Together with GTP and initiator methionyl-tRNA, translation initiation factor eIF2 forms a ternary complex that binds the 40S ribosome and then scans an mRNA to select the AUG start codon for protein synthesis. Here, we show that a human X-chromosomal neurological disorder characterized by intellectual disability and microcephaly is caused by a missense mutation in eIF2? (encoded by EIF2S3), the core subunit of the heterotrimeric eIF2 complex. Biochemical studies of human cells overexpressing the eIF2? mutant and of yeast eIF2? with the analogous mutation revealed a defect in binding the eIF2? subunit to eIF2?. Consistent with this loss of eIF2 integrity, the yeast eIF2? mutation impaired translation start codon selection and eIF2 function in vivo in a manner that was suppressed by overexpressing eIF2?. These findings directly link intellectual disability to impaired translation initiation, and provide a mechanistic basis for the human disease due to partial loss of eIF2 function.
