Required for normal mitochondrial ribosome function and mitochondrial translation (PubMed:22238375, PubMed:23171548). May play a role in ribosome biogenesis by preventing premature association of the 28S and 39S ribosomal subunits (Probable). Interacts with mitochondrial ribosomal protein uL14m (MRPL14), probably blocking formation of intersubunit bridge B8, preventing association of the 28S and 39S ribosomal subunits (Probable). 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 (Probable). May also participate in the assembly and/or regulation of the stability of the large subunit of the mitochondrial ribosome (PubMed:22238376, PubMed:23171548). May function as a ribosomal silencing factor (Probable). {ECO:0000269|PubMed:22238375, ECO:0000269|PubMed:22238376, ECO:0000269|PubMed:23171548, ECO:0000305|PubMed:22829778, ECO:0000305|PubMed:28892042}. This is the function of MALSU1 (mitochondrial assembly of ribosomal large subunit 1, ENSG00000156928).