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

Abstract

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

“…Most notably, Lys126 is well positioned to point its side chain directly toward the center of the D-ring and block the channel filter. The same electrostatic interactions have been observed in the recent structure of the human uniplex obtained in low [Ca 2+ ] ( Fan et al, 2020 ) as well as the structure of a holocomplex formed by the beetle MCU-EMRE and the human MICU1-MICU2 under resting [Ca 2+ ] conditions ( Wang et al, 2020 ). In both studies, it has also been shown that mutations of these positively charged residues in MICU1 destabilize the MCU-MICU1 interactions and mitigate the blocking of MCU by MICU1.…”

“…However, to understand the molecular mechanisms of the uniplex assembly and its Ca 2+ -dependent gating necessitates the structural determination of the four-component MCU holocomplex. To this end, several groups have recently reported the structures of the MCU holocomplex in various states, including the structures of the human MCU holocomplex in low and high [Ca 2+ ] ( Fan et al, 2020 ), the structure of human MCU holocomplex in the absence of Ca 2+ ( Zhuo et al, 2020 ), and the structure of a holocomplex formed by the beetle MCU-EMRE and the human MICU1-MICU2 under resting [Ca 2+ ] conditions ( Wang et al, 2020 ). Here, we also present the cryo-EM structures of the human MCU holocomplex in the apo and Ca 2+ -bound states.…”

Cryo-EM structure of the calcium release-activated calcium channel Orai in an open conformation

“…Supporting this, there are no additional hydrogen bonds between the glutamate residues and other amino acids that might stabilize their conformations. This is in marked contrast to another highly selective Ca 2+ channel, the mitochondrial Ca 2+ uniporter, in which the conformations of a ring of glutamate residues in its selectivity filter are stabilized by van der Waals interactions and hydrogen bonds with other amino acids (Baradaran, Wang, Siliciano, & Long, 2018;Fan et al, 2018;Fan et al, 2020;Nguyen et al, 2018; Jacewicz, Delgado, Baradaran, & Long, 2020;Wang et al, 2019;Yoo et al, 2018).…”

Cryo-EM structure of the calcium release-activated calcium channel Orai in an open conformation

“…For example, there are no additional hydrogen bonds between the glutamate residues and other amino acids that might stabilize their conformations. This is in marked contrast to another highly selective Ca 2+ channel, the mitochondrial Ca 2+ uniporter, in which the conformations of a ring of glutamate residues in its selectivity filter are stabilized by van der Waals interactions and hydrogen bonds with other amino acids (Baradaran, Wang, Siliciano, & Long, 2018; C. Fan et al, 2018; M. Fan et al, 2020; Nguyen et al, 2018; C. Wang, Jacewicz, Delgado, Baradaran, & Long, 2020; Y. Wang et al, 2019; Yoo et al, 2018).…”