Assembly of ERMES Subunit Mdm12 and its Interaction with Synthetic Lipid Bilayers

The endoplasmic reticulum—mitochondria encounter structure (ERMES) is a tethering complex that mediates the close apposition of ER and mitochondrial membranes. The ERMES complex consists of at least four proteins: the ER transmembrane protein Mmm1, the mitochondrial outer membrane (OM) protein Mdm34, the OM β-barrel protein Mdm10, and the cytosolic protein Mdm12. Accumulating evidence suggests that the ERMES complex provides the molecular basis for non-vesicular phospholipid transport between the ER and mitochondria. Yet how these subunits assemble into a quaternary membrane tether is poorly understood, and the mechanism of ERMES-mediated phospholipid transport has yet to be revealed. Both of these aspects are necessary prerequisites for developing a thorough picture of not only lipid transport at ER—mitochondria contact sites, but of non-vesicular lipid transport in general. Here we show that full-length Mdm12 of S. cerevisiae exists primarily as a disulfide-mediated dimer in dynamic equilibrium. We also demonstrate novel features of Mdm12—bilayer interaction, in particular that Mdm12 induces fusion, lysis, solubilization, and aggregation of synthetic lipid bilayers. The results show that the reductant DTT and calcium both modulate the kinetics of Mdm12 lipid binding. Structural aspects of Mdm12 lipid-binding specificity are also addressed by in silico phospholipid docking