ATP HYDROLYSIS BY INITIATION FACTOR-4A IS REQUIRED FOR TRANSLATION INITIATION IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae translation initiation factor eIF-4A, an RNA helicase of the Asp-Glu-Ala-Asp (DEAD) box protein family, was mutated in the putative ATP binding site and expressed in Escherichia coli. Mutant proteins with alanine at position 66 replaced by glycine [eIF-4A(A66G)] or valine [eIF-4A(A66V)] were purified from Escherichia coli extracts and analyzed in vitro for activity in ATP crosslinking, ATP hydrolysis, RNA helicase, and translation assays. The results show that in vitro ATP hydrolysis activity, RNA helicase activity, and translation activity of eIF-4A correlate with in vivo activity of the factor. Whereas eIF-4A(A66G) showed wild-type activity in all assays, eIF-4A(A66V) was active in ATP crosslinking but inactive in ATP hydrolysis and RNA helicase assays. In vitro translation was supported by wild-type eIF-4A and eIF-4A(A66G) but not by eIF-4A(A66V). The results show that, for their translation, the majority of mRNAs from Saccharomyces cerevisiae including an mRNA with the initiator AUG positioned 8 nucleotides downstream of the cap structure require eIF-4A that is able to hydrolyze ATP.