Molecular characterization of the N-terminal domains of the human mitochondrial calcium uniporter (MCU) and its paralog, MCUb

Mitochondrial calcium (Ca2+) homeostasis is essential for cell survival and death. The mitochondrial calcium uniporter(MCU) homo-tetramer is a selective Ca2+ channel responsible for mitochondrial Ca2+ uptake. Its paralog, MCUb, can form a Ca2+-impermeable hetero-tetramer with MCU exerting a dominant-negative function. We report the structural modeling for MCUb N-terminal domain (NTD), which showed 83% sequence homology with MCU NTD. The MCUb NTD model structure forms a stable globular domain via a hydrophobic interior and contains conserved putative phosphorylation and ubiquitination sites. The MCUb NTD structure is similar to that of MCU NTD with a root mean square deviation (RMSD) of 0.3 Å for 96 Cα atoms. The MCUb NTD forms a stable heterodimer with MCU NTD mainly by electrostatic interaction, although both NTDs are stable as monomers. Besides, our cross-linking results reveal that both NTDs form high-order oligomers. Our findings suggest that stable heterodimer formation of MCU-MCUb NTDs affects MCU NTD clustering, resulting in inefficient clustering of MCU for efficient Ca2+ permeation at the high [Ca2+] microdomain.