Modulation of the helicase activity of eIF4A by eIF4B, eIF4H, and eIF4F

Eukaryotic initiation factor (eIF) 4A is a DEAD box RNA helicase that works in conjunction with eIF4B, eIF4H, or as a subunit of eIF4F to unwind secon dary structure in the 5 ' -untranslated region of mRNA, which facilitates b inding of the mRNA to the 40 S ribosomal subunit. This study demonstrates h ow the helicase activity of eIF4A is modulated by eIF4B, eIF4H, or as a sub unit of eIF4F. Results indicate that a linear relationship exists between t he initial rate or amplitude of unwinding and duplex stability for all fact or combinations tested. eIF4F, like eIF4A, behaves as a non-processive heli case. Either eIF4B or eIF4H stimulated the initial rate and amplitude of eI F4A-dependent duplex unwinding, and the magnitude of stimulation is depende nt on duplex stability. Furthermore, eIF4A (or eIF4F) becomes a slightly pr ocessive helicase in the presence of eIF4B or eIF4H. All combinations of fa ctors tested indicate that the rate of duplex unwinding is equivalent in th e 5 ' --> 3 ' and 3 ' --> 5 ' directions. However, the optimal rate of unwi nding was dependent on the length of the single-stranded region of the subs trate when different combinations of factors were used. The combinations of eIF4A, eIF4A + eIF4B, eIF4A + eIF4H, and cIF4F showed differences in their ability to unwind chemically modified duplexes. A simple model of how eIF4 B or eIF4H affects the duplex unwinding mechanism of eIF4A is proposed.