MEHMO syndrome mutation EIF2S3-I259M impairs initiator Met-tRNAiMet binding to eukaryotic translation initiation factor eIF2

The heterotrimeric eukaryotic translation initiation factor (eIF) 2 plays critical roles in delivering initiator Met-tRNA(i)(Met) to the 40S ribosomal subunit and in selecting the translation initiation site. Genetic analyses of patients with MEHMO syndrome, an X-linked intellectual disability syndrome, have identified several unique mutations in the EIF2S3 gene that encodes the subunit of eIF2. To gain insights into the molecular consequences of MEHMO syndrome mutations on eIF2 function, we generated a yeast model of the human eIF2-I259M mutant, previously identified in a patient with MEHMO syndrome. The corresponding eIF2-I318M mutation impaired yeast cell growth and derepressed GCN4 expression, an indicator of defective eIF2-GTP-Met-tRNA(i)(Met) complex formation, and, likewise, overexpression of human eIF2-I259M derepressed ATF4 messenger RNA translation in human cells. The yeast eIF2-I318M mutation also increased initiation from near-cognate start codons. Biochemical analyses revealed a defect in Met-tRNA(i)(Met) binding to the mutant yeast eIF2 complexes in vivo and in vitro. Overexpression of tRNA(i)(Met) restored Met-tRNA(i)(Met) binding to eIF2 in vivo and rescued the growth defect in the eIF2-I318M strain. Based on these findings and the structure of eIF2, we propose that the I259M mutation impairs Met-tRNA(i)(Met) binding, causing altered control of protein synthesis that underlies MEHMO syndrome.