Skeletal muscle mass is regulated by activity, metabolism, and the availability of nutrients. During
muscle atrophy, MNK2 expression increases. We found that MNK2 (
mitogen-activated protein kinase-interacting
kinase 2), but not MNK1, inhibited
proteins involved in promoting
protein synthesis, including
eukaryotic translation initiation factor 4G (
eIF4G) and
mammalian target of rapamycin (mTOR). Phosphorylation at
serine 1108 (Ser¹¹⁰⁸) of
eIF4G, which is associated with enhanced protein translation, is promoted by
insulin-like growth factor 1 and inhibited by
rapamycin or
starvation, suggesting that phosphorylation of this residue is regulated by mTOR. In cultured myotubes,
small interfering RNA (
siRNA) knockdown of MNK2 increased
eIF4G Ser¹¹⁰⁸ phosphorylation and overcame
rapamycin's inhibitory effect on this phosphorylation event. Phosphorylation of Ser¹¹⁰⁸ in
eIF4G, in gastrocnemius muscle, was increased in mice lacking MNK2, but not those lacking MNK1, and this increased phosphorylation was maintained in MNK2-null animals under
atrophy conditions and upon
starvation. Conversely, overexpression of MNK2 decreased
eIF4G Ser¹¹⁰⁸ phosphorylation. An
siRNA screen revealed that
serine-
arginine-rich
protein kinases linked increased MNK2 activity to decreased
eIF4G phosphorylation. In addition, we found that MNK2 interacted with mTOR and inhibited phosphorylation of the mTOR target, the ribosomal
kinase p70S6K (70-kD
ribosomal protein S6 kinase), through a mechanism independent of the
kinase activity of MNK2. These data indicate that MNK2 plays a unique role, not shared by its closest paralog MNK1, in limiting protein translation through its negative effect on
eIF4G Ser¹¹⁰⁸ phosphorylation and
p70S6K activation.