| Eukaryotic mRNA translation initiation is a complex process involving assembly of many eukaryotic initiation factors (eIF), ribosomal recruitment, and initiation codon selection. In the most general case, cellular translation begins with the recognition of the cap structure [7-methyl G(5′ )ppp(5′)N] on the 5′ end of the mRNA by eIF4F. eIF4F consists of the cap-binding protein, eIF4E, an ATP-dependent RNA helicase, eIF4A, and the bridging molecule, eIF4G. eIF4E-binding proteins (4EBPs) are general inhibitors of cap-dependent translation that disrupt assembly of eIF4F by sequestering eIF4E from eIF4G. During picornaviral infection, cellular, cap-dependent translation is inhibited, while viral translation occurs through a cap-independent mechanism. eIF4G recognizes the picornaviral type II internal ribosomal entry site (IRES) allowing assembly of the ribosome at the translation start site. The structures of eIFE/7-methyl-GDP binary complex, two ternary complexes of eIF4E/7-methyl-GDP with the eIF4E-recognition motifs from eIF4G and 4E-BP1, and the eIF4A/picornaviral, type II RES binding domain of eIF4G have been determined by X-ray crystallography. eIF4 bound to 7-methyl-GDP resembles a cupped hand, and consists of a curved, 8-stranded antiparallel β-sheet, backed by three long α-helices. 7-methyl-GDP resides in a narrow cap-binding slot on the molecule's concave surface formed by two loops and a short α-helix. 7-methyl-gaunosines interacts with the sidechains of two conserved tryptophans through π-π stacking, a van der Waals contact between its N7-methyl group and a third conserved tryptophan, and forms three hydrogen bonds to a backbone amino group and the sidechain of a conserved glutamate. The eIF4E-recognition motifs of eIF4G and the 4E-BPs undergo disorder-to-order transitions, adopting L-shaped, extended chain/α-helical conformations when they interact with a phylogenetically-invariant portion of the convex surface of eIF4E. The structure of the eIF4A/IRES binding domain of eIF4G has an overall crescent shape, consisting of five repeats of two antiparallel α-helices forming a double layer of α-helices. The fold of eIF4G is similar to that of the HEAT repeat containing proteins importin β and the PR65/A subunit of protein phosphatase 2A. The implications of these results on the mechanism and regulation of cap-dependent and cap-independent translation are discussed. |
