Regulation of translation initiation by FRAP/mTOR.

Regulation of protein synthesis in eukaryotes plays a critical role in development, differentiation, cell cycle progression, cell growth, and apoptosis (Mathews et al. 2000). Translational control allows for a more rapid response than transcriptional modulation because no mRNA synthesis, processing, or transport is required, and can be used to coordinate gene expression in systems that lack transcriptional regulation, such as reticulocytes or platelets (Weyrich et al. 1998; Mathews et al. 2000). Translational control plays a particularly important role in early developmental processes, when localized translation is utilized to establish polarity (Wickens et al. 2000), and localized translation in neurons may be critical for learning and memory (e.g., Casadio et al. 1999). Following transcription, processing, and nucleocytoplasmic export, mRNAs are competent for translation. However, two transcripts present in identical quantities may be translated at very different rates. This phenomenon is caused, in part, by the fact that the ribosome does not bind to mRNA directly, but must be recruited to mRNA by the concerted action of a large number of eukaryotic translation initiation factors (eIFs). This recruitment step, also referred to as the initiation phase, is a complex process that culminates in the positioning of a charged ribosome (that is, an 80S ribosome loaded with an initiator tRNA in its P site) at an initiation codon (for review, see Hershey and Merrick 2000). As discussed further below, the recruitment process is rate-limiting for translation in many cases, and is subject to exquisite regulation. The structure mGpppN (or the cap, where m is a methyl group and N any nucleotide) is present at the 5 end of all nuclear transcribed mRNAs, and plays an important role in the initiation process. The cap is recognized by the initiation factor eIF4E. eIF4E, via an interaction with a large scaffolding protein termed eIF4G, directs the translational machinery to the 5 end of the mRNA. eIF4E and eIF4G function as components of a trimeric complex, termed eIF4F, which also contains the RNA helicase eIF4A (Gingras et al. 1999b; Hershey and Merrick 2000; Fig. 1). eIF4G also establishes intermolecular contacts with several other components of the translational machinery, including the multisubunit, ribosome-associated initiation factor eIF3 (Hentze 1997; Hershey and Merrick 2000). Importantly, optimal binding of the 40S ribosomal subunit is thought to require a region of single-stranded mRNA (Sonenberg 1993; Gingras et al. 1999b; Hershey and Merrick 2000). Thus, once eIF4F binds to the cap, eIF4A (in conjunction with an associated ubiquitous cofactor, eIF4B) is thought to unwind any inhibitory secondary structure present in the cap-proximal 5 untranslated region (5 UTR). Through its interaction with eIF3 and its ability to bind mRNA in a sequence nonspecific fashion, eIF4G bridges the mRNA to the 40S ribosomal subunit. Once the 40S subunit is bound to mRNA, the ribosome and associated factors are believed to scan in a 5 to 3 direction, until an initiation codon in the proper sequence context is encountered (Kozak 1989; Jackson 2000). When an initiation codon is recognized, and a codon/anticodon interaction established, the initiation factors dissociate from the small ribosomal subunit and allow for the joining of a 60S ribosomal subunit. In this way, a single ribosome is directed to a start codon, and protein synthesis can commence. Treatment of cells with mitogens, hormones, or growth factors generally leads to an increase in translation. Conversely, nutrient deprivation or environmental stresses such as heat shock, osmotic shock, or UV irradiation generally reduce protein synthetic rates (for review, see Kleijn et al. 1998; Gingras et al. 1999b; Mathews et al. 2000; Schneider 2000). In mammalian cells, these changes in translation rates are often correlated with changes in the level or activity of eIF4F, resulting in differences in the rate of ribosomal recruitment to mRNA. That is, growing or stimulated cells contain high levels of eIF4F, whereas in quiescent or stressed cells low eIF4F levels are detected. Mammalian eIF4F formation is regulated by a family of translation repressors, the eIF4E binding proteins (4E-BPs; Lin et al. 1994; Pause et al. 1994). The 4E-BPs constitute a family of three small polypeptides that compete with eIF4G for These authors contributed equally to this work. Corresponding author. E-MAIL nsonen@med.mcgill. ca; FAX (514) 398-1287. Article and publication are at www.genesdev. org/cgi/doi/10.1101/ gad.887201.