Cyclin‐dependent kinase 5 mediated phosphorylation of GluProRS induces translational silencing of inflammatory gene expression

Abstract

The bifunctional aminoacyl‐tRNA synthetase GluProRS has a regulated noncanonical activity in translational control of multiple gamma‐interferon (IFN)‐inducible mRNAs. GluProRS is a component of the heterotetrameric GAIT (Gamma‐inteferon Activated Inhibitor of Translation) complex that binds the GAIT element in the 3’UTR of ceruloplasmin and VEGF mRNA and blocks their translation. Here we investigate the activation of GluProRS by phosphorylation, and its role in GAIT complex formation and function. In human monocytes, we have identified Ser886 and Ser999 in the non‐catalytic linker domain of GluProRS as the sites of phosphorylation upon induction with IFN. Phosphorylation of the two sites is catalyzed independently by two distinct kinases. Cyclin‐dependent kinase 5 (Cdk5), also known as neuronal CDC2‐like kinase (NCLK), phosphorylates Ser886. We have also elucidated an IFN‐activated kinase cascade in which ERK‐2 activates the Cdk5 activator protein, p35 (NCK5a) that activates Cdk5, the proximal kinase that phosphorylates GluProRS. Cdk5‐mediated phosphorylation induces rapid release of GluProRS from its residence in the tRNA multisynthetase complex and facilitates its interaction with the other GAIT proteins to form the active GAIT complex. Suppression of Cdk5 expression or activity inhibits GluProRS phosphorylation as well as the translational silencing function. The non‐neuronal activity of Cdk5 and its activator proteins, and the identification of GluProRS as a substrate in monocytes clearly indicate a unique extraneuronal function of Cdk5 in translational control of inflammatory gene expression. Research support: P01 HL29582 and AHA.