Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation

Serine 209, not serine 53, is the major site of phosphorylation in initiation factor eIF-4E in serum-treated Chinese hamster ovary cells

Ser-53 has previously been considered the major phosphorylation site in eukaryotic initiation factor (eIF)-4E, and this appeared to be supported by studies using a S53A mutant. Recently, however, several lines of evidence have indicated that Ser-53 might not be the true phosphorylation site. This prompted us to re-examine the phosphorylation site in eIF-4E using factor purified from 32P-labeled, serum-treated Chinese hamster ovary cells. Isoelectric focusing and phosphoamino acid analysis indicated the existence of a single phosphorylated serine. Edman degradation of the major radiolabeled tryptic product from 32P-labeled eIF-4E showed that the phosphorylated site was positioned three residues from the N terminus of this peptide. There are three serines in the sequence of eIF-4E that are three residues away from a tryptic cleavage site (i.e. lysine or arginine). 32P-Labeled eIF-4E was digested with trypsin, Lys-C, or trypsin followed by Glu-C and subjected to two-dimensional mapping; the data obtained eliminated two of these potential sites, leaving Ser-209. Comigration of the synthetic peptide SGS(P)209TTK with the radiolabeled tryptic product on (i) reverse-phase chromatography and (ii) two-dimensional mapping at different pH values confirmed that Ser-209 is the major phosphorylation site in eIF-4E in serum-stimulated Chinese hamster ovary cells.

Phosphorylation of eukaryotic protein synthesis initiation factor 4E by insulin-stimulated protamine kinase

Insulin-stimulated protamine kinase (cPK) and protein kinase C (PKC) phosphorylated eukaryotic protein synthesis initiation factor 4E (eIF-4E) on serine and threonine residues located on an identical tryptic fragment as judged by two-dimensional phosphopeptide mapping. With cPK and PKC, the apparent Km for eIF-4E was about 1.2 and 50 microM, respectively. Relative to recombinant human eIF-4E, cPK exhibited about 100% and < or = 5% activity with eIF-4ES209A and eIF-4ET210A, respectively, and eIF-4ES209A was phosphorylated exclusively on threonines. Bovine kidney eIF-4E enhanced up to 1.8-fold globin synthesis in m7GTP-Sepharose-treated reticulocyte lysates. In contrast, following incubation with cPK, these eIF-4E preparations stimulated globin synthesis up to 6-fold. Compared to the dephosphorylation of the cPK-modified serine on eIF-4E, reticulocyte lysates and highly purified protein phosphatase 2A exhibited marked preference for the cPK-modified threonine. The results indicate that cPK phosphorylates eIF-4E on Ser209 and Thr210, that the hydroxyl group or phosphorylation of Thr210 is necessary for cPK to act on Ser209, and that Ser209 phosphorylation activates reticulocyte globin synthesis. The results suggest that cPK could contribute to the insulin-stimulated phosphorylation of eIF-4E, but that protein phosphatase 2A may confer the site specificity of this response.

The translational repression mediated by the platelet-derived growth factor 2/c-sis mRNA leader is relieved during megakaryocytic differentiation

Expression of the platelet-derived growth factor 2/c-sis gene is highly restricted and controlled at multiple levels. Its structured mRNA leader, which is unusually long (1022 nucleotides), serves as a potent translational inhibitor. One of the sites of PDGF2 synthesis is megakaryocytes, implying that PDGF2 translation efficiency is modulated during megakaryocytic differentiation. To study the role of the mRNA leader as a translational cis-modulator, the hybrid T7/vaccinia cytoplasmic expression system was used to disconnect between determinants controlling transcription, alternative splicing, and mRNA stability from those controlling translation. Chimeric transcripts in which the human PDGF2/c-sis mRNA leader positioned in frame upstream of a reporter gene were used to determine whether the mRNA leader can confer variable translational efficiencies during differentiation. It is demonstrated that there is a time window during megakaryocytic differentiation of K562 cells in which the strong translational inhibition by PDGF2/c-sis mRNA leader is relieved. The time course of the translational repression relief is similar to that of PDGF2/c-sis transcriptional induction during the differentiation process. A 179-nucleotides CG-rich fragment immediately upstream of the initiator AUG codon is necessary for coffering stringent modulation of the translational efficiency. In NIH3T3 overexpressing translation initiation factor eIF4E, the inhibitory effect of the mRNA leader of c-sis is not relieved, suggesting that the changes in the translational machinery during megakaryocytic differentiation are beyond eIF4E activity. The possible involvement of a 5'-end-independent translational mechanism is discussed.

