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The binding of the eukaryotic initiation factor 4E (eIF4E) to the mRNA 5= cap structure is a rate-limiting step in mRNA translation initiation. eIF4E promotes ribosome recruitment to the mRNA. In Drosophila, the eIF4E homologous protein (d4EHP) forms a complex with binding partners to suppress the translation of distinct mRNAs by competing with eIF4E for binding the 5= cap structure. This repression mechanism is essential for the asymmetric distribution of proteins and normal embryonic development in Drosophila. In contrast, the physiological role of the mammalian 4EHP (m4EHP) was not known. In this study, we have identified the Grb10-interacting GYF protein 2 (GIGYF2) and the zinc finger protein 598 (ZNF598) as components of the m4EHP complex. GIGYF2 directly interacts with m4EHP, and this interaction is required for stabilization of both proteins. Disruption of the m4EHP-GIGYF2 complex leads to increased translation and perinatal lethality in mice. We propose a model by which the m4EHP-GIGYF2 complex represses translation of a subset of mRNAs during embryonic development, as was previously reported for d4EHP.

Exported mRNAs are targeted for translation or can undergo degradation by several decay mechanisms. The 533 degradation machinery localizes to cytoplasmic P bodies (PBs). We followed the dynamic properties of PBs in vivo and investigated the mechanism by which PBs scan the cytoplasm. Using proteins of the decapping machinery, we asked whether PBs actively scan the cytoplasm or whether a diffusion-based mechanism is sufficient. Live-cell imaging showed that PBs were anchored mainly to microtubules. Quantitative single-particle tracking demonstrated that most PBs exhibited spatially confined motion dependent on microtubule motion, whereas stationary PB pairs were identified at the centrosome. Some PBs translocated in long-range movements on microtubules. PB mobility was compared with mitochondria, endoplasmic reticulum, peroxisomes, SMN bodies, and stress granules, and diffusion coefficients were calculated. Disruption of the microtubule network caused a significant reduction in PB mobility together with an induction of PB assembly. However, FRAP measurements showed that the dynamic flux of assembled PB components was not affected by such treatments. FRAP analysis showed that the decapping enzyme Dcp2 is a nondynamic PB core protein, whereas Dcp1 proteins continuously exchanged with the cytoplasm. This study reveals the mechanism of PB transport, and it demonstrates how PB assembly and disassembly integrate with the presence of an intact cytoskeleton. INTRODUCTIONGene expression begins with the synthesis of mRNA molecules in the nucleus. After processing events, transcripts are exported to the cytoplasm where they can face several posttranscriptional fates, elicited by a balance between cytoplasmic translation and mRNA degradation pathways. Quality control pathways regulate the degradation of mRNAs and facilitate their sequestration or translational repression (Meyer et al., 2004). In eukaryotes, mRNA degradation typically begins with deadenylation, and then either of two major pathways is used. The exosome protein complex degrades mRNAs in the 3Ј35Ј direction, whereas the 5Ј33Ј direction involves other factors, including a decapping enzyme followed by the Xrn1 exonuclease (Parker and Song, 2004).The removal of the cap structure irreversibly marks the mRNA for degradation. The decapping process is tightly regulated biochemically and spatially. It was first discovered that the Xrn1 nuclease localizes in discrete cytoplasmic foci in eukaryotic cells (Bashkirov et al., 1997). Some years later, a decapping protein termed Dcp2 was found to colocalize in Xrn1-foci (Ingelfinger et al., 2002;Lykke-Andersen, 2002;van Dijk et al., 2002), finally leading to the understanding that both yeast (Sheth and Parker, 2003) and mammalian cells (Cougot et al., 2004) contain discrete areas in which mRNA decapping and 5Ј33Ј degradation can occur (Sheth and Parker, 2006). These cytoplasmic foci are now widely known as P bodies (PBs), and they have been referred to as Dcpbodies, processing bodies, mRNA-decay foci, and GW182 bodies. Since ...

In eukaryotes, a surveillance mechanism known as nonsensemediated decay (NMD) degrades the mRNA when a prematuretermination codon (PTC) is present. NMD requires translation to read the frame of the mRNA and detect the PTC. During pre-mRNA splicing, the exon-exon junction complex (EJC) is recruited to a region 20 -24 nt upstream of the exon junction on the mature mRNA. The presence of a PTC upstream from the EJC elicits NMD. Eukaryotic initiation factor 4A (eIF4A) III is a nuclear protein that interacts physically or functionally with translation initiation factors eIF4G and eIF4B, respectively, and shares strikingly high identity with the initiation factors eIF4AI͞II. Here we show that siRNA against eIF4AIII, but not against eIF4AI͞II, inhibits NMD. Moreover, eIF4AIII, but not eIF4AI, is specifically recruited to the EJC during splicing. The observations that eIF4AIII is loaded onto the mRNA during splicing in the nucleus, has properties related to a translation initiation factor, and functions in NMD raises the possibility that eIF4AIII substitutes for eIF4AI͞II during NMD.