Fellow’s research: Self-association of a repressor protein regulates protein biosynthesis inside a cell


25 Jul 2019

Fellow’s research: Self-association of a repressor protein regulates protein biosynthesis inside a cell

 

Dr Purusharth I Rajyaguru, Intermediate Fellow

Indian Institute of Science, Bengaluru

In our recent study, we observed that the activity of a protein synthesis repressor is regulated by binding to itself. The binding occurs through a region in its sequence rich in amino acids, arginine and glycine. Since, misregulated protein synthesis underlies a variety of human diseases including many cancers and neurodegenerative disorders, our recent research may improve molecular understanding of these disease states.

Proteins are the building blocks of the cell. Synthesizing proteins is a complex and energy-intensive process. The tightly regulated step of reading the information on the messenger RNA (mRNA) and using it to build proteins is called  ‘mRNA translation’. The cell decides what protein must be synthesized and how much of it, based on the physiological needs of the organism. Not all the proteins are synthesized all the time, thanks to ‘translation repressors’ that keep the translation process under check. The intriguing question is what regulates the translation repressors! It is also important to note here that the initiation of protein translation is considered to be the most regulated step of translation.

Using baker’s yeast (Saccharomyces cerevisiae) as our model system, we have tried to understand the regulation of translation repressor proteins. It is worthy to note that yeast is the simplest eukaryote to work with and the mechanisms of biological processes identified in yeast are similar in higher organisms.

Left: Unstressed yeast cells expressing Scd6GFP. Right: Sodium azide treated yeast cells expressing Scd6GFP. The ‘dots’ or ‘foci’ represent RNA granules (sites of translation repression).

Several proteins like Scd6, Sbp1, Npl3, and Khd1 have been identified as a class of translation repressors. They control translation of protein from mRNA by targeting the translation initiation factor protein, eIF4G, which brings the two ends of the mRNA together to begin the process of translation. In our current study, we tried to understand what prevents Scd6, one such translation repressor, from binding to eIF4G when the target mRNAs need to be translated.

Scd6 has several repeats of RG/RGGs (i.e. regions rich in the amino acids arginine {R} and glycine {G}) and Glutamine/Asparagine in its C terminus, thus making it a low-complexity amino acid sequence. We found out that Scd6 binds to itself through this RGG-rich region, which we found is sufficient for this self-interaction. The self-associated form of Scd6 competes with eIF4G binding thereby preventing it from targeting the translation initiation complex.

However, a post-translational modification of Scd6 protein that adds a methyl group to arginine (arginine methylation) disrupts the self-interaction of Scd6 and acts as a switch in regulating the protein function. The self-association mediated regulation of RGG-motif translation repressors could be a general phenomenon. We found that Sbp1, another translation repressor, also bound self via its RGG-repeat sequences and the interaction was tuned by arginine methylation.

Scd6 binds self through its RGG domain. Self-interaction is curtailed upon arginine methylation of the Scd6 RGG motif by Hmt1 (predominant methyltransferases in yeast). This promotes Scd6-eIF4G interaction leading to translation repression. (‘-CH3’represents arginine methylation).

This work, which is in collaboration with Dr Tanweer Hussain (India Alliance Intermediate Fellow, Molecular Reproduction Development and Genetics, Indian Institute of Science) helps us understand the contribution of RGG-motifs and their self-association to the fundamental process of mRNA fate determination. Since proteins with sequence and functional similarity (orthologue) to Scd6 are present in all higher organisms, including humans, we are currently exploring if the findings reported by us are conserved in higher organisms. RGG-motif proteins are reported to function in various cellular processes and it would be interesting to explore if the property of self-association is common to RGG-motif proteins involved in other cellular processes as well. 

Our long-term goal is to understand the mechanistic basis of regulation of mRNA translation by understanding the mRNA specificity of translation repressors and physiological-cue-based regulation of mRNAs. This crucial biological information will help us to better understand regulation of protein synthesis inside a cell, which may ultimately aid in the mechanistic understanding of disease development and progression.

Reference:

RGG-motif self-association regulates eIF4G-binding translation repressor protein Scd6. Gopalakrishna Poornima, Ravishankar Mythili, Priyabrata Nag, Sabnam Parbin, Praveen Kumar Verma, Tanweer Hussain & Purusharth I Rajyaguru. RNA Biology. May 2019