Component of the ERMES/MDM complex, which serves as a molecular tether to connect the endoplasmic reticulum and mitochondria (PubMed:19556461, PubMed:22250200). 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 (PubMed:19556461, PubMed:27469264). MDM12 is required for the interaction of the ER-resident membrane protein MMM1 and the outer mitochondrial membrane-resident beta-barrel protein MDM10. The MDM12-MMM1 subcomplex functions in the major beta-barrel assembly pathway that is responsible for biogenesis of all mitochondrial 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 (By similarity) (PubMed:17410204). Component of the ER-mitochondria encounter structure (ERMES) or MDM complex, composed of MMM1, MDM10, MDM12 and MDM34 (PubMed:13679517, PubMed:17410204, PubMed:19556461). 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 (PubMed:28479252, PubMed:29078410). Interacts with PUF3 (By similarity) (PubMed:17210948). 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. Present with 1070 molecules/cell in log phase SD medium. Belongs to the MDM12 family.