Signal Transduction Pathways That Regulate Eukaryotic Protein Synthesis

Abstract

eukaryotic initiation factor amino acids adenylate cyclase calmodulin diacylglycerol eukaryotic elongation factor eEF2 kinase epidermal growth factor extracellular signal-regulated kinase guanine nucleotide exchange factor G-protein-coupled receptors glycogen synthase kinase-3 inositol insulin receptor substrate mitogen-activated protein kinase mitogen-activated protein/ERK kinase MAPK-interacting kinase mammalian target of rapamycin 70-kDa ribosomal S6 kinase 90-kDa ribosomal S6 kinase PtdIns-dependent kinase pleckstrin homology phosphorylated, heat- and acid-stable protein phosphatidylinositol 3-kinase protein kinase B protein kinase C phospholipase C phosphatidylinositol receptor kinase substrates receptor tyrosine kinase Src homology domain 2 SH2-containing phosphotyrosine phosphatase-2 terminal oligopyrimidine untranslated region The last several years have witnessed an explosion in the published literature on two topics, the pathways that transduce extracellular signals to their intracellular targets and modification of the core translational apparatus in response to these signals. Most of these pathways result in cell growth and cell division. Synthesis of the entire complement of proteins is necessary to double the cell size, but synthesis of the so-called “growth-regulated” proteins (1Baserga R. Cancer Res. 1990; 50: 6769-6771PubMed Google Scholar) is needed for cell division. This article summarizes recent advances in our understanding of how a single mitogenic stimulus can simultaneously lead to an increase in both global and growth-regulated protein synthesis. The three stages of protein synthesis are catalyzed by initiation, elongation, and release factors (Ref. 2Merrick W.C. Hershey J.W.B. Hershey J.W.B. Mathews M.B. Sonenberg N. Translational Control. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1996: 31-69Google Scholar; a guide to current and previous nomenclature can be found in Ref. 3Clark B.F.C. Grunberg-Manago M. Gupta N.K. Hershey J.W.B. Hinnebusch A.G. Jackson R.J. Maitra U. Mathews M.B. Merrick W.C. Rhoads R.E. Sonenberg N. Spremulli L. Trachsel H. Voorma H.O. Biochimie ( Paris ). 1996; 78: 11-12Crossref Scopus (41) Google Scholar). A ternary complex of eIF2·GTP·Met-tRNAi1 binds to the 40 S ribosomal subunit to form the 43 S initiation complex (Fig.1). The eIF4 factors plus poly(A)-binding protein recognize the 5′-terminal cap or 3′-terminal poly(A) tract of mRNA, unwind mRNA secondary structure, and transfer it to the 43 S initiation complex, resulting in the 48 S initiation complex. Scanning for the first initiation codon in good sequence context requires eIF4A and the presence of eIF1 and eIF1A (4Pestova T.V. Borukhov S.I. Hellen C.U.T. Nature. 1998; 394: 854-859Crossref PubMed Scopus (320) Google Scholar). Then eIF5 stimulates GTP hydrolysis by eIF2, after which the initiation factors are replaced by the 60 S subunit to form the 80 S initiation complex. The released eIF2·GDP is recycled to eIF2·GTP by the GEF eIF2B. The first elongator aminoacyl-tRNA is brought to the A-site by eEF1, followed by a cycle of GTP hydrolysis and exchange analogous to that of eIF2. Translocation is catalyzed by eEF2, again with a GTP hydrolysis cycle. The binding of growth factors to the extracellular domain of RTKs causes a conformational change that induces oligomerization and activation of the intracellular protein Tyr kinase domain (Ref. 5Marshall C.J. Cell. 1995; 80: 179-185Abstract Full Text PDF PubMed Scopus (4245) Google Scholar; Fig. 2). Substrates for the kinase can be either the RTK itself or a separate RKS. The SH2 domains of several different signaling molecules dock to the resulting Tyr(P)s in a sequence-specific manner, thereby activating separate downstream signaling cascades. These receptors are coupled to heterotrimeric G-proteins (6Vaughan M. J. Biol. Chem. 1998; 273: 17297Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). Dissociation of the G-protein subunits activates AC, PLC, and other downstream effectors. PLC hydrolyzes PtdIns(4,5)P2 to DAG and Ins(1,4,5)P3. These are docking proteins for downstream effectors of RTKs (7White M.F. Mol. Cell. Biochem. 1998; 182: 3-11Crossref PubMed Scopus (625) Google Scholar). The best studied RKS are the insulin receptor substrates, which include IRS-1, IRS-2, IRS-3, Gab-1, and p62DOK. Members of the IRS family bind to insulin receptor via an NH2-terminal PH domain and a Tyr(P)-binding domain. The COOH-terminal portions of the proteins contain numerous Tyr phosphorylation sites. IRS-1 alone provides docking sites for PI3-K, SH-PTP2, Grb-2, Fyn, Nck, and Crk. This phosphatase contains two SH2 domains, and enzyme activity is maximally activated when both are occupied by Tyr(P)-containing peptides (8Pluskey S. Wandless T.J. Walsh C.T. Shoelson S.E. J. Biol. Chem. 1995; 270: 2897-2900Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar). SH-PTP2 is activated by docking to EGF receptor, platelet-derive