Bidirectional RNA helicase activity of eucaryotic translation initiation factors 4A and 4F.

TLDR

Ribosome binding to eukaryotic mRNAs requires eIF‑4A, eIF‑4B, and eIF‑4F with ATP hydrolysis, and these factors are thought to unwind 5′‑proximal secondary structure to facilitate binding. The study aimed to directly demonstrate RNA unwinding by initiation factors. A simple helicase assay revealed that eIF‑4A or eIF‑4F together with eIF‑4B possess RNA helicase activity. The helicase activity is bidirectional, with 5′‑to‑3′ unwinding stimulated by the 5′ cap and 3′‑to‑5′ unwinding cap‑independent, highlighting distinct cap‑dependent and cap‑independent ribosome‑binding mechanisms.

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.