Biochemical Characterization of Pathogenic Mutations in Human Mitochondrial Methionyl-tRNA Formyltransferase [Enzymology]

October 6th, 2014 by Sinha, A., Kohrer, C., Weber, M. H. W., Masuda, I., Mootha, V. K., Hou, Y.–M., RaȷBhandary, U. L.

N-formylation of initiator methionyl-tRNA (Met-tRNAMet) by methionyl-tRNA formyltransferase (MTF) is important for translation initiation in bacteria, mitochondria and chloroplasts. Unlike all other translation systems, the metazoan mitochondrial system is unique in using a single methionine tRNA (tRNAMet) for both initiation and elongation. A portion of Met-tRNAMet is formylated for initiation while the remainder is used for elongation. Recently, we showed that compound heterozygous mutations within the nuclear gene encoding human mitochondrial MTF (mt-MTF) significantly reduced mitochondrial translation efficiency leading to combined OXPHOS deficiency and Leigh syndrome in two unrelated patients. Patient P1 has a stop codon mutation in one of the MTF genes and a S209L mutation in the other MTF gene. P2 has a S125L mutation in one of the MTF genes and the same S209L mutation as P1 in the other MTF gene. Here, we have investigated the effect of mutations at S125 and S209 on activities of human mt-MTF and of the corresponding mutations, A89 or A172, respectively, on activities of E. coli MTF. The S125L mutant has 653 fold lower activity, whereas the S209L mutant has 36 fold lower activity. Thus, both patients depend upon residual activity of the S209L mutant to support low levels of mitochondrial protein synthesis. We discuss the implications of these and other results on whether the effect of the S209L mutation on mitochondrial translational efficiency is due to reduced activity of the mutant mt-MTF and/or reduced levels of the mutant mt-MTF.