MICU1 and MICU2 Finely Tune the Mitochondrial Ca2+ Uniporter by Exerting Opposite Effects on MCU Activity

Abstract

SummaryMitochondrial calcium accumulation was recently shown to depend on a complex composed of an inner-membrane channel (MCU and MCUb) and regulatory subunits (MICU1, MCUR1, and EMRE). A fundamental property of MCU is low activity at resting cytosolic Ca2+ concentrations, preventing deleterious Ca2+ cycling and organelle overload. Here we demonstrate that these properties are ensured by a regulatory heterodimer composed of two proteins with opposite effects, MICU1 and MICU2, which, both in purified lipid bilayers and in intact cells, stimulate and inhibit MCU activity, respectively. Both MICU1 and MICU2 are regulated by calcium through their EF-hand domains, thus accounting for the sigmoidal response of MCU to [Ca2+] in situ and allowing tight physiological control. At low [Ca2+], the dominant effect of MICU2 largely shuts down MCU activity; at higher [Ca2+], the stimulatory effect of MICU1 allows the prompt response of mitochondria to Ca2+ signals generated in the cytoplasm.

Strategic Positioning and Biased Activity of the Mitochondrial Calcium Uniporter in Cardiac Muscle

“…Higher molecular mass (ϳ720 kDa) complexes have also been described, and their existence depends on EMRE and MICU1/2 (3,4,34). Consistently, size exclusion chromatography suggested very large complexes incorporating a major portion of the MCU and EMRE pool to be on a par with the blue dextran (2 MDa).…”

Reduced mitochondrial Ca2+ transients stimulate autophagy in human fibroblasts carrying the 13514A>G mutation of the ND5 subunit of NADH dehydrogenase

“…MICU2 is a negative MCU modulator as it forms the MICU1/ MICU2 heterodimer, thus sequestering MICU1 and preventing the MICU1 homodimer stimulatory activity on MCU, as we recently demonstrated. 19 Indeed, we detected the presence of MICU1 homodimers in control but not in Pat#1 fibroblasts (Supplementary Figure S3h). Such an imbalance of MCU regulators could explain the reduced efficiency of mitochondrial Ca 2+ uptake in 13514A4G cells.…”

“…In addition, we recently reported that MICU1 homodimer and MICU1/MICU2 heterodimer regulates MCU sensitivity to extramitochondrial [Ca 2+ ] and exert opposite effects on the activity of the channel (stimulatory for MICU1 and inhibitory for MICU2). 19 The analysis of the ratio between MICU1/MICU1 and MICU1/MICU2 dimers in nonreducing conditions showed that two bands were present in control fibroblasts, corresponding to the MICU1/MICU2 heterodimer and the MICU1 homodimer. Conversely, Pat#1 cells lacked the MICU1 homodimer (Supplementary Figure S3h), which is responsible for the rapid and efficient mitochondrial Ca 2+ uptake.…”

“…The pore region is composed of MCU, its isoform MCUb 14 and essential MCU regulator (EMRE). 15 The channel is gated by the Ca 2+ -sensitive proteins mitochondrial Ca 2+ uptake 1 (MICU1) and MICU2 [16][17][18][19] and further regulated by the SLC25A23 protein. 20 As to its cellular function, mitochondrial Ca 2+ has been shown to stimulate ATP production by positive regulation of three key dehydrogenases of the tricarboxylic acid cycle 21 and of the ETC.…”

The EF-Hand Ca2+ Binding Protein MICU Choreographs Mitochondrial Ca2+ Dynamics in Arabidopsis

“…The recent identification of several protein families making up the mitochondrial Ca 2+ uniporter complex (MCUC) in mammals, including the pore-forming MCU (MCU and MCUb;Baughman et al, 2011;De Stefani et al, 2011;Raffaello et al, 2013), and the regulatory, associated MICUs (MICU1, 2, and 3; Perocchi et al, 2010;Plovanich et al, 2013), EMRE (Sancak et al, 2013), and MCUR1/CCDC90A (although potentially not directly; Mallilankaraman et al, 2012b;Paupe et al, 2015), has spurred intense research across the biomedical disciplines Mallilankaraman et al, 2012a;Csordás et al, 2013;Hoffman et al, 2013;Marchi et al, 2013;Pan et al, 2013;Raffaello et al, 2013;Kovács-Bogdán et al, 2014;Logan et al, 2014;Patron et al, 2014;Wang et al, 2014). While some genes of the complex components appear not to be present in plants (e.g., MCUb and EMRE), others have multiple homologs (six for the functional poreforming subunit MCU in Arabidopsis [Stael et al, 2012] and maize [Zea mays; Meng et al, 2015] and two for MCUR1/CCDC90A in Arabidopsis), suggesting potential functional modification and/or differentiation.…”