Involved in the regulation of mitochondrial fission mediated by DNM1L (PubMed:29083303). Positively regulates mitochondrial translation (PubMed:30215512). May play a role in ribosome biogenesis by preventing premature association of the 28S and 39S ribosomal subunits (Probable). Identified in a complex composed of MALSU1, MIEF1 upstream open reading frame protein and NDUFAB1; within the trimeric complex, MIEF1 upstream open reading frame protein functions as a bridging scaffold that interacts with MALSU1 on one side, and with NDUFAB1 on the other side (PubMed:28892042, PubMed:30215512). Within the complex, MALSU1 mediates interaction with the mitochondrial large ribosomal subunit (PubMed:28892042). Interacts with ribosomal proteins MRPL4 and MRPS27 (PubMed:30215512). Product of the upstream open reading frame of this bicistronic gene. Belongs to the complex I LYR family.