eIF3: A ConnecTOR of S6K1 to the Translation Preinitiation Complex

In the November 18 issue of Cell, Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar discover an unidentified function for the eIF3 translation initiation factor as a scaffold for the dynamic associations of many preinitiation complex components, including the growth-regulating kinases mTOR and S6K1. In the November 18 issue of Cell, Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar discover an unidentified function for the eIF3 translation initiation factor as a scaffold for the dynamic associations of many preinitiation complex components, including the growth-regulating kinases mTOR and S6K1. The role of the mammalian target of rapamycin (mTOR) and S6K1 signaling pathway in translational control remains, despite its considerable study, poorly understood. mTOR is an evolutionarily conserved serine/threonine kinase that exists in two multiprotein complexes and responds to growth factor and nutrient signals to control cell growth and survival. One of these complexes, called raptor-mTOR, phosphorylates S6K1 in a manner that is sensitive to the immunosuppressant drug rapamycin. The initial demonstration of the inhibitory effects of rapamycin on translation regulators (Beretta et al., 1996Beretta L. Gingras A. Svitkin Y. Hall M. Sonenberg N. EMBO J. 1996; 15: 658-664Crossref PubMed Scopus (582) Google Scholar, Chung et al., 1992Chung J. Kuo C. Crabtree G. Blenis J. Cell. 1992; 69: 1227-1236Abstract Full Text PDF PubMed Scopus (988) Google Scholar, Kuo et al., 1992Kuo C. Chung J. Fiorentino D. Flanagan W. Blenis J. Crabtree G. 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Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar deliver many of these insights in a plentiful display of regulated and temporally defined interactions between mTOR, S6K1, S6K1 substrates, and components of the translation preinitiation complex. Their work provides key new information on the signaling relay from mTOR and S6K1 to the initiation of mRNA translation. Although S6K1 has multiple translational targets (Hay and Sonenberg, 2004Hay N. Sonenberg N. Genes Dev. 2004; 18: 1926-1945Crossref PubMed Scopus (3247) Google Scholar), the question of how S6K1 is localized to its substrates during protein synthesis has been largely unexplored. To address this question, Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar used an affinity-based purification method to identify interactors of S6K1. This work revealed the interaction of S6K1 with several of the proteins that make up the eIF3 translation initiation factor. During the process of translation preinitiation, eIF3 associates with a large complex of proteins called the preinitiation complex (PIC), which includes the small ribosomal subunit. eIF3 is composed of 13 subunits, and, among its many functions, it is thought to play a pivotal role in tethering the PIC to the 5′ cap structure of mRNA via its interaction with the eIF4F protein complex (Unbehaun et al., 2004Unbehaun A. Borukhov S. Hellen C. Pestova T. Genes Dev. 2004; 18: 3078-3093Crossref PubMed Scopus (168) Google Scholar, Gebauer and Hentze, 2004Gebauer F. Hentze M. Nat. Rev. Mol. Cell Biol. 2004; 5: 827-835Crossref PubMed Scopus (664) Google Scholar). Thus, the identification of eIF3 family members as S6K1 interactors represented a potentially important connection between the mTOR pathway and an integral component of the translational preinitiation apparatus. After confirming the interaction between endogenous S6K1 and eIF3, the authors set upon uncovering potential mechanisms that regulate this interaction. They found that multiple stimuli, including growth factors and nutrients, disrupt the association of S6K1, but not the related S6K2, with eIF3 in a rapamycin-sensitive fashion. As rapamycin treatment leads to hypophosphorylation and inactivation of S6K1, the authors reasoned that the activity of S6K1 might regulate its binding to eIF3. Indeed, mutants that eliminate S6K1 activity constitutively bind eIF3, whereas active S6K1 mutants are incapable of binding eIF3. Because the rapamycin sensitivity of the eIF3-S6K1 interaction suggested the involvement of the raptor-mTOR complex, the authors tested if these proteins might also associate with eIF3. This was the case for raptor-mTOR, but the authors found that the pattern of association of raptor-mTOR with eIF3 was opposite to that seen with S6K1; i.e., insulin