Crystal structure of MICU2 and comparison with MICU1 reveal insights into the uniporter gating mechanism.

The mitochondrial uniporter is a Ca-channel complex resident within the organelle's inner membrane. In mammalian cells the uniporter's activity is regulated by Ca due to concerted action of MICU1 and MICU2, two paralogous, but functionally distinct, EF-hand Ca-binding proteins. Here we present the X-ray structure of the apo form of MICU2 at 2.5-Å resolution. The core structure of MICU2 is very similar to that of MICU1. It consists of two lobes, each containing one canonical Ca-binding EF-hand (EF1, EF4) and one structural EF-hand (EF2, EF3). Two molecules of MICU2 form a symmetrical dimer stabilized by highly conserved hydrophobic contacts between exposed residues of EF1 of one monomer and EF3 of another. Similar interactions stabilize MICU1 dimers, allowing exchange between homo- and heterodimers. The tight EF1-EF3 interface likely accounts for the structural and functional coupling between the Ca-binding sites in MICU1, MICU2, and their complex that leads to the previously reported Ca-binding cooperativity and dominant negative effect of mutation of the Ca-binding sites in either protein. The N- and C-terminal segments of the two proteins are distinctly different. In MICU2 the C-terminal helix is significantly longer than in MICU1, and it adopts a more rigid structure. MICU2's C-terminal helix is dispensable in vitro for its interaction with MICU1 but required for MICU2's function in cells. We propose that in the MICU1-MICU2 oligomeric complex the C-terminal helices of both proteins form a central semiautonomous assembly which contributes to the gating mechanism of the uniporter.