L13a Blocks 48S Assembly: Role of a General Initiation Factor in mRNA-Specific Translational Control

Abstract

Transcript-specific translational control restricts macrophage inflammatory gene expression. The proinflammatory cytokine interferon-gamma induces phosphorylation of ribosomal protein L13a and translocation from the 60S ribosomal subunit to the interferon-gamma-activated inhibitor of translation (GAIT) complex. This complex binds the 3'UTR of ceruloplasmin mRNA and blocks its translation. Here, we elucidate the molecular mechanism underlying repression by L13a. Translation of the GAIT element-containing reporter mRNA is sensitive to L13a-mediated silencing when driven by internal ribosome entry sites (IRESs) that require initiation factor eIF4G, but is resistant to silencing when driven by eIF4F-independent IRESs, demonstrating a critical role for eIF4G. Interaction of L13a with eIF4G blocks 43S recruitment without suppressing eIF4F complex formation. eIF4G attack, e.g., by virus, stress, or caspases, is a well-known mechanism of global inhibition of protein synthesis. However, our studies reveal a unique mechanism in which targeting of eIF4G by mRNA-bound L13a elicits transcript-specific translational repression.

Expression of Truncated Eukaryotic Initiation Factor 3e (eIF3e) Resulting from Integration of Mouse Mammary Tumor Virus (MMTV) Causes a Shift from Cap-dependent to Cap-independent Translation

“…We proposed that free eIF3e inhibits protein synthesis by interfering with the interaction between eIF3 and eIF4G (20). Ribosomal protein L13a also blocks 43 S PIC recruitment by interacting with eIF4G at the eIF3-binding site in an interferon-␥-dependent mechanism (21). Overexpression of eIF3e, but not eIF3j, blocks the interferon-␥-dependent binding between L13a and eIF4G.…”

RGG motif proteins: Modulators of mRNA functional states

“…Moreover, the RGG motifcontaining protein Nsr1 binds eIF4G in vivo and in purified form and represses translation (Rajyaguru P and Parker R, unpublished observation). It is interesting to note that proteins L13a 19 and Khd1, 20 which also target eIF4G to repress translation, contain RGG-/RGX motif. The functional significance of RGG motifs in these proteins is not understood.…”

“…42 Moreover, the ability of eIF4G-binding RGG motif proteins to bind specifically distinct subsets of mRNAs would lead to transcript-specific repression by these proteins. For example, Khd1 specifically promotes repression of ASH1 20 mRNA and, L13a is required for regulation of ceruloplasmin 19 mRNA.…”

“…eIF4G can provide a scaffold to recruit proteins that negatively affect translation, such as RGG motif proteins, 1 Khd1 20 and L13a. 19 The scaffolding role of eIF4G in promoting translation initiation is well known. Similarly role of eIF4G in splicing by recruiting splicing factors has been demonstrated.…”

Trading translation with RNA-binding proteins

“…Translational repression seems to require the formation of a transcript closed loop in order to place the GAIT complex in proximity to the 59 end (Mazumder et al 2001). L13a then binds to eIF4G, impeding its interaction with eIF3 and blocking 43S complex recruitment (Kapasi et al 2007). …”