title: Bidirectional RNA helicase activity of eucaryotic translation initiation factors 4A and 4F.
abstract: The mechanism of ribosome binding to eucaryotic mRNAs is not well understood, but it requires the participation of eucaryotic initiation factors eIF-4A, eIF-4B, and eIF-4F and the hydrolysis of ATP. Evidence has accumulated in support of a model in which these initiation factors function to unwind the 5'-proximal secondary structure in mRNA to facilitate ribosome binding. To obtain direct evidence for initiation factor-mediated RNA unwinding, we developed a simple assay to determine RNA helicase activity, and we show that eIF-4A or eIF-4F, in combination with eIF-4B, exhibits helicase activity. A striking and unprecedented feature of this activity is that it functions in a bidirectional manner. Thus, unwinding can occur either in the 5'-to-3' or 3'-to-5' direction. Unwinding in the 5'-to-3' direction by eIF-4F (the cap-binding protein complex), in conjunction with eIF-4B, was stimulated by the presence of the RNA 5' cap structure, whereas unwinding in the 3'-to-5' direction was completely cap independent. These results are discussed with respect to cap-dependent versus cap-independent mechanisms of ribosome binding to eucaryotic mRNAs.
authors.affiliation: Department of Biochemistry, McGill University, Montreal, Quebec, Canada.
authors.forename: F
authors.initials: F
authors.lastname: Rozen
authors.forename: I
authors.initials: I
authors.lastname: Edery
authors.forename: K
authors.initials: K
authors.lastname: Meerovitch
authors.forename: T E
authors.initials: TE
authors.lastname: Dever
authors.forename: W C
authors.initials: WC
authors.lastname: Merrick
authors.forename: N
authors.initials: N
authors.lastname: Sonenberg
mesh_terms: D000818:Animals; D001483:Base Sequence; D002474:Cell-Free System; D039601:Eukaryotic Initiation Factor-4A; D039562:Eukaryotic Initiation Factor-4F; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D010442:Peptide Chain Initiation, Translational; D010448:Peptide Initiation Factors / Q000378:metabolism*; D012315:RNA Caps / Q000378:metabolism; D012316:RNA Nucleotidyltransferases / Q000378:metabolism*; D012330:RNA, Double-Stranded / Q000378:metabolism*; D011817:Rabbits; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D039562:Eukaryotic Initiation Factor-4F; D010448:Peptide Initiation Factors; D012315:RNA Caps; D012330:RNA, Double-Stranded; D039601:Eukaryotic Initiation Factor-4A; D012316:RNA Nucleotidyltransferases