy consuming organ whose function strictly depends on appropriate Ca²⁺ signaling.

Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca2+ uptake

Ca2+ uptake by mitochondria regulates bioenergetics, apoptosis, and Ca2+ signaling. The primary pathway for mitochondrial Ca2+ uptake is the mitochondrial calcium uniporter (MCU), a Ca2+-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca2+ uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca2+ uptake is tightly regulated to maintain low matrix [Ca2+] and prevent opening of the permeability transition pore and cell death, while meeting dynamic cellular energy demands. How this is accomplished is controversial. Here we define a regulatory mechanism of MCU-channel activity in which cytoplasmic Ca2+ regulation of intermembrane space-localized MICU1/2 is controlled by Ca2+-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca2+ that permeates through the channel pore regulates Ca2+ affinities of coupled inhibitory and activating sensors in the matrix. Ca2+ binding to the inhibitory sensor within the MCU amino terminus closes the channel despite Ca2+ binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex disables matrix Ca2+ regulation of channel activity. Our results demonstrate how Ca2+ influx into mitochondria is tuned by coupled Ca2+-regulatory mechanisms on both sides of the inner mitochondrial membrane.

The structure of the MICU1-MICU2 complex unveils the regulation of the mitochondrial calcium uniporter

The MICU1-MICU2 heterodimer regulates the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake. Herein, we present two crystal structures of the MICU1-MICU2 heterodimer, in which Ca²⁺ -free and Ca²⁺ -bound EF-hands are observed in both proteins, revealing both electrostatic and hydrophobic interfaces. Furthermore, we show that MICU1 interacts with EMRE, another regulator of MCU, through a Ca²⁺ -dependent alkaline groove. Ca²⁺ binding strengthens the MICU1-EMRE interaction, which in turn facilitates Ca²⁺ uptake. Conversely, the MICU1-MCU interaction is favored in the absence of Ca²⁺ , thus inhibiting the channel activity. This Ca²⁺ -dependent switch illuminates how calcium signals are transmitted from regulatory subunits to the calcium channel and the transition between gatekeeping and activation channel functions. Furthermore, competition with an EMRE peptide alters the uniporter threshold in resting conditions and elevates Ca²⁺ accumulation in stimulated mitochondria, confirming the gatekeeper role of the MICU1-MICU2 heterodimer. Taken together, these structural and functional data provide new insights into the regulation of mitochondrial calcium uptake.

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

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.

Structures reveal gatekeeping of the mitochondrial Ca2+ uniporter by MICU1-MICU2

The mitochondrial calcium uniporter is a Ca2+-gated ion channel complex that controls mitochondrial Ca2+ entry and regulates cell metabolism. MCU and EMRE form the channel while Ca2+-dependent regulation is conferred by MICU1 and MICU2 through an enigmatic process. We present a cryo-EM structure of an MCU-EMRE-MICU1-MICU2 holocomplex comprising MCU and EMRE subunits from the beetle Tribolium castaneum in complex with a human MICU1-MICU2 heterodimer at 3.3 Å resolution. With analogy to how neuronal channels are blocked by protein toxins, a uniporter interaction domain on MICU1 binds to a channel receptor site comprising MCU and EMRE subunits to inhibit ion flow under resting Ca2+ conditions. A Ca2+-bound structure of MICU1-MICU2 at 3.1 Å resolution indicates how Ca2+-dependent changes enable dynamic response to cytosolic Ca2+ signals.

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

Structure and mechanism of the mitochondrial Ca2+ uniporter holocomplex

Mitochondria take up Ca²⁺ through the mitochondrial calcium uniporter complex to regulate energy production, cytosolic Ca²⁺ signalling and cell death^1,2. In mammals, the uniporter complex (uniplex) contains four core components: the pore-forming MCU protein, the gatekeepers MICU1 and MICU2, and an auxiliary subunit, EMRE, essential for Ca²⁺ transport^3-8. To prevent detrimental Ca²⁺ overload, the activity of MCU must be tightly regulated by MICUs, which sense changes in cytosolic Ca²⁺ concentrations to switch MCU on and off^9,10. Here we report cryo-electron microscopic structures of the human mitochondrial calcium uniporter holocomplex in inhibited and Ca²⁺-activated states. These structures define the architecture of this multicomponent Ca²⁺-uptake machinery and reveal the gating mechanism by which MICUs control uniporter activity. Our work provides a framework for understanding regulated Ca²⁺ uptake in mitochondria, and could suggest ways of modulating uniporter activity to treat diseases related to mitochondrial Ca²⁺ overload.

Structure of the MICU1-MICU2 heterodimer provides insights into the gatekeeping threshold shift

Mitochondrial calcium uptake proteins 1 and 2 (MICU1 and MICU2) mediate mitochondrial Ca2+ influx via the mitochondrial calcium uniporter (MCU). Its molecular action for Ca2+ uptake is tightly controlled by the MICU1-MICU2 heterodimer, which comprises Ca2+ sensing proteins which act as gatekeepers at low [Ca2+] or facilitators at high [Ca2+]. However, the mechanism underlying the regulation of the Ca2+ gatekeeping threshold for mitochondrial Ca2+ uptake through the MCU by the MICU1-MICU2 heterodimer remains unclear. In this study, we determined the crystal structure of the apo form of the human MICU1-MICU2 heterodimer that functions as the MCU gatekeeper. MICU1 and MICU2 assemble in the face-to-face heterodimer with salt bridges and me-thio-nine knobs stabilizing the heterodimer in an apo state. Structural analysis suggests how the heterodimer sets a higher Ca2+ threshold than the MICU1 homodimer. The structure of the heterodimer in the apo state provides a framework for understanding the gatekeeping role of the MICU1-MICU2 heterodimer.