Identification and characterization of the methyl arginines in the fragile X mental retardation protein Fmrp
April Stetler1, Claudia Winograd, Joyce Sayegh
1Department of Cell and Developmental Biology, University of Illinois, Urbana-Champaign, IL 61801, USA.
Abstract:
Fragile X syndrome is the most common form of inherited mental retardation and is caused by the absence of expression of the FMR1 gene. The protein encoded by this gene, Fmrp, is an RNA-binding protein that binds a subset of mRNAs and regulates their translation, leading to normal cognitive function. Although the association with RNAs is well established, it is still unknown how Fmrp finds and assembles with its RNA cargoes and how these activities are regulated. We show here that Fmrp is post-translationally methylated, primarily on its arginine-glycine-glycine box. We identify the four arginines that are methylated and show that cellular Fmrp is monomethylated and asymmetrically dimethylated. We also show that the autosomal paralog Fxr1 and the Drosophila ortholog dFmr1 are methylated post-translationally. Recombinant protein arginine methyl transferase 1 (PRMT1) methylates Fmrp on the same arginines in vitro as in cells. In vitro methylation of Fmrp results in reduced binding to the minimal RNA sequence sc1, which encodes a stem loop G-quartet structure. Our data identify an additional mechanism, arginine methylation, for modifying Fmrp function and suggest that methylation occurs to limit or modulate RNA binding by Fmrp.
Insights
Fragile X syndrome protein (FMRP) undergoes arginine methylation, a new modification impacting its RNA binding. This methylation, particularly on its RGG box, may regulate FMRP
Area of Science:
- Molecular Biology
- Neuroscience
- Genetics
Background:
- Fragile X syndrome, a leading inherited intellectual disability cause, stems from FMR1 gene silencing.
- The FMR1 gene product, FMRP, is an RNA-binding protein crucial for cognitive function.
- Mechanisms governing FMRP's RNA cargo binding and regulation remain incompletely understood.
Purpose of the Study:
- To investigate post-translational modifications of FMRP.
- To determine the impact of these modifications on FMRP's RNA-binding capabilities.
Main Methods:
- Analysis of FMRP methylation in cellular and in vitro systems.
- Identification of methylated arginine residues within FMRP.
- Assessment of RNA binding affinity following methylation.
Main Results:
- FMRP is post-translationally methylated on its arginine-glycine-glycine (RGG) box.
- Specific arginines were identified as methylation sites, with cellular FMRP exhibiting mono- and asymmetric dimethylation.
- In vitro methylation by PRMT1 reduced FMRP's binding to a specific RNA sequence (sc1).
- Methylation was also observed in FMRP paralogs (Fxr1) and orthologs (dFmr1).
Conclusions:
- Arginine methylation represents a novel regulatory mechanism for FMRP function.
- This modification appears to modulate or limit FMRP's interaction with specific RNAs.
- Understanding FMRP methylation offers new insights into Fragile X syndrome pathogenesis.
