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Factor-dependent processivity in human eIF4A DEAD-box helicase

Unwinding RNA for protein synthesis During the first steps of protein synthesis, the small subunit of the ribosome scans the 5′ end of the mRNA, looking for the protein start codon. This process involves one of the translation initiation factors, eIF4A, which helps to remove any RNA structures that might impede the ribosome's search. García-García et al. used single-molecule optical trap assays to show that eIF4A, in combination with two other translation initiation factors, is able to continuously and directionally unwind a double-stranded RNA hairpin. The factors unwound RNA in steps roughly equal to a turn of the RNA double helix. Science, this issue p. 1486 A complex of three protein translation initiation factors is able to unwind messenger RNA continuously and directionally. During eukaryotic translation initiation, the small ribosomal subunit, assisted by initiation factors, locates the messenger RNA start codon by scanning from the 5′ cap. This process is powered by the eukaryotic initiation factor 4A (eIF4A), a DEAD-box helicase. eIF4A has been thought to unwind structures formed in the untranslated 5′ region via a nonprocessive mechanism. Using a single-molecule assay, we found that eIF4A functions instead as an adenosine triphosphate–dependent processive helicase when complexed with two accessory proteins, eIF4G and eIF4B. Translocation occurred in discrete steps of 11 ± 2 base pairs, irrespective of the accessory factor combination. Our findings support a memory-less stepwise mechanism for translation initiation and suggest that similar factor-dependent processivity may be shared by other members of the DEAD-box helicase family.