Structure of MICU from non-metazoan Dictyostelium discoideum reveals unique characteristics

In most eukaryotes, the mitochondrial calcium uniporter (MCU) mediates Ca²⁺ influx into the mitochondrial matrix through a process regulated by MICUs and the EMRE. In Dictyostelium discoideum, a model organism for amoebozoans that lack an EMRE, the MCU complex consists solely of the MCU and MICU. Most likely, therefore, the mechanism by which DdMICU regulates the DdMCU differs from the extensively studied metazoan MCU-EMRE-MICU system. Here, we report the crystal structure of Ca²⁺-bound DdMICU at 2.5 Å resolution. Unlike human MICUs, which contain two Ca²⁺-binding EF-hand motifs, DdMICU possesses three EF-hand motifs, each with a submicromolar Ca²⁺ binding affinity. The overall structure of DdMICU is comparable to that of human MICUs, and their well-conserved dimer interface interactions are similar. In addition to the face-to-face dimer observed in human MICUs, DdMICU forms a head-to-head dimer with multimeric states that equilibrate between tetrameric and dimeric forms, depending on the solution ionic strength. Moreover, the C-helix of DdMICU plays a critical role in membrane binding. These findings provide a molecular basis for the unique mechanism regulating Ca²⁺ uptake by MICUs in an EMRE-free system and offer insight into the evolution and functional diversity of the MCU complex in non-metazoan organisms.