Ribosomal RACK1: Protein Kinase C βII Modulates Intramolecular Interactions between Unstructured Regions of Eukaryotic Initiation Factor 4G (eIF4G) That Control eIF4E and eIF3 Binding

The receptor for activated C kinase (RACK1), a conserved constituent of eukaryotic ribosomes, mediates phosphorylation of eukaryotic initiation factor 4G1(S1093) [eIF4G1(S1093)] and eIF3a(S1364) by protein kinase C beta II (PKC beta II) (M. I. Dobrikov, E. Y. Dobrikova, and M. Gromeier, Mol Cell Biol 38:e00304-18, 2018, https://doi.org/10.1128/MCB.00304-18). RACK1:PKC beta II activation drives a phorbol ester-induced surge of global protein synthesis and template-specific translation induction of PKC-Raf-extracellular signal-regulated kinase 1/2 (ERK1/2)-responsive genes. For unraveling mechanisms of RACK1:PKC beta II-mediated translation stimulation, we used sequentially truncated eIF4G1 in coimmunoprecipitation analyses to delineate a set of autoinhibitory elements in the N-terminal unstructured region (surrounding the eIF4E-binding motif) and the interdomain linker (within the eIF3-binding site) of eIF4G1. Computer-based predictions of secondary structure, mutational analyses, and fluorescent titration with the beta-sheet dye thioflavin T suggest that eIF4G1(S1093) modulates a 4-stranded beta-sheet composed of antiparallel beta-hairpins formed by the autoinhibitory elements in eIF4G1's unstructured regions. The intact beta-sheet "locks" the eIF4G configuration, preventing assembly with eIF3/40S ribosomal subunits. Upon PKC stimulation, activated RACK1:PKC beta II phosphorylates eIF4G(S1093) in the tight 48S initiation complex, possibly facilitating dissociation/recycling of eIF4F.