The GAIT system: a gatekeeper of inflammatory gene expression

Abstract

Functionally related genes are coregulated by specific RNA-protein interactions that direct transcript-selective translational control. In myeloid cells, interferon (IFN)-gamma induces formation of the heterotetrameric, IFN-gamma-activated inhibitor of translation (GAIT) complex comprising glutamyl-prolyl tRNA synthetase (EPRS), NS1-associated protein 1 (NSAP1), ribosomal protein L13a and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This complex binds defined 3' untranslated region elements within a family of inflammatory mRNAs and suppresses their translation. IFN-gamma-dependent phosphorylation, and consequent release of EPRS and L13a from the tRNA multisynthetase complex and 60S ribosomal subunit, respectively, regulates GAIT complex assembly. EPRS recognizes and binds target mRNAs, NSAP1 negatively regulates RNA binding, and L13a inhibits translation initiation by binding eukaryotic initiation factor 4G. Repression of a post-transcriptional regulon by the GAIT system might contribute to the resolution of chronic inflammation.

Figure 1

Secondary structures of GAIT elements. Secondary structures of functional 3′UTR GAIT elements as predicted by Mfold: ceruloplasmin (Cp, nt 78–106), vascular endothelial growth factor-A (VEGF-A, nt 358–386), death-associated protein kinase (DAPK, nt 1141–1169), zipper-interacting protein kinase (ZIPK, nt 174–206) and chemokine C-C motif ligand 22 (CCL22, nt 433–462) mRNAs. Nucleotides essential for Cp-GAIT activity are shown in red.

Figure 2

GAIT-mediated translational control. IFN-γ treatment elicits an intricate translational regulatory pathway in which the GAIT complex modulates the expression of its target mRNAs. (i) IFN-γ induces phosphorylation and release of EPRS (green) from the tRNA multisynthetase complex (yellow). Phosphorylated EPRS (P-EPRS) interacts with NSAP1 (light red) and forms an inactive, pre-GAIT complex. (ii) Later, L13a (dark blue) is phosphorylated and released from the 60S ribosomal subunit. Phosphorylated L13a (P-L13a), with GAPDH (brown), joins the pre-GAIT complex to form the functional GAIT complex. (iii) The complex binds the GAIT element in the 3′UTR of target transcripts circularized by simultaneous interactions of poly(A)-binding protein (PABP, gray) with eIF4G (orange) and the poly(A) tail. (iv) P-L13a interacts with eIF4G in the translation initiation complex to block recruitment of the eIF3 (light blue)-containing 43S ribosomal complex subunit and repress translation. Cap-binding protein, eIF4E, is shown (dark orange).

Figure 3

Early and late negative regulatory pathways of IFN-γ-driven gene expression programs. STAT1 and the GAIT complex regulate early transcription-induction (light red) and late translation-inhibition (light blue) regulons, respectively, with an intersection in both regulons (light violet). Dark red ellipses within the transcription-induction regulon denote early-IFN-γ-induced transcripts repressed by negative regulators of the JAK–STAT pathway (black); violet ellipses represent stable, IFN-γ-induced transcripts that evade transcriptional inhibition but are repressed by L13a (dark green) in the GAIT complex. Light blue ellipses denote transcripts induced by JAK–STAT-independent pathways, for example transcriptional induction by C/EBP-β. Abbreviations: IFN-γ-R, IFN-γ-receptor; PTP, protein tyrosine phosphatase.