8.A.45 The Essential MCU Regulator EMRE (EMRE) Family The mitochondrial uniporter (1.A.77) is a highly selective calcium channel in the organelle's inner membrane. Its molecular components include the two EF-hand-containing calcium-binding proteins, mitochondrial calcium uptake 1 (MICU1) and MICU2 as well as the oligomeric pore-forming subunit, mitochondrial calcium uniporter (MCU) in animals. Sancak et al. 2013 achieved a full molecular characterization of the uniporter holocomplex. Quantitative mass spectrometry of the affinity-purified complex revealed the presence of MICU1 and MICU2 (TC#8.A.44), MCU and its paralog MCUb (1.A.77), and an essential MCU regulator (EMRE), a 10-kilodalton protein with a single transmembrane domain. In its absence, uniporter channel activity was lost despite intact MCU expression and oligomerization. EMRE was required for the interaction of MCU with MICU1 and MICU2. Hence, EMRE is essential for in vivo uniporter current and bridges the calcium-sensing role of MICU1 and MICU2 with the calcium-conducting role of MCU (Sancak et al. 2013). EMRE interacts with MCU and MICU via its transmembrane helices. It maintains tight MICU regulation of the MCU pore, a role that involves EMRE binding to MICU1 using its conserved C-terminal polyaspartate tail. EMRE ensures that all transport-competent uniporters are tightly regulated, responding appropriately to a dynamic intracellular Ca²⁺ landscape (Tsai et al. 2016). It is essential for MCU activity in mice (Liu et al. 2020). EMRE controls MCU activity via its transmembrane helix, while using an N-terminal PKP motif to strengthen binding with MCU. Opening of MCU requires hydrophobic interactions mediated by MCU residues near the pore's luminal end. Enhancing these interactions by single mutation allows human MCU to transport Ca²⁺ without EMRE. EMRE may thus facilitate MCU opening by stabilizing the open state in a conserved MCU gating mechanism, present also in non-metazoan MCU homologs (Van Keuren et al. 2020). Effects of cardiac mitochondrial calcium uniporter inactivation via EMRE deletion have been documented (Chapoy Villanueva et al. 2023).
References:
Chapoy Villanueva, H., J.H. Sung, J.A. Stevens, M.J. Zhang, P.M. Nelson, L.S. Denduluri, F. Feng, T.D. O''Connell, D. Townsend, and J.C. Liu. (2023). Distinct effects of cardiac mitochondrial calcium uniporter inactivation via EMRE deletion in the short and long term. J Mol. Cell Cardiol 181: 33-45.
Liu, J.C., N.C. Syder, N.S. Ghorashi, T.B. Willingham, R.J. Parks, J. Sun, M.M. Fergusson, J. Liu, K.M. Holmström, S. Menazza, D.A. Springer, C. Liu, B. Glancy, T. Finkel, and E. Murphy. (2020). EMRE is essential for mitochondrial calcium uniporter activity in a mouse model. JCI Insight 5:.
MacEwen, M.J., A.L. Markhard, M. Bozbeyoglu, F. Bradford, O. Goldberger, V.K. Mootha, and Y. Sancak. (2020). Evolutionary divergence reveals the molecular basis of EMRE dependence of the human MCU. Life Sci Alliance 3:.
Sancak, Y., A.L. Markhard, T. Kitami, E. Kovács-Bogdán, K.J. Kamer, N.D. Udeshi, S.A. Carr, D. Chaudhuri, D.E. Clapham, A.A. Li, S.E. Calvo, O. Goldberger, and V.K. Mootha. (2013). EMRE is an essential component of the mitochondrial calcium uniporter complex. Science 342: 1379-1382.
Tsai, M.F., C.B. Phillips, M. Ranaghan, C.W. Tsai, Y. Wu, C. Willliams, and C. Miller. (2016). Dual functions of a small regulatory subunit in the mitochondrial calcium uniporter complex. Elife 5:.
Vais, H., R. Payne, U. Paudel, C. Li, and J.K. Foskett. (2020). Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca uptake. Proc. Natl. Acad. Sci. USA 117: 21731-21739.
Van Keuren, A.M., C.W. Tsai, E. Balderas, M.X. Rodriguez, D. Chaudhuri, and M.F. Tsai. (2020). Mechanisms of EMRE-Dependent MCU Opening in the Mitochondrial Calcium Uniporter Complex. Cell Rep 33: 108486.
Examples:
TC#	Name	Organismal Type	Example
8.A.45.1.1	The single-pass Essential MCU REgulator membrane protein with aspartate-rich tail of 107 aas, EMRE or MICU1/2 (Sancak et al. 2013). Also called the chromosome 22 ORF32 protein. Functions to interlink MICU1 and 2 to MCU in the mitorchondrial uniporter complex (TC# 1.A.77 (Vais et al. 2020). In amoeba and fungi, MCU homologs are sufficient to form a functional calcium channel, whereas human MCU exhibits a strict requirement for EMRE for conductance. EMRE directly interacts with human MCU at its transmembrane domains as well as the 10 aa interaction domain (MacEwen et al. 2020). EMRE stabilizes the EDD of MCU, permitting both channel opening and calcium conductance, consistent with recently published structures of MCU-EMRE (MacEwen et al. 2020). See the family description of MCU (1.A.77) for more deetails.	Animals	EMRE of Homo sapiens

8.A.45.1.2	Mitochondrial portein SMDT1 homologue	Animals	SMDT1 homologue of Drosophila melanogaster

8.A.45.1.3	Tag-299 protein of 90 aas	Animals	Tag-299 as Caenorhabditis elegans

8.A.45.1.4	EMRE homologue of 91 aas	Animals	*EMRE homologue of Strongylocentrotus purpuratus* (Purple sea urchin)**

8.A.45.1.5	Uncharacterized protein of 81 aas	Echthyosporea	UP of Capsaspora owczarzaki

8.A.45.1.6	EMRE regulatory subunit of CMU (TC# 1.A.77.1.17). MCU and EMRE form the minimal functional unit of the mitochondrial calcium uniporter complex in metazoans. Wang et al. 2020 functionally reconstituted an MCU-EMRE complex from the red flour beetle, Tribolium castaneum, and determined a cryo-EM structure of the complex at 3.5 Å resolution. They demonstrated robust Ca²⁺ uptake into proteoliposomes containing the purified complex. Uptake depended on EMRE and also on mitochondrial cardiolipin. The structure reveals a tetrameric channel with a single ion pore. EMRE is located at the periphery of the transmembrane domain and associates primarily with the first TMS of MCU. Coiled-coil and juxtamembrane domains within the matrix portion of the complex adopt markedly different conformations than in a structure of a human MCU-EMRE complex, suggesting that the structures represent different conformations of these functionally similar metazoan channels (Wang et al. 2020).		EMRE of Tribolium castaneum

Examples:
TC#	Name	Organismal Type	Example
8.A.45.2.1	Uncharacterized protein of 163 aas.	Choanoflagellida	UP of Monosiga brevicollis

8.A.45.2.2	Uncharacterized protein of 89 aas	Choanoflagellida	UP of Salpingoeca rosetta