Structure of the MICU1–MICU2 heterodimer provides insights into the gatekeeping threshold shift

Abstract

Mitochondrial calcium uptake proteins 1 and 2 (MICU1 and MICU2) mediate mitochondrial Ca2+ influx via the mitochondrial calcium uniporter (MCU). Its molecular action for Ca2+ uptake is tightly controlled by the MICU1–MICU2 heterodimer, which comprises Ca2+ sensing proteins which act as gatekeepers at low [Ca2+] or facilitators at high [Ca2+]. However, the mechanism underlying the regulation of the Ca2+ gatekeeping threshold for mitochondrial Ca2+ uptake through the MCU by the MICU1–MICU2 heterodimer remains unclear. In this study, we determined the crystal structure of the apo form of the human MICU1–MICU2 heterodimer that functions as the MCU gatekeeper. MICU1 and MICU2 assemble in the face-to-face heterodimer with salt bridges and methionine knobs stabilizing the heterodimer in an apo state. Structural analysis suggests how the heterodimer sets a higher Ca2+ threshold than the MICU1 homodimer. The structure of the heterodimer in the apo state provides a framework for understanding the gatekeeping role of the MICU1–MICU2 heterodimer.

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

“…The interfacial residues are conserved in MICU3 ( Figure 4—figure supplement 2 ), which suggests that the neuron-specific MICU1-MICU3 heterodimer has a similar arrangement. As would be expected under resting [Ca 2+ ] conditions, all four Ca 2+ -binding EF-hand motifs of the MICU1-MICU2 heterodimer (‘EF1’ and ‘EF4’ of both subunits) are in the apo state and the heterodimer is superimposable with a previous apo structure of MICU1-MICU2 alone ( Park et al, 2020 ; Figure 1—figure supplement 6 , Figure 4—figure supplement 3A ).…”

“…Patron et al, 2019;Payne et al, 2017;Phillips et al, 2019). While structures of MICU1-3 and MCU-EMRE components have been determined (Kamer, Jiang, Kaushik, Mootha, & Grabarek, 2019;Park et al, 2020;C. Wang, Baradaran, & Long, 2020;L.…”

“…At the ~100 nM resting level of [Ca 2+ ] cytosol , MICU1-MICU2 and MICU1-MICU3 heterodimers prevent ion conduction through the channel, and they permit it when Ca 2+ levels rise ( Csordás et al, 2013 ; Kamer et al, 2017 ; Kamer and Mootha, 2014 ; Liu et al, 2016 ; Mallilankaraman et al, 2012b ; Paillard et al, 2017 ; Patron et al, 2014 ; Patron et al, 2019 ; Payne et al, 2017 ; Phillips et al, 2019 ). While structures of MICU1-3 and MCU-EMRE components have been determined ( Kamer et al, 2019 ; Park et al, 2020 ; Wang et al, 2020 ; Wang et al, 2014 ; Wang et al, 2019 ; Wu et al, 2019 ; Xing et al, 2019 ), it is unclear how MICU proteins interact with the channel and the mechanism by which they confer Ca 2+ -dependent control to the channel is enigmatic. Further it is not known why MICU1, in particular, is necessary among MICU1-3 for regulating the channel ( Kamer and Mootha, 2014 ; Patron et al, 2014 ; Patron et al, 2019 ; Payne et al, 2017 ; Xing et al, 2019 ).…”

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

“…MICU1 and MICU2 dimerize through a face-to-face contact in which the EF-1 motif of one subunit interacts with the EF-3 of the other ( Figure 2c ). This face-to-face dimerization mode has been commonly observed in the crystal structures of MICU1 and MICU2 homodimers as well as the MICU1-MICU2 heterodimer ( Park et al, 2020 ; Wang et al, 2014 ; Wu et al, 2019 ; Xing et al, 2019 ). Major Ca 2+ -induced conformational changes also occur at the interface between MICU1 and MICU2 as will be further discussed later.…”

“…Despite their low overall resolutions, the EM density for the MCU-EMRE subcomplex is well defined in all of the maps and can be perfectly modeled by rigid-body docking of the previously determined structure with minor adjustments ( Wang et al, 2019 ). In addition, multiple crystal structures of human MICU1 and MICU2 homodimer in both the apo and Ca 2+ -bound states, as well as the apo MICU1-MICU2 heterodimer have been determined ( Kamer et al, 2019 ; Park et al, 2020 ; Wang et al, 2014 ; Wu et al, 2019 ; Xing et al, 2019 ), thus facilitating our model building for the MICU1 and MICU2 part of the uniplex ( Figure 1—figure supplement 6 ). As the structures of the individual MICU1, MICU2 and MCU-EMRE subcomplex have all been extensively described before, our discussion will be focused on how MICU1 and MICU2 interact with MCU-EMRE to form a uniplex assembly in a Ca 2+ -dependent manner.…”