1.A.77 The Mg2+ /Ca2+ Uniporter (MCU) Family
Mitochondrial Ca2+ homeostasis plays a key role in the regulation of aerobic metabolism and cell survival. Mitochondrial Ca2+ uptake occurs via the ruthenium red sensitive Ca2+ uniporter (MCU), and efflux occurs via a Na+/Ca2+ exchanger (mNCX). De Stefani et al. (2011) and Baughman et al. (2011) simultaneously reported the identity of a protein (named MCU) that shares tissue distribution with MICU1 (also known as CBARA1), a uniporter regulator that is present in organisms in which mitochondrial Ca2+ uptake was demonstrated and whose sequence includes two transmembrane domains. Short interfering RNA (siRNA) silencing of MCU reduced mitochondrial Ca2+ uptake, and MCU overproduction increased the matrix Ca2+ concentration evoked by inositol 1,4,5-trisphosphate-generating agonists. The purified MCU protein showed channel activity in planar lipid bilayers, with electrophysiological properties and inhibitor sensitivities of the uniporter 90. A mutant MCU, in which two negatively charged residues of the putative pore-forming region were replaced, had no channel activity and reduced agonist-dependent matrix Ca2+ concentration transients when overexpressed. Thus, MCU is the channel responsible for ruthenium-red-sensitive mitochondrial Ca2+ uptake. Distant prokaryotic homologues have been identified (A. Lee and MH Saier, unpublished results). A small (10 KDa) protein with 1 TMS, EMRE, is required for interaction of MCU with MICU1 and 2 (see figure4F in Sancak et al. 2013). The entire complex includes MICU1, MICU2, EMRE and MCU, and has a molecular size of 480,000 Daltons. The MICU family has TC# 8.A. and functions in regulation. The EMRE family has TC# and funtions to interconnect MUC and MICU in the complex (Sancak et al. 2013).
Mitochondrial calcium uniporter, MCU, forms oligomers in the mitochondrial inner membrane, physically interacts with MICU1 (1.A.76.1.1), and resides within a large molecular weight complex. Silencing MCU in cultured cells or in vivo in mouse liver severely abrogates mitochondrial Ca2+ uptake, whereas mitochondrial respiration and membrane potential remain fully intact. MICU1 (TC# 8.A.44.1.1) is an essential component of the MCU system, and serves as the gatekeeper of MCU-mediated Ca2+ uptake that is essnetial to prevent Ca2+ overload and associated stress (Mallilankaraman et al., 2012a; Mallilankaraman et al., 2012b). The oligomeric channel can incorporate an inhibitory subunit, MCUb, that exerts a dominant-negative effect on channel formation (Raffaello et al., 2013). Also essential is the EMRE subunit (TC# 8.A.45.1.1) which binds MICU1 via transmembrane helices to control Ca2+ transport activity (Tsai et al. 2016).
MCU has two predicted transmembrane helices, which are separated by a highly conserved linker facing the intermembrane space. Acidic residues in this linker are required for its full activity. However, an S259A point mutation retained function but confered resistance to Ru360, the most potent inhibitor of the uniporter (Baughman et al., 2011). MCU homologues and their cytoplasmic regulatory protein partners with two EF-hand motifs, MICU, are both present in many but not all eukaryotes having MCU (Bick et al., 2012). The phylogenies and domain orders of MCU Ca2+ channel homologues have been reported (Bick et al., 2012).
The generalized reaction catalyzed by MCU is:
Ca2+ (cytoplasm) → Ca2+ (mitochondrial matrix)
The atp operon of alkaliphilic Bacillus pseudofirmus OF4, as in most prokaryotes, contains the eight structural genes for the F-ATPase (ATP synthase), which are preceded by an atpI gene that encodes a membrane protein with 2 TMSs. A tenth gene, atpZ, has been found in this operon, which is upstream of and overlapping with atpI (Hicks et al., 2003). Most Bacillus species, and some other bacteria, possess atpZ homologues. AtpZ is predicted to be a membrane protein with a hairpin topology. Deletion of atpZ, atpI, or atpZI from B. pseudofirmus OF4 led to a requirement for a greatly increased concentration of Mg2 for growth at pH 7.5. Either atpZ, atpI, or atpZI complemented the similar phenotype of a triple mutant of Salmonella typhimurium(MM281), which is deficient in Mg2+ uptake. atpZ and atpI, separately and together, increased the Mg2+ -sensitive 45Ca2+ uptake by vesicles of an Escherichia coli mutant that is defective in Ca2+ and Na+ efflux. Hicks et al. (2003) hypothesized that AtpZ and AtpI, as homooligomers, and perhaps as heterooligomers, are Mg2+ transporters, Ca2+ transporters, probable Mg2+/Ca2+ channel proteins. Such proteins could provide Mg2+ , which is required by ATP synthase, and also support charge compensation, when the enzyme is functioning in the hydrolytic direction e.g., during cytoplasmic pH regulation. AtpZ and AtpI have 2 and 4 TMSs respectively.
The Na+ F1FO ATP synthase of the anaerobic acetogenic bacterium Acetobacterium woodii has a unique hybrid rotor that contains nine copies of a FO-like c subunit with 1 ion binding site in the 2TMS protein, and one copy of a VO-like c(1) subunit with one ion binding site in four transmembrane helices. Brandt et al. (2013) cloned and expressed its Na+ F1FO ATP synthase operon in E. coli. A Δatp mutant of E. coli produced a functional, membrane-bound Na+ F1FO ATP synthase that was purified in a single step after inserting a His(6)-tag to its β subunit. The purified enzyme was competent in Na+ transport and contained the FOVO hybrid rotor in the same stoichiometry as in A. woodii. Deletion of the atpI gene from the A. woodii operon resulted in a loss of the c ring and a mis-assembled Na+ F1FO ATP synthase. AtpI from E. coli did not substitute for AtpI from A. woodii. Thus, the native AtpI is required for assembly of the hybrid rotor.
The uniporter is membrane potential dependent and sensitive to ruthenium red and its derivative Ru360. It has high conductance and selectivity. Ca2+ entry into mitochondria activates the tricarboxylic acid cycle and seems to be crucial for matching the production of ATP in mitochondria with its cytosolic demand. MCU is the pore-forming subunit of the uniporter holocomplex. Oxenoid et al. 2016 reported the structure of the pore domain of MCU from Caenorhabditis elegans, determined using nuclear magnetic resonance (NMR) and electron microscopy (EM). MCU is a homo-oligomer in which the second transmembrane helix forms a hydrophilic pore. The channel assembly displays a unique ion channel architecture and is stabilized by a coiled-coil motif protruding into the mitochondrial matrix. The critical DXXE motif forms the pore entrance, which features two carboxylate rings; these rings appear to form the selectivity filter (Oxenoid et al. 2016).
The generalized reaction believed to be catalyzed by (AtpZ)n, (AtpI)n and (AtpZ)n-x AtpIx is:
Mg2+ or Ca2+ (out) ⇌ Mg2+ or Ca2+ (in)