Translation, Pdcd4 and eIF4A

Pdcd4 (programmed cell death 4) has received considerable attention as a tumor suppressor protein in recent years, however, its molecular function is still poorly understood. Pdcd4 is a nuclear-cytoplasmic shuttling and RNA-binding protein, which is involved in the control of translation of specific mRNAs. Pdcd4 interacts with the eukaryotic translation initiation factor eIF4A, an RNA helicase that plays a critical role in cap-dependent translation by melting stable RNA secondary structures in the 5’-untranslated regions (UTRs) of mRNAs [1,2]. It has been shown that Pdcd4 inhibits the helicase activity of eIF4A, suggesting that it suppresses translation of mRNAs with highly structured 5’-UTRs [3]. This idea was supported by analyzing the effect of Pdcd4 on artificial RNA constructs containing stable hairpin structures in the 5’-UTR and, more recently, confirmed by demonstrating that translation of p53 mRNA (whose 5’-UTR forms very stable secondary structures) is suppressed by Pdcd4 via an eIF4A-dependent mechanism [4] (Figure 1a). Recent findings indicate that the role of Pdcd4 in translation is more complex and involves an additional, entirely different inhibitory mechanism. It was shown that Pdcd4 suppresses the translation of the c-myb and A-myb mRNAs even when the eIF4A binding site was destroyed by mutation [5,6]. Instead, the RNA-binding activity of Pdcd4 was required to suppress translation of these RNAs, suggesting that direct RNA-binding by Pdcd4 plays a key role. Additional work revealed that the nucleotide sequences responsible for Pdcd4-induced translation suppression were located in the coding regions of c-myb and A-myb mRNAs. Furthermore, in vitro RNAbinding studies demonstrated that the “Pdcd4 response regions” of the c-myb and A-myb mRNAs are able to form secondary structures which were preferentially bound by Pdcd4. Overall, these experiments suggested that Pdcd4 is able to suppress translation by a novel mechanism, which involves direct binding of Pdcd4 to specific target RNAs. How does Pdcd4 suppress translation from within the coding region? When the c-myb coding region was placed under the control of the Hepatitis C virus internal ribosomal entry site (HCV-IRES) Pdcd4 failed to suppress translation [5]. Because the HCV-IRES does not depend on the translation initiation factors required for capdependent initiation, this suggested that Pdcd4 suppresses translation of c-myb mRNA by interfering with one of these factors (except eIF4A) at translation initiation. However, when the c-myb and A-myb “Pdcd4 response regions” were fused to GFP RNA to ask if they are able to convey Pdcd4-responsiveness onto a heterologous RNA, a surprising observation was made. It was indeed found that Pdcd4 was able to suppress translation of the recombinant RNA, but only when a continuous open reading frame extended from the GFP coding sequence into the added c-myb or A-myb sequences. In other words, Pdcd4 suppressed the translation of the recombinant RNAs only when the binding region for Pdcd4 was itself part of the open reading frame. This observation was confirmed by introducing in-frame translational stop codons into the authentic c-myb and A-myb coding regions upstream of the “Pdcd4 response regions”. This completely abolished the inhibitory effect of Pdcd4, again indicating that Pdcd4 supresses translation only when the sequence to which it binds is part of the translated region [6]. Thus, truncating the coding region by a single stop codon is sufficient to abrogate Pdcd4-dependent inhibition. A straightforward explanation for this observation is that Pdcd4 suppresses translation of these RNAs at the elongation step (Figure 1b). Editorial