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From Proteopedia

Revision as of 21:00, 18 June 2023

Structure and functions

Fragile X messenger ribonucleoprotein (FMRP) is encoded by the fragile X messenger ribonucleoprotein 1 (FMR1) gene, located in the X chromosome, and is associated with the fragile X syndrome (FXS), Fragile X Tremor/Ataxia Syndrome (FXTAS) and Premature Ovarian Failure (POF1). FMRP functions as a synaptic regulator by binding to mRNAs and inhibiting its translation, therefore regulating the synthesis of proteins in the synapse. It is also an RNA binding protein, which is responsible for the transportation of mRNAs to the cytoplasm. The FMRP can also bind to its own FMR1 transcripts, possibly as a self-regulatory mechanism. The FMRP contains domains related to its RNA binding function, either in the N-terminal or C-terminal domain; the Agenet and the KH-motif are located in the N-terminal domain, and they, respectively, exerce functions in binding to methylated lysin and RNA binding; and the RGG box, in the C-terminal domain, acts as a binding to RNA, especifically to G-quadruplexes, a secondary RNA structure. ^[1] The protein has 20 non-redundant isoforms and the most common is isoform 7, and the longest isoform contains 632 aminoacids. The structure shown contains 209 aminoacids and was expressed in E. coli and viewed through X-ray diffraction with a 3 Å resolution.^[2]

Overall structure

RGG motif

A motif that is going to be explored is the RGG motif, which the protein uses to bind to guanine G-quadruplexes, a structure that consists of nucleic acid folding in which four guanines arrange in a planar conformation stabilized by Hoogsteen-trype hydrogen bonds, named tetrad. FMRP RGG motifs seem to prefer binding to specific structures, not linear motifs.

Different domains and motifs mediate the RNA binding mechanism, and the exon 15-encoded RGG (arginine - glycine - glycine) motif is one of them. The FMRP RGG motif is located in the C-terminal region of the protein and is well conserved in vertebrates. To easily represent the RGG motif binding to RNA, this motif will be highlighted in the scene Crystal structure of the complex between the human FMRP RGG motif and G-quadruplex RNA The RGG motif binds to G-quadruplexes when it adopts a sharp turn and specifically binds to guanines from two consecutive G-C base pairs in the duplex-quadruplex junction. Several tetrads can stack in a single G-quadruplex structure and be stabilized further by potassium cations, in the case of FMRP targets, whereas they are destabilized by lithium cations.

The regulation of particular mRNAs and the binding of FMRP with ribosomes and proteins depend on this interaction between the RGG motif and G-quadruplex RNA.

Therefore, understanding the interaction RGG-RNA is important for the comprehension of FMRP and FXS. The structure being represented on the right represents the FMRP RGG motif and the G-quadruplex secondary structure in the RNA. The protein structure was obtained by X-ray diffraction with a resolution of 3 Å. ^[3]

N-terminal domain and central portion

The N-terminal domain (NTD) of the FMRP is the first 215 aminoacids of the protein, containing two in tandem Agenet domains and one KH domain. The KH1 and KH2 motifs are located in the protein's central region. The NTD is important for the functions of the protein as a RBP and its participation in the ribonucleoprotein (RNP) complex.

The NTD was shown to adopt independent folding, and its tertiary structure was found to be compatible with two tandem agenet domains. Both of the Agenet domains resemble those of Argonaute, a protein involved in RNA interference. The KH domain folding is similar to other RBPs domains, which play an important role in RNA binding and protein-protein interactions. It has been demonstrated that the binds to methylated lysine due to its aromatic cage in the structure; specifically, the Agenet motif 2 binds to trimethylated lysine.

There are three in tandem . Many RNA-binding proteins contain the KH motif, a conserved RNA-binding domain. It is most likely involved in RNA binding and regulation, as seen in other proteins that also contain KH motifs. To give a more thorough response addressing the precise role played by the KH theme in FMRP, additional details would be needed. Overall, the FMRP NTD plays an important role as an RNA-binding protein, and its involvement in RNP complexes and its specific domains and motifs allow it to bind to specific RNAs and regulate their translation. ^[4]

The image of the N-terminal domain was obtained by X-ray diffraction with a 3.19 Å resolution and expressed in E. coli.

Associated diseases

Trinucleotides repeats (CGG) are located in CpG islands in the 5' untranslated region (UTR) of the gene related to the expression of the gene. Individuals carrying up to 44 repeats of trinucleotides are of common aleles. Individuals that have between 44 and 55 repeats are known to carry the premutation, usually associated with FXTAS and PFO1. However, when the repeat expansion is above 55 the individual is carrying the full mutation, which leads to the silencing of the gene, due to methylation, therefore there is absence or reduced levels of the FMRP, causing abnormal synaptic development and symptons associated with the FXS.

References

1. ↑ SUARDI, G. A. M.; HADDAD, L. A. Chapter Three - FMRP ribonucleoprotein complexes and RNA homeostasis.[1]
2. ↑ Hu, Y., Chen, Z., Fu, Y., He, Q., Jiang, L., Zheng, J., Gao, Y., Mei, P., Chen, Z. and Ren, X. (2015). The amino-terminal structure of human fragile X mental retardation protein obtained using precipitant-immobilized imprinted polymers. Nature Communications, [online] 6(1), p.6634. [2]
3. ↑ VASILYEV, N. et al. Crystal structure reveals specific recognition of a G-quadruplex RNA by a β-turn in the RGG motif of FMRP. Proceedings of the National Academy of Sciences, v. 112, n. 39, p. E5391–E5400, 15 set. 2015.[3]
4. ↑ MYRICK, L. K. et al. Human FMRP contains an integral tandem Agenet (Tudor) and KH motif in the amino terminal domain. Human Molecular Genetics, v. 24, n. 6, p. 1733–1740, 20 nov. 2014.[4]