Eukaryotic initiation factor 4F

Eukaryotic initiation factor 4F (eIF4F) is a heterotrimeric protein complex that binds the 5' cap of messenger RNAs (mRNAs) to promote eukaryotic translation initiation. The eIF4F complex is composed of three non-identical subunits: the DEAD-box RNA helicase eIF4A, the cap-binding protein eIF4E, and the large 'scaffold' protein eIF4G. The mammalian eIF4F complex was first described in 1983, and has been a major area of study into the molecular mechanisms of cap-dependent translation initiation ever since.eIF4F is important for recruiting the small ribosomal subunit (40S) to the 5' cap of mRNAs during cap-dependent translation initiation. Components of the complex are also involved in cap-independent translation initiation; for instance, certain viral proteases cleave eIF4G to remove the eIF4E-binding region, thus inhibiting cap-dependent translation.Structures of eIF4F components have been solved individually and as partial complexes by a variety of methods, but no complete structure of eIF4F is currently available.In mammals, the eIF4E•G•A trimeric complex can be directly purified from cells, while only the two subunit eIF4E•G can be purified from yeast cells. eIF4E binds the m7G 5' cap and the eIF4G scaffold, connecting the mRNA 5' terminus to a hub of other initiation factors and mRNA. The interaction of eIF4G•A is thought to guide the formation of a single-stranded RNA landing pad for the 43S preinitiation complex (43S PIC) via eIF4A's RNA helicase activity.The eIF4E subunit of eIF4F is an important target of mTOR signaling through the eIF4E binding protein (4E-BP). Phosphorylation of 4E-BPs by mTOR prevents their binding to eIF4E, freeing eIF4E to bind eIF4G and participate in translation initiation.