Heterotrimeric GAIT Complex Drives Transcript-Selective Translation Inhibition in Murine Macrophages

The gamma interferon (IFN-γ)-activated inhibitor of translation (GAIT) complex in human myeloid cells is heterotetrameric, consisting of glutamyl-prolyl-tRNA synthetase (EPRS), NS1-associated protein 1 (NSAP1), ribosomal protein L13a, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The complex binds a structural GAIT element in the 3′ untranslated region ofVEGF-A and other inflammation-related transcripts and inhibits their translation. EPRS is dually phosphorylated by cyclin-dependent kinase 5 (Cdk5) at Ser886 and then by a Cdk5-dependent-AGC kinase at Ser999 ; L13a is phosphorylated at Ser77 by death-associated protein kinases DAPK and ZIPK. Because profound differences in inflammatory responses between mice and humans are known, we investigated the GAIT system in mouse macrophages. The murine GAIT complex is heterotrimeric, lacking NSAP1. As in humans, IFN-γ activates the mouse macrophage GAIT system via induced phosphorylation of EPRS and L13a. Murine L13a is phosphorylated at Ser77 by the DAPK-ZIPK cascade, but EPRS is phosphorylated only at Ser999 . Loss of EPRS Ser886 phosphorylation prevents NSAP1 incorporation into the GAIT complex. However, the triad of Ser999 -phosphorylated EPRS, Ser77 -phosphorylated L13a, and GAPDH forms a functional GAIT complex that inhibits translation of GAIT target mRNAs. Thus, translational control by the heterotrimeric GAIT complex in mice exemplifies the distinctive species-specific responses of myeloid cells to inflammatory stimuli.