Component of the ERMES/MDM complex, which serves as a molecular tether to connect the endoplasmic reticulum (ER) and mitochondria. Components of this complex are involved in the control of mitochondrial shape and protein biogenesis, and function in nonvesicular lipid trafficking between the ER and mitochondria. The MDM12-MMM1 subcomplex functions in the major beta-barrel assembly pathway that is responsible for biogenesis of all outer membrane beta-barrel proteins, and acts in a late step after the SAM complex. The MDM10-MDM12-MMM1 subcomplex further acts in the TOM40-specific pathway after the action of the MDM12-MMM1 complex. Essential for establishing and maintaining the structure of mitochondria and maintenance of mtDNA nucleoids. Homodimer. Component of the ER-mitochondria encounter structure (ERMES) or MDM complex, composed of MMM1, MDM10, MDM12 and MDM34. A MMM1 homodimer associates with one molecule of MDM12 on each side in a pairwise head-to-tail manner, and the SMP-LTD domains of MMM1 and MDM12 generate a continuous hydrophobic tunnel for phospholipid trafficking. The ERMES/MDM complex localizes to a few discrete foci (around 10 per single cell), that represent mitochondria-endoplasmic reticulum junctions. These foci are often found next to mtDNA nucleoids. The SMP-LTD domain is a barrel-like domain that can bind various types of glycerophospholipids in its interior and mediate their transfer between two adjacent bilayers. Belongs to the MMM1 family.