Functions of the 3′UTR and 5′UTR in translation initiation. (a) eIF4E bound to the 5′UTR, and poly(A) binding protein (PABP) bound to the 3′ poly(A) tail, are linked through their binding to eIF4G. The resulting looped complex facilitates recruitment of other translational regulators and ribosomal subunits for translation initiation. (b) For maternal mRNAs such as that derived from the Xenopus gene mos, eIF4E is initially sequestered by interaction with Maskin and CPE-binding protein (CPEB), with translation suppressed. Activation involves phosphorylation of CPEB, which in association with the cleavage and polyadenylation specificity factor (CPSF) promotes lengthening of the poly(A) tail and recruitment of PABP and eIF4G; eIFG displaces Maskin, allowing translation to start.