Dominant negative mutants of mammalian translation initiation factor eIF-4A define a critical role for eIF-4F in cap-dependent and cap-independent initiation of translation.

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Abstract
Eukaryotic translation initiation factor-4A (eIF-4A) plays a critical role in binding of eukaryotic mRNAs to ribosomes. It has been biochemically characterized as an RNA-dependent ATPase and RNA helicase and is a prototype for a growing family of putative RNA helicases termed the DEAD box family. It is required for mRNA-ribosome binding both in its free form and as a subunit of the cap binding protein complex, eIF-4F. To gain further understanding into the mechanism of action of eIF-4A in mRNA-ribosome binding, defective eIF-4A mutants were tested for their abilities to function in a dominant negative manner in a rabbit reticulocyte translation system. Several mutants were demonstrated to be potent inhibitors of translation. Addition of mutant eIF-4A to a rabbit reticulocyte translation system strongly inhibited translation of all mRNAs studied including those translated by a cap-independent internal initiation mechanism. Addition of eIF-4A or eIF-4F relieved inhibition of translation, but eIF-4F was six times more effective than eIF-4A, whereas eIF-4B or other translation factors failed to relieve the inhibition. Kinetic experiments demonstrated that mutant eIF-4A is defective in recycling through eIF-4F, thus explaining the dramatic inhibition of translation. Mutant eIF-4A proteins also inhibited eIF-4F-dependent, but not eIF-4A-dependent RNA helicase activity. Taken together these results suggest that eIF-4A functions primarily as a subunit of eIF-4F, and that singular eIF-4A is required to recycle through the complex during translation. Surprisingly, eIF-4F, which binds to the cap structure, appears to be also required for the translation of naturally uncapped mRNAs.