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      Phosphorylation of glutamyl-prolyl tRNA synthetase by cyclin-dependent kinase 5 dictates transcript-selective translational control.

      Proceedings of the National Academy of Sciences of the United States of America
      Amino Acyl-tRNA Synthetases, genetics, metabolism, Cyclin-Dependent Kinase 5, Humans, Immunoblotting, Interferon-gamma, pharmacology, Nerve Tissue Proteins, Phosphorylation, drug effects, Protein Binding, Protein Biosynthesis, RNA Interference, Reverse Transcriptase Polymerase Chain Reaction, Ribosomal Proteins, Serine, Transcription, Genetic, U937 Cells, Vascular Endothelial Growth Factor A

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          Abstract

          Cyclin-dependent kinase 5 (Cdk5) is an atypical but essential member of the Cdk kinase family, and its dysregulation or deletion has been implicated in inflammation-related disorders by an undefined mechanism. Here we show that Cdk5 is an indispensable activator of the GAIT (IFN-γ-activated inhibitor of translation) pathway, which suppresses expression of a posttranscriptional regulon of proinflammatory genes in myeloid cells. Through induction of its regulatory protein, Cdk5R1 (p35), IFN-γ activates Cdk5 to phosphorylate Ser(886) in the linker domain of glutamyl-prolyl tRNA synthetase (EPRS), the initial event in assembly of the GAIT complex. Cdk5/p35 also induces, albeit indirectly via a distinct kinase, phosphorylation of Ser(999), the second essential event in GAIT pathway activation. Diphosphorylated EPRS is released from its residence in the tRNA multisynthetase complex for immediate binding to NS1-associated protein and subsequent binding to ribosomal protein L13a and GAPDH. The mature heterotetrameric GAIT complex binds the 3' UTR GAIT element of VEGF-A and other target mRNAs and suppresses their translation in myeloid cells. Inhibition of Cdk5/p35 inhibits both EPRS phosphorylation events, prevents EPRS release from the tRNA multisynthetase complex, and blocks translational suppression of GAIT element-bearing mRNAs, resulting in increased expression of inflammatory proteins. Our study reveals a unique role of Cdk5/p35 in activation of the major noncanonical function of EPRS, namely translational control of macrophage inflammatory gene expression.

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