RNA-binding protein BrunoL1 mediates translational stimulation via interactions with eIF3

Protein synthesis occurs when mRNA is translated into amino acids. RNA-binding proteins interact with specific mRNAs in order to enhance or repress translation of those mRNAs. A known group of RNA binding proteins, the Bruno family, typically repress translation of mRNAs that contain Bruno Response Elements (BREs). Our lab has found that Xenopus BrunoL1 is expressed in endoderm progenitor cells and is required for proliferation and differentiation of these cells. Surprisingly, unlike its other family members, BrunoL1 enhances translation of specific mRNAs including cyclinA2. Yet it is unclear how BrunoL1 does this. To identify the BrunoL1 complex that mediates translational activation we performed an immunoprecipitation/mass spec experiment to identify BrunoL1 partner proteins that are involved in translational stimulation. From this experiment we identified five subunits of eukaryotic initiation factor 3 (eIF3). eIF3 is composed of thirteen subunits and is the largest initiation complex involved in translation. Acting as a scaffold for the preinitiation complex for translation, eIF3 binds to the small ribosomal unit and to eIF4. However, it is not known how eIF3 functions in the enhancement of translation. Through the use of cloning and in situ hybridization, the function of these eIF3 subunits in translational stimulation by BrunoL1 was examined. Cells that actively expressed BrunoL1 also actively expressed eIF3 subunits a, b, i, l, and m during various developmental stages, suggesting that BrunoL1 interacts with these subunits to enhance translation of specific mRNAs at these stages. My findings may be a model for how other RNA-binding proteins enhance translation.