Required for normal mitochondrial ribosome function and mitochondrial translation. May play a role in ribosome biogenesis by preventing premature association of the 28S and 39S ribosomal subunits. Interacts with mitochondrial ribosomal protein L14 (MRPL14), probably blocking formation of intersubunit bridge B8, preventing association of the 28S and 39S ribosomal subunits. Addition to isolated mitochondrial ribosomal subunits partially inhibits translation, probably by interfering with the association of the 28S and 39S ribosomal subunits and the formation of functional ribosomes. May also participate in the assembly and/or regulation of the stability of the large subunit of the mitochondrial ribosome. May function as a ribosomal silencing factor. Associates with the mitochondrial ribosome large subunit (39S) via interaction with MRPL12 and/or MRPL14. The interaction generates steric hindrance that is expected to prevent premature association of the 28S and 39S ribosomal subunits. 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. Interacts with MRPL12 and MRPL14. Colocalizes with MRPL12 and/or MRPL14. Belongs to the Iojap/RsfS family.