Structural Characterization of the N-Terminal Domain of the Dictyostelium discoideum Mitochondrial Calcium Uniporter

The mitochondrial calcium uniporter (MCU) plays a critical role in mitochondrial calcium uptake into the matrix. In metazoans, the uniporter is a tightly regulated multicomponent system, including the pore-forming subunit MCU and several regulators (MICU1, MICU2, and Essential MCU REgulator, EMRE). The calcium-conducting activity of metazoan MCU requires the single-transmembrane protein EMRE. Dictyostelium discoideum (Dd), however, developed a simplified uniporter for which the pore-forming MCU (DdMCU) alone is necessary and sufficient for calcium influx. Here, we report a crystal structure of the N-terminal domain (NTD) of DdMCU at 1.7 angstrom resolution. The DdMCU-NTD contains four helices and two strands arranged in a fold that is completely different from the known structures of other MCU-NTD homologues. Biochemical and biophysical analyses of DdMCU-NTD in solution indicated that the domain exists as high-order oligomers. Mutagenesis showed that the acidic residues Asp60, Glu72, and Glu74, which appeared to mediate the interface II, as observed in the crystal structure, participated in the self-assembly of DdMCU-NTD. Intriguingly, the oligomeric complex was disrupted in the presence of calcium. We propose that the calcium-triggered dissociation of NTD regulates the channel activity of DdMCU by a yet unknown mechanism.