Background:
MICU1 is an EF-hand domain containing Ca
2+
-sensor regulating the mitochondrial Ca
2+
uniporter channel and mitochondrial Ca
2+
uptake. MICU1-null mouse and fly models display perinatal lethality with disorganized mitochondrial architecture. Interestingly, these phenotypes are distinct from other mtCU loss-of-function models (
MCU, MICU2, EMRE, MCUR1
) and thus are likely not explained solely by changes in matrix Ca
2+
content. Using size-exclusion proteomics and co-immunofluorescence, we found that MICU1 localizes to mitochondrial complexes lacking MCU. These observations suggest that MICU1 may have additional cellular functions independent of the MCU.
Methods:
Biotin-based proximity labeling and proteomics, protein biochemistry, live-cell Ca
2+
imaging, electron microscopy, confocal and super-resolution imaging were utilized to identify and validate MICU1 novel functions.
Results:
The expression of a MICU1-BioID2 fusion protein in
MCU
+/+
and
MCU
-/-
cells allowed the identification of the total vs. MCU-independent MICU1 interactome. LC-MS analysis of purified biotinylated proteins identified the mitochondrial contact site and cristae organizing system (MICOS) components Mitofilin (MIC60) and Coiled-coil-helix-coiled-coil helix domain containing 2 (CHCHD2) as MCU independent novel MICU1 interactors. We demonstrate that MICU1 is essential for proper organization of the MICOS complex and that MICU1 ablation results in altered cristae organization, mitochondrial ultrastructure, mitochondrial membrane dynamics, membrane potential, and cell death signaling. We hypothesize that MICU1 is a MICOS Ca
2+
- sensor since perturbing MICU1 is sufficient to modulate cytochrome c release independent of Ca
2+
uptake across the inner mitochondrial membrane.
Conclusions:
Here, we provide the first experimental evidence of an intermembrane space Ca
2+
- sensor regulating mitochondrial membrane dynamics, independent of changes in matrix Ca
2+
content. This study provides a novel paradigm to understand Ca
2+
-dependent regulation of mitochondrial structure and function and may help explain the mitochondrial remodeling reported to occur in numerous disease states.
Studies on mitochondrial structure and function in Physarum polycephalum. V. Behaviour of mitochondrial nucleoids throughout mitochondrial division cycle.
