Structural and Biochemical Studies of the Non-metazoan MCU Complex

Abstract

The mitochondrial calcium uniporter (MCU) mediates Ca2+ influx into the mitochondrial matrix, a process regulated by MICUs and EMRE. In Dictyostelium discoideum, a model organism for amoebozoans lacking of an EMRE, the MCU complex comprises exclusively the MCU and MICU. Consequently, the mechanism by which DdMICU modulates DdMCU is likely distinct from the widely studied metazoan MCU-EMRE-MICU system. In this study, I determined the crystal structure of Ca2+-bound DdMICU at a resolution of 2.5 Å. In contrast to human MICUs, which feature two Ca2+-binding EF-hand motifs, DdMICU contains three EF-hand motifs, each exhibiting a sub- micromolar Ca2+ binding affinity. The overall structure of DdMICU is comparable to that of human MICUs, and its highly conserved dimer interface interactions exhibit consistency. In addition to the face-to-face dimer identified in human MICUs, DdMICU produces a head-to-head dimer with multimeric states that equilibrate between tetrameric and dimeric forms, depending on the ionic strength of the solution. Furthermore, the C-helix of DdMICU is essential for membrane binding. Using the DdMICU structure together with sequence- and structure-based models of DdMCU, I constructed a non-metazoan MCU complex model that highlights potential MCU–MICU interactions and stoichiometries in the absence of EMRE. These findings elucidate the molecular basis for the distinctive mechanism regulating Ca2+ uptake by MICUs in an EMRE-deficient system and enhance understanding of the evolution and functional variety of the MCU complex in non-metazoan organisms.