Mitogen‐activated protein (MAP) kinases bind tightly to many of their physiologically relevant substrates. We have identified a new subfamily of murine serine/threonine kinases, whose members, MAP kinase‐interacting kinase 1 (Mnk1) and Mnk2, bind tightly to the growth factor‐regulated MAP kinases, Erk1 and Erk2. Mnk1, but not Mnk2, also binds strongly to the stress‐activated kinase, p38. Mnk1 complexes more strongly with inactive than active Erk, implying that Mnk and Erk may dissociate after mitogen stimulation. Erk and p38 phosphorylate Mnk1 and Mnk2, which stimulates their in vitro kinase activity toward a substrate, eukaryotic initiation factor‐4E (eIF‐4E). Initiation factor eIF‐4E is a regulatory phosphoprotein whose phosphorylation is increased by insulin in an Erk‐dependent manner. In vitro, Mnk1 rapidly phosphorylates eIF‐4E at the physiologically relevant site, Ser209. In cells, Mnk1 is post‐translationally modified and enzymatically activated in response to treatment with either peptide growth factors, phorbol esters, anisomycin or UV. Mitogen‐ and stress‐mediated Mnk1 activation is blocked by inhibitors of MAP kinase kinase 1 (Mkk1) and p38, demonstrating that Mnk1 is downstream of multiple MAP kinases. Mnk1 may define a convergence point between the growth factor‐activated and one of the stress‐activated protein kinase cascades and is a candidate to phosphorylate eIF‐4E in cells.

Initiation factor eIF4E binds to the 5-cap of eukaryotic mRNAs and plays a key role in the mechanism and regulation of translation. It may be regulated through its own phosphorylation and through inhibitory binding proteins (4E-BPs), which modulate its availability for initiation complex assembly. eIF4E phosphorylation is enhanced by phorbol esters. We show, using specific inhibitors, that this involves both the p38 mitogen-activated protein (MAP) kinase and Erk signaling pathways. Cell stresses such as arsenite and anisomycin and the cytokines tumor necrosis factor-␣ and interleukin-1␤ also cause increased phosphorylation of eIF4E, which is abolished by the specific p38 MAP kinase inhibitor, SB203580. These changes in eIF4E phosphorylation parallel the activity of the eIF4E kinase, Mnk1. However other stresses such as heat shock, sorbitol, and H 2 O 2 , which also stimulate p38 MAP kinase and increase Mnk1 activity, do not increase phosphorylation of eIF4E. The latter stresses increase the binding of eIF4E to 4E-BP1, and we show that this blocks the phosphorylation of eIF4E by Mnk1 in vitro, which may explain the absence of an increase in eIF4E phosphorylation under these conditions.

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 32 P-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 32 P-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).

We have examined the effects of widely used stress-inducing agents on protein synthesis and on regulatory components of the translational machinery. The three stresses chosen, arsenite, hydrogen peroxide and sorbitol, exert their effects in quite different ways. Nonetheless, all three rapidly ( 30 min) caused a profound inhibition of protein synthesis. In each case this was accompanied by dephosphorylation of the eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1) and increased binding of this repressor protein to eIF4E. Binding of 4E-BP1 to eIF4E correlated with loss of eIF4F complexes. Sorbitol and hydrogen peroxide each caused inhibition of the 70-kDa ribosomal protein S6 kinase, while arsenite activated it. The effects of stresses on the phosphorylation of eukaryotic elongation factor 2 also differed: oxidative stress elicited a marked increase in eEF2 phosphorylation, which is expected to contribute to inhibition of translation, while the other stresses did not have this effect. Although all three proteins (4E-BP1, p70 S6 kinase and eEF2) can be regulated through the mammalian target of rapamycin (mTOR), our data imply that stresses do not interfere with mTOR function but act in different ways on these three proteins. All three stresses activate the p38 MAP kinase pathway but we were able to exclude a role for this in their effects on 4E-BP1. Our data reveal that these stress-inducing agents, which are widely used to study stress-signalling in mammalian cells, exert multiple and complex inhibitory effects on the translational machinery.Keywords: stress; initiation; elongation factor; mRNA translation; S6 kinase.The control of mRNA translation in mammalian cells involves the regulation of a range of components of the translational machinery, principally by changes in their phosphorylation, leading to modulation of their activities or their abilities to interact with one another [1,2].Initiation factor 4E (eIF4E) plays a key role in mRNA translation and its control in eukaryotic cells. eIF4E binds to the 5¢ cap structure (containing 7-methylguanosine triphosphate; m 7 GTP) which is present at the 5¢ end of all cellular cytoplasmic mRNAs [3,4]. eIF4E can be regulated by its own phosphorylation (which occurs at a single major site (Ser209) [5,6]; and by binding proteins (4E-BPs) that modulate its availability for initiation complex formation (reviewed in [7]). eIF4E forms a complex termed eIF4F, which also contains the translation factors eIF4G (formerly called p220) and eIF4A. eIF4A has ATP-dependent RNA helicase activity thought to be required to unwind regions of self-complementary secondary structure in the 5¢ UTRs of certain mRNAs [4,8]. Such secondary structure inhibits translation and therefore mRNAs with 5¢ UTRs that contain significant secondary structure are often poorly translated. In contrast to many other cellular mRNAs, translation of heat shock protein mRNAs appears to be relatively cap-independent (reviewed in [9-11]), and translation of the mRNA for the stress-protein BiP/grp78 occ...

There is mounting evidence that in fat and other insulin-sensitive cells activation of protein synthesis may involve the dissociation of a protein (4E-BP1) from eukaryotic initiation factor (eIF)-4E thus allowing formation of the eIF-4F complex. This study compares the effects of insulin and epidermal growth factor (EGF) on the phosphorylation of 4E-BP1 in fat-cells (followed by gel-shift assays and incorporation of 32P) and on its association with eIF-4E. Several lines of evidence suggest that mitogenactivated protein kinase (MAP kinase) is not involved in these effects of insulin. Insulin causes much more extensive phosphorylation and dissociation of 4E-BP1 from eIF-4E than EGF, although EGF activates MAP kinase to a much greater extent than insulin. Moreover, MAP kinase does not phosphorylate 4E-BP1 when it is complexed with eIF-4E. In contrast, insulin activates the 40S ribosomal protein S6 kinase (p70S6K) 18-fold compared with a 2-fold activation by EGF, and the time course of this activation is similar to the phosphorylation and dissociation of 4E-BP1. Rapamycin, a specific inhibitor of the activation of this latter kinase, inhibits dissociation of 4E-BP1 from eIF-4E in cells incubated with insulin but reveals a phosphorylated from of 4E-BP1 which remains bound to eIF-4E. It is concluded that in rat epididymal fat-cells, the effects of insulin on 4E-BP1 involves multiple phosphorylation events. One phosphorylation event is rapamycin-insensitive, occurs only on bound 4E-BP1 and does not initiate dissociation. The second event does result in dissociation and is blocked by rapamycin, suggesting that the p70S6K signalling pathway is involved: p70S6K itself is probably not involved directly as this kinase does not phosphorylate 4E-BP1 in vitro.