stimulation promotes the association of raptor-mTOR to eIF3, whereas rapamycin inhibits the interaction. In addition to linking eIF3 to this upstream regulator of S6K1 activation, the authors also demonstrate a previously unknown interaction between an S6K1 substrate, the initiation factor eIF4B, and eIF3. Moreover, they found that the ability of eIF4B to bind eIF3 depends on the activation of S6K1. The correlation between mTOR and S6K activation and the binding or dissociation of PIC components with eIF3 and the 5′ cap was temporally examined during a time course following insulin stimulation. In a beautiful experiment, the authors demonstrate that, within minutes of stimulation, raptor-mTOR rapidly associates with eIF3, whereas S6K1 is rapidly released from eIF3. Also interestingly, S6 phosphorylation reaches a maximum level at a later time, leading the authors to suggest that the full activation of S6K1 occurs following its release from eIF3, a model corroborated by the similar kinetics of the S6K1-activated association of eIF4B with eIF3 and the 5′ cap. Finally, Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar demonstrate that the mTOR-dependent translational signaling they characterize biochemically contributes to cap-dependent translation within cells. Using a luciferase-based translation reporter, the authors show that expression of S6K1 and eIF4B mutants containing phosphomimetic residues at their rapamycin-sensitive phosphorylation sites stimulate cap-dependent translation in a manner comparable to overexpression of eIF4E, a known promoter of cap-dependent translation (Hay and Sonenberg, 2004Hay N. Sonenberg N. Genes Dev. 2004; 18: 1926-1945Crossref PubMed Scopus (3247) Google Scholar). The multitude of signaling events described by Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar and summarized in Figure 1 provide a significant framework for thinking about the involvement of mTOR and S6K1 in the initiation of translation. In the future, it will be important to further delineate the mechanistic bases of the interactions of eIF3 with its newly described partners: mTOR, raptor, S6K1, and eIF4B. Questions still surround the correlation between mTOR and S6K1 activation and their respective association and dissociation from eIF3: does raptor-mTOR binding to eIF3 physically displace S6K1 from eIF3? And how does S6K1 activation alter the ability of eIF3 and S6K1 to interact? Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar focused much of their work on defining mTOR and S6K1 interactions with the eIF3 subunit, eIF3B, which leads one to wonder how mTOR and raptor may interact with other components of the PIC. One potential molecular explanation for the raptor-mTOR-eIF3 interactions that bears consideration is that both mTOR and raptor are HEAT-domain-containing proteins, as are other components that bind eIF3 in the PIC, namely eIF5 and eIF2b. Considering this, it seems reasonable to suggest that the HEAT domains in these proteins could be their common site of attachment to the PIC. The work of Holz et al., 2005Holz M. Ballif B. Gygi S. Blenis J. Cell. 2005; 123: 569-580Abstract Full Text Full Text PDF PubMed Scopus (818) Google Scholar is a standout contribution to our understanding of one of the cell's fundamental processes, the regulation and assembly of the translation preinitiation complex. They provide many first demonstrations of the direct interactions and regulated behavior of mTOR, S6K1, and eIF3 throughout the process of translation initiation. With these insights in hand, many of the mysteries of how mTOR and S6K1 exert their control over translation are now much closer to being solved. mTOR and S6K1 Mediate Assembly of the Translation Preinitiation Complex through Dynamic Protein Interchange and Ordered Phosphorylation EventsHolz et al.CellNovember 18, 2005In BriefIn response to nutrients, energy sufficiency, hormones, and mitogenic agents, S6K1 phosphorylates several targets linked to translation. However, the molecular mechanisms whereby S6K1 is activated, encounters substrate, and contributes to translation initiation are poorly understood. We show that mTOR and S6K1 maneuver on and off the eukaryotic initiation factor 3 (eIF3) translation initiation complex in a signal-dependent, choreographed fashion. When inactive, S6K1 associates with the eIF3 complex, while the S6K1 activator mTOR/raptor does not. Full-Text PDF Open Archive