| Members in the YjgF/YER057c/UK114 family are highly conserved in archaea, bacteria and eukarya. All members of this have similar secondary structures and a molecular mass of approximately 15 kDa. They share a highly conserved primary sequence with an identity from 47 to 78%, but possess diverse biological functions. The human homolog hp14.5 has been proposed as a putative translation inhibitor that can inhibit cell-free protein synthesis in the rabbit reticulocyte lysate system, whereas the rat homolog rp14.5 has endoribonuclease activity, and the goat homolog UK114 has tumor antigen activity. Biological functions of other homologs include the calpain activation in the bovine, the purine regulation activity in Bacillus subtilis, and photosynthesis and chromoplastogenesis in plants. A line of evidences suggested that subtle sequence changes attribute the functional divergence among the members. To date, about twenty structures of this family have been determined, all of which adopt a trimeric structure with three clefts, each of which is characterized by 6–9 signature residues. These conserved clefts are able to bind diverse ligands, including L-threonine, L-serine, 2-ketobutyrate, ethylene glycol, or propionate. However, the physiological significance of these members remains unclear. The homolog from the yeast Saccharomyces cerevisiae was named Mmf1, for mitochondrial matrix factor. Mmf1 was proposed to be involved in isoleucine biosynthesis and intact mitochondria maintenance.Tandem affinity purification assays indicated that Mmf1 physically interacted with a mitochondrial acidic matrix protein Mam33, suggesting their putative functional relevance. Mam33 was found to be involved in sorting cytochrome b2 to the mitochondrial intermembrane space via binding to its signal peptide. Deletion of MAM33 gene would lead to a slower growth rate of yeast in glycerol medium but not in glucose medium, suggesting that Mam33 may participate in the mitochondrial oxidative phosphorylation. Mam33 was identified as a homo-trimer or tetramer and shared a sequence identity of approximately 24–35% with the homologs from human, Leishmania major, Trypanosoma brucei, and Caenorhabditis elegans. The human homolog of Mam33 was termed splicing factor 2-associated protein p32, which was proposed to be involved in the maintenance of mitochondrial oxidative phosphorylation. The human p32 executes diverse functions via binding to various partners. Up to date, a total of 82 protein partners were identified to have physical or genetic interactions with yeast Mam33. Among them, 23 partners are mitochondrial ribosomal proteins. This led us to presume that, with the assistance of Mmf1, Mam33 may take part in mitochondrial ribosomal protein synthesis.To figure out the structural insights into the interaction pattern between Mmf1 and Mam33, we attempted to solve the crystal structure of their complex. We purified the complex of Mam33–Mmf1, but we were unable to obtain its crystal. Nevertheless, we solved the crystal structures of individual Mmf1 and Mam33 at 1.74 and 2.10 (A|°), respectively. In vitro biochemical assays demonstrated that Mmf1 and Mam33 can form a complex with a molar ratio of 2 to 1. Structural analysis revealed the positively charged surfaces of Mmf1 trimer are complementary to the negatively charged patches of Mam33 trimer. In addition, a docking model of Mmf1–Mam33 complex provides the structural basis for the putative binding mode of these two proteins.
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