Required for mitochondrial morphology. With UPS1 and UPS2, controls phospholipid metabolism in the mitochondrial intermembrane space. Interacts with MDM35. Lacks the two major intermembrane space-targeting signals, bipartite presequences and cysteine motifs, and import is mediated by another IMS protein, MDM35. Present with 672 molecules/cell in log phase SD medium. Belongs to the slowmo family.