Features of the Catalytic Domains and C Termini of the MAPK Signal-integrating Kinases Mnk1 and Mnk2 Determine Their Differing Activities and Regulatory Properties

Abstract

The MAPK signal-integrating kinases Mnk1 and Mnk2 are closely related but show marked differences in their basal activities and regulation. Both possess, within their C termini, motifs for binding to MAPKs, although these differ between Mnk1 and Mnk2. Mnk2 shows much higher activity in unstimulated cells than Mnk1, whose activity is greatly increased, e.g. by stimulation of the MEK/ ERK pathway. Such increases are sensitive to blockade of that pathway, whereas the activation state of Mnk2 is relatively insensitive to inhibition of upstream signaling. Here we have studied the roles of features in their catalytic domains and C termini in determining their regulatory properties and basal activities. Mnk2 can bind to phosphorylated, active ERK, whereas Mnk1 cannot. Such binding apparently protects ERK against dephosphorylation and inactivation. The high basal activity of Mnk2 and its binding to (phospho)ERK requires features both of the catalytic domain and of the C terminus. For example, within the catalytic region an aspartate in Mnk2 plays a key role. Mutation to alanine inactivates Mnk2. In the C terminus, features within the MAPK-binding motif and to either side of it, including potential phosphorylation sites, affect MAPK binding and activity. The association of Mnks with the scaffold protein eukaryotic initiation factor 4G is negatively modulated by Mnk activity. These data indicate that multiple features determine the activities of the Mnks and thus impact on their ability to phosphorylate physiological substrates such as eukaryotic initiation factor 4E.

Mnk kinases in cytokine signaling and regulation of cytokine responses

“…Although Mnk1 and Mnk2 share a high degree of homology, there are differences in their regulation by MAPKs and in their basal kinase activity. Mnk1 can bind both p38 MAPK and Erk while Mnk2 can only bind Erk [7]. Engagement of Mnk kinases by external stimulu can be mediated by either p38 MAPK or Erk or both p38 MAPK and Erk [77].…”

“…Although most studies do not focus on differences in the functions of Mnk1 and Mnk2, there is emerging evidence that variances in the amino acid sequence of the catalytic domains and the C-terminal regions of the kinases contributes to differences in their activities [7]. Mnk1 has lower basal activity as compared to Mnk2, while Mnk2 activity is more resistant to upstream inhibition of either p38 MAPK or Erk [7].…”

“…Mnk1 has lower basal activity as compared to Mnk2, while Mnk2 activity is more resistant to upstream inhibition of either p38 MAPK or Erk [7]. An Asp residue in the catalytic domain of Mnk2 is necessary for its enhanced catalytic activity, while mutating the corresponding amino acid residue in the Mnk1 catalytic domain to Asp enhances the catalytic activity of Mnk1 [7]. Interestingly, swapping the Mnk1 and Mnk2 C terminal domains results in decreased activity in Mnk2 but fails to enhance Mnk1 activity [7].…”

“…An Asp residue in the catalytic domain of Mnk2 is necessary for its enhanced catalytic activity, while mutating the corresponding amino acid residue in the Mnk1 catalytic domain to Asp enhances the catalytic activity of Mnk1 [7]. Interestingly, swapping the Mnk1 and Mnk2 C terminal domains results in decreased activity in Mnk2 but fails to enhance Mnk1 activity [7]. Mnk1 can interact with both p38 MAPK and Erk whereas Mnk2 cannot bind p38 MAPK and, compared to Mnk1, exhibits enhanced binding ability for phosphorylated Erk [7].…”

“…Interestingly, swapping the Mnk1 and Mnk2 C terminal domains results in decreased activity in Mnk2 but fails to enhance Mnk1 activity [7]. Mnk1 can interact with both p38 MAPK and Erk whereas Mnk2 cannot bind p38 MAPK and, compared to Mnk1, exhibits enhanced binding ability for phosphorylated Erk [7]. Such enhanced binding of Mnk2 to Erk appears to be partially mediated by a glutamine (Gln) residue in the MAPK binding domain, but this is not sufficient to completely explain the observed differences and additional mechanisms may be involved [7].…”