Understanding the Regulation of mRNA Translation Repression by RGG-motif Proteins

Abstract

Messenger RNA translation is one of the most energy expensive steps in the process of gene expression and therefore translation regulation is critical for maintaining cellular homeostasis. In contrast to transcriptional regulation, rapid cellular changes in protein levels and thus physiological homeostasis can be achieved by regulation of mRNAs at translation level. At every stage of its life cycle, mRNA associates with RNA-binding proteins. The dynamic remodeling of mRNPs plays a major role in translation control and deciding mRNA fate. In yeast, a conserved translation initiation factor eIF4G, which promotes translation activation, was discovered to act as a nexus for binding of negative regulators of translation. A subset of RGG-motif containing proteins bind to eIF4G and repress mRNA translation in yeast. RGG-motifs are low-complexity sequences that contain repeats of arginine and glycine residues. Translation repressor protein Scd6, targets eIF4G1 but does not alter the eIF4F complex. It represses mRNA translation by inhibiting the 48S complex formation on mRNA. Although the mechanism of repression by Scd6 protein has been reported, the regulatory events that may control the repression activity of RGG-motif proteins was not known. In our first study, we uncovered the role of arginine methylation in modulating the translation repression activity of a conserved RGG-motif protein, Scd6. We found that Hmt1, a conserved arginine methyltransferase, binds Scd6 and methylates it at the RGG-motif. Scd6-eIF4G1 interaction was compromised upon deletion of Hmt1 or mutating arginines in the RGG-motif. Based on these results, we concluded that arginine methylation is a key event that augments the mRNA translation repression activity of Scd6 by promoting its interaction with eIF4G1. This study led us to ask an intriguing question about mechanism(s) that could keep the repression activity of Scd6 in check when the target mRNAs need to be translated. Scd6 C-terminal is rich in RG/RGG and Q/N repeats. Such repeat sequences render a low complexity to Scd6 C-terminus. In our second study, we have uncovered the role of RGG-motif in translation control through its self-association that competes with eIF4G binding. We demonstrated that Scd6 binds itself in an RNA-independent manner both in vivo and in vitro. We further elucidated that Scd6 self-interaction competes with eIF4G1 binding and arginine methylation of Scd6 RGG-motif by Hmt1 negatively affects self-association. For our third study in which we elucidate the role of RGG-motif containing mRNA export factor Gbp2 in translation control. Our study found that Gbp2 directly associates with eIF4G1 via its RGG-motif and RGG-motif of Gbp2 is important for its polysome association. We found that purified Gbp2 directly represses mRNA translation of a luciferase reporter mRNA, in vitro. We demonstrated the ability of Gbp2 to repress mRNA translation in vivo using a GFP-reporter mRNA tethering assay. Put together, this study hints at the role of an RGG-motif export factor in translation control.