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.