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

Mitochondrial Ca²⁺ uptake plays an important role in cellular physiology such as modulating ATP production, regulating cytoplasmic Ca²⁺ dynamics, and triggering cell death, and is mediated by the mitochondrial calcium uniporter, a highly selective calcium channel localized to the inner mitochondrial membrane. In humans, the uniporter functions as a holocomplex consisting of MCU, EMRE, MICU1 and MICU2, among which MCU and EMRE form a subcomplex and function as the conductive channel while MICU1 and MICU2 are EF-hand proteins that regulate the channel activity in a Ca²⁺ dependent manner. Here we present the EM structures of the human mitochondrial calcium uniporter holocomplex (uniplex) in the presence and absence of Ca²⁺, revealing distinct Ca²⁺ dependent assembly of the uniplex. In the presence of Ca²⁺, MICU1 and MICU2 form a heterotetramer of MICU1-(MICU2)2-MICU1 and bridge the dimeric form of the MCU-EMRE subcomplex through electrostatic interactions between MICU1 and EMRE, leaving the MCU channel pore unblocked. In the absence of Ca²⁺, multiple uniplex assemblies are observed but is predominantly occupied by the MICU1 subunit from a MICU1-MICU2 heterodimer blocking the MCU channel pore. Our structural observations suggest that Ca²⁺ changes the dimerization interaction between MICU1 and MICU2, which in turn determines how the MICU1-MICU2 subcomplex interacts with the MCU-EMRE channel and, consequently, changes the distribution of the uniplex assemblies between the blocked and unblocked states.