Abstract
The mitochondrial calcium uniporter is a Ca2+ channel that regulates intracellular Ca2+ signaling, oxidative phosphorylation, and apoptosis. It contains the pore-forming MCU protein, which possesses a DIME sequence thought to form a Ca2+ selectivity filter, and also regulatory EMRE, MICU1, and MICU2 subunits. To properly carry out physiological functions, the uniporter must stay closed in resting conditions, becoming open only when stimulated by intracellular Ca2+ signals. This Ca2+-dependent activation, known to be mediated by MICU subunits, is not well understood. Here, we demonstrate that the DIME-aspartate mediates a Ca2+-modulated electrostatic interaction with MICU1, forming an MICU1 contact interface with a nearby Ser residue at the cytoplasmic entrance of the MCU pore. A mutagenesis screen of MICU1 identifies two highly-conserved Arg residues that might contact the DIME-Asp. Perturbing MCU-MICU1 interactions elicits unregulated, constitutive Ca2+ flux into mitochondria. These results indicate that MICU1 confers Ca2+-dependent gating of the uniporter by blocking/unblocking MCU.
Highlights
The mitochondrial calcium uniporter is a multi-subunit Ca2+-activated Ca2+ channel complex located in the inner mitochondrial membrane (IMM)
We performed co-immunoprecipitation (CoIP) experiments to examine complex formation between human MICU1 and various MCU homologues in MCU/EMRE-KO HEK 293 cells (Tsai et al, 2016)
The EMRE gene is deleted because EMRE can bind both MCU and MICU1 (Figure 1) (Tsai et al, 2016; Sancak et al, 2013), and would complicate assessment of direct MCU-MICU1 contacts
Summary
The mitochondrial calcium uniporter is a multi-subunit Ca2+-activated Ca2+ channel complex located in the inner mitochondrial membrane (IMM). The uniporter is tightly regulated by intracellular Ca2+ signals It stays quiescent in resting cellular conditions, and becomes activated only when IMS Ca2+ increases to low micromolar levels (Csordas et al, 2013; Mallilankaraman et al, 2012). This Ca2+-dependent gating is mediated by Phillips et al eLife 2019;8:e41112. Mutations that disrupt the MCU-MICU1 interaction severely perturbs Ca2+-regulation of the uniporter These results led to a molecular mechanism in which MICUs open or close the uniporter in response to intracellular Ca2+ signals by physically blocking or unblocking the MCU pore
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