Hormone-induced meiotic maturation in Xenopus oocytes occurs independently of p70s6k activation and is associated with enhanced initiation factor (eIF)-4F phosphorylation and complex formation

Hormone-induced meiotic maturation of the Xenopus oocyte is regulated by complex changes in protein phosphorylation. It is accompanied by a stimulation in the rate of translation, manifest at the level of polypeptide chain initiation. At laser times in the maturation process, this reflects an increased ability for mRNA to interact with the 40 S ribosomal subunit. In mammalian cells there is growing evidence for the regulation of translation by phosphorylation of ribosomal protein S6 and of initiation factors responsible for the binding of mRNA to ribosomes. In this report, we show that although the 70 kDa form of S6 kinase is activated within 1.5 hours in response to progesterone or insulin, a time critical for protein synthesis, its activation is not required for hormone-induced stimulation of translation rates or maturation. In response to progesterone, activation of translation occurs in parallel with enhanced phosphate labelling of eIF-4 alpha and eIF-4 gamma and eIF-4F complex formation, events which are thought to facilitate the interaction of eIF-4F with the mRNA cap structure. However, with insulin, activation of translation occurs prior to detectable de novo phosphorylation of eIF-4F, although a small enhancement of turnover of phosphate on eIF-4 alpha may occur at this early time. With either hormone, enhanced phosphate labelling of eIF-4 alpha is shown to reflect activation of eIF-4 alpha kinase(s), which coincides temporally with activation of p42 MAP and p90rsk kinases. The possible role of initiation factor modification on increased translation rates during meiotic maturation is discussed.

Proteolytic cleavage of initiation factor eIF-4 gamma in the reticulocyte lysate inhibits translation of capped mRNAs but enhances that of uncapped mRNAs

Infection of cells with the foot-and-mouth-disease virus, a member of the picornavirus family, results in the shut-off of host protein synthesis. A major contributory mechanism is the proteolytic destruction of the gamma subunit of the complex eIF-4, which functions in translation to promote the binding of the 43S ribosomal preinitiation complex to the 5' end of the cellular mRNA molecules bearing a 5' terminal cap structure. Picornavirus RNA molecules, which are uncapped, use a distinct mechanism for translational initiation, which can operate in the absence, or at low levels, of eIF-4. The proteolysis of eIF-4 gamma in cells infected by foot-and-mouth-disease virus results from expression of a virus-encoded cysteine proteinase known as Leader (or L) protease. We have used a transcription plasmid encoding this protease as a tool to deplete in vitro translation systems of eIF-4 gamma in order to elucidate in more detail the role of this polypeptide in the control of translation. Using in vitro transcribed mRNAs we have observed a marked contrast between capped and uncapped transcripts in the response of their translation to the proteolysis of eIF-4 gamma. Translation of capped mRNAs is, as expected, severely impaired, and is restored by addition of eIF-4 complex containing the intact gamma-subunit. On the other hand, translation of uncapped transcripts, normally inefficient, is substantially enhanced. The data suggest that the translation of uncapped mRNAs may be stimulated in this system by one or more of the proteolytic degradation products of eIF-4 gamma.

Cap binding complexes and cellular growth control

The cap-binding complex eIF-4F plays a major role in the control of translation initiation, and overexpression of its limiting subunit, eIF-4E, leads to the deregulation of cellular growth. The recent cloning of eIF-4E binding proteins (4E-BPs) has uncovered a previously unsuspected pathway for the regulation of eIF-4E activity, through sequestration of eIF-4E as a complex with 4E-BPs.

Signal transduction mechanisms in the regulation of protein synthesis

Control of polypeptide synthesis plays an important role in cell proliferation and translation rates generally reflect the growth state of the cultured eukaryotic cell. Physiological regulation of protein synthesis is almost always exerted at the level of polypeptide chain initiation, with the binding of mRNA to the small ribosomal subunit a rate-limiting step in many cell systems. Studies have indicated key roles in the regulation of protein synthesis for the structural features of mRNA molecules and phosphorylation of initiation factors which catalyse this process. This review focuses on translational regulation at the level of mRNA binding to the ribosome and the role of phosphorylation of initiation factors in mediating both quantitative and qualitative control. The identity of putative kinases which may mediate these processes is addressed and a possible model for the role of a transient activation of initiation factors in cell growth or differentiation is presented.

Regulation of translation and cell growth by eIF-4E

This review discusses the regulation of a key controlling step in the initiation of protein synthesis, the binding of mRNA to ribosomes. Particular focus is given to the phosphorylation of the cap-binding factor, eIF-4E, and the role of this factor in the regulation of cell growth.

Expression of initiation factor genes in mammalian cells

This review focuses on how cells establish the levels of initiation factors, within the broader context of determining levels of the translational machinery. Most initiation factor polypeptides are moderately abundant proteins with concentrations approaching those of ribosomes. eIF4A and eIF5A are more abundant than ribosomes, whereas eIF4F alpha and eIF2B are considerably less abundant than the other factors. The cloning of cDNAs generates hybridization probes for monitoring the levels and activities of factor mRNAs, and the cloning of their genes is just beginning to provide insight into promoter structures and regulation. Initiation factor gene expression appears to be coordinately regulated in many cases, and preferential synthesis is seen in mitogen-activated T-cells. The gene for eIF2 alpha has been best characterized, and mechanisms that provide for the coordinated synthesis of eIF2 subunits are emerging. Recombinant DNA methods also allow investigators to manipulate the levels of expression of specific factor genes by overexpression or antisense repression. Such approaches provide a means to investigate in vivo the mechanisms of action of the initiation factors and their roles in regulating translation rates.