The Ras/Raf/ERK pathway culminates in the activation of the Mnks (especially Mnk1a). Mnk1a can also be activated by p38 MAPK downstream of cytokines or stressful stimuli. Mnk binds to eIF4G and phosphorylates eIF4E within the eIF4F complex. The functional consequences of phosphorylation of eIF4E are unclear but are believed to favour the translation of certain mRNAs. Stimulation of PI3K, e.g., by growth factors, initiates a cascade of events: PDK1 activates AKT which phosphorylates TSC2, thereby inactivating the TSC1/TSC2 complex. Rheb, the small G protein, is no longer inhibited by the GAP (GTPase-activating protein) activity of TSC2 and the resulting Rheb-GTP activates mTORC1 via unknown mechanisms. mTORC1 promotes the activation of the translational activator S6K and the hierarchical phosphorylation of 4E-BP. Hyperphosphorylated 4E-BP is released from eIF4E, thereby allowing eIF4E to bind to eIF4G. Activated S6K phosphorylates eIF4B and PDCD4, effects which promote the helicase activity of eIF4A. The dashed lines indicate possible links.

(A) Interactions of CGP57380 (green) within the ATP binding domain, and (B) schematic presentation of the corresponding interactions. (C) Interactions of cercosporamide (green) within the ATP binding domain, and (D) schematic presentation of the corresponding interactions. The DFD motif: pink sticks; Hinge region: orange sticks; Gate keeper: red sticks; Inhibitors: green sticks. Hydrogen bonds: dashed lines; Hydrophobic region I: shaded pink; Hydrophobic region II: shaded light blue; Allosteric pocket: shaded light beige. The likely binding posts were generated by Autodock 4.0 (http://w3.to/autodock). The structure of the Mnk2-KRD228G–Staurosporine complex (PDB2HW7) was used as the initial template. The dimensions of the active site box were chosen to be large enough to encompass the entire ATP binding pocket and allosteric site. Docking calculations were carried out using the Lamarckian genetic algorithm (LGA). A maximum number of 2,500,000 energy evaluations were used. Each docking experiment consisted of 100 independent runs.

(i) The eIF4F complex including eIF4E, eIF4A and eIF4G is recruited to the mRNA via an interaction between eIF4E and the 5'-cap of the mRNA, which includes a 7-methylguanosine moiety; (ii) eIF4B binds to eIF4G and enhances the helicase activity of eIF4A; (iii) secondary structure (stem loops) in the 5'-untranslated region of the mRNA, which can inhibit translation initiation, are ‘unwound’ by eIF4A/eIF4B; (iv) the 40S ribosomal subunit is recruited to mRNA via an interaction between eIF4G and eIF3. Together with other translation factors, the 40S subunit then locates the start codon (‘AUG’) via ‘scanning’. The recruitment of the 40S subunit likely occurs earlier in the process than is depicted here.

(A) Sequence alignment of kinase domains inserts the DFD motif (shaded box); Thr209/Thr214, the MAPK phosphorylation sites (P); and the kinase inserts (shaded box). (B) The layout indicates the arrangement of the known functional domains (as labelled). NLS: nuclear localization signal; NES: nuclear export signal; eIF4G, the scaffolding protein of the translation initiation complex eIF4F that binds Mnk1 and Mnk2.

Residues around the DFD motif in either the DFG/D-IN or -OUT conformations are displayed as green sticks and the residues in the ATP-hinge region are shown as orange sticks. The gate keeper residues are indicated by red sticks. (A) The DFG/D-OUT conformation of wild-type Mnk2-KR is indicated with Phe227 and Asp228 poking into the ATP binding cleft (PDB 2AC3). The DFG/D-OUT conformation not only positions Phe227 and Asp228 in the ATP binding cleft, but also obstructs access to this cleft from the front. (B) In the Mnk2-KRD228G (Asp228Gly mutant), the DFG motif is found in both DFD/G-IN and DFD/G-OUT conformation and Phe227 flaps in/out shown by the green curly arrow (PDB 2AC5). (C) The DFG/D-OUT conformation of wild-type Mnk1-KR (PDB 2HW6), and (D) the structure of Mnk2-KRD228G–Staurosporine complex (PDB 2HW7). Staurosporine is displayed as a pink stick structure. The dashed lines indicate hydrogen bonds.

(A) 3D view of the interactions. The inhibitor is shown in green sticks. Residues around the inhibitor are shown in orange sticks. Red dashed lines indicated the hydrogen bond interactions or electrostatic interactions. (B) Schematic presentation of the corresponding interactions. Hydrogen bonds: dashed lines; Hydrophobic region I: shaded pink; Hydrophobic region II: shaded light blue; Allosteric pocket: shaded light beige. The binding post was generated by the following procedures: the DFD/G-IN structure of the Mnk2 (PDB 2AC3) was used as the initial template; the conformation of Phe227 was modified to make the ATP binding pocket accessible for the inhibitor.