Structural Insights into Mitochondrial Calcium Uniporter Regulation by Divalent Cations

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Physiology

Background:

• Mitochondrial calcium (Ca2+) uptake is crucial for cell death and energy production.
• The mitochondrial calcium uniporter (MCU) controls this flux, but its regulatory mechanisms are not fully understood at the atomic level.

Purpose of the Study:

• To elucidate the atomic mechanisms governing the regulation of the mitochondrial calcium uniporter (MCU).
• To investigate the role of the N-terminal matrix domain and its interaction with divalent cations in MCU activity.

Main Methods:

• Crystal structure determination of the N-terminal matrix domain of human MCU.
• Site-directed mutagenesis to disrupt the acidic face of the matrix domain.
• Biochemical assays to measure MCU activity and cation binding.
• Experiments involving mitochondrial Mg2+ loading and Ca2+ extrusion blockade.

Main Results:

• The crystal structure revealed a β-grasp-like fold with a negatively charged cluster interacting with divalent cations (Ca2+, Mg2+).
• Cation binding destabilizes the matrix domain's oligomerization, favoring a monomeric state.
• Mutations disrupting the acidic face mimicked cation binding effects, weakening oligomerization and decreasing MCU activity.
• Elevated mitochondrial Mg2+ or blocked Ca2+ extrusion reduced MCU Ca2+-uptake rates.

Conclusions:

• The N-terminal matrix domain of MCU contains an acidic patch that acts as a regulatory site.
• Binding of Ca2+ and Mg2+ to this patch inhibits human MCU activity.
• This mechanism provides insight into the control of mitochondrial calcium homeostasis.