RESUMO
G quadruplex structures have been predicted by bioinformatics to form in the 5'- and 3'-untranslated regions (UTRs) of several thousand mature mRNAs and are believed to play a role in translation regulation. Elucidation of these roles has primarily been focused on the 3'-UTR, with limited focus on characterizing the G quadruplex structures and functions in the 5'-UTR. Investigation of the affinity and specificity of RNA binding proteins for 5'-UTR G quadruplexes and the resulting regulatory effects have also been limited. Among the mRNAs predicted to form a G quadruplex structure within the 5'-UTR is the survival motor neuron domain containing 1 (SMNDC1) mRNA, encoding a protein that is critical to the spliceosome. Additionally, this mRNA has been identified as a potential target of the fragile X mental retardation protein (FMRP), whose loss of expression leads to fragile X syndrome. FMRP is an RNA binding protein involved in translation regulation that has been shown to bind mRNA targets that form G quadruplex structures. In this study we have used biophysical methods to investigate G quadruplex formation in the 5'-UTR of SMNDC1 mRNA and analyzed its interactions with FMRP. Our results show that SMNDC1 mRNA 5'-UTR forms an intramolecular, parallel G quadruplex structure comprised of three G quartet planes, which is bound specifically by FMRP both in vitro and in mouse brain lysates. These findings suggest a model by which FMRP might regulate the translation of a subset of its mRNA targets by recognizing the G quadruplex structure present in their 5'-UTR, and affecting their accessibility by the protein synthesis machinery.
Assuntos
Regiões 5' não Traduzidas , Química Encefálica , Proteína do X Frágil da Deficiência Intelectual/química , Quadruplex G , Fatores de Processamento de RNA/química , Proteínas do Complexo SMN/química , Sequência de Aminoácidos , Animais , Sequência de Bases , Sítios de Ligação , Encéfalo/metabolismo , Encéfalo/patologia , Proteína do X Frágil da Deficiência Intelectual/genética , Proteína do X Frágil da Deficiência Intelectual/metabolismo , Síndrome do Cromossomo X Frágil/genética , Síndrome do Cromossomo X Frágil/metabolismo , Síndrome do Cromossomo X Frágil/patologia , Expressão Gênica , Regulação da Expressão Gênica , Humanos , Camundongos , Ligação Proteica , Biossíntese de Proteínas , Domínios e Motivos de Interação entre Proteínas , Fatores de Processamento de RNA/genética , Fatores de Processamento de RNA/metabolismo , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Proteínas do Complexo SMN/genética , Proteínas do Complexo SMN/metabolismo , Spliceossomos/genética , Spliceossomos/metabolismo , Termodinâmica , Transcrição GênicaRESUMO
Fragile X syndrome, the most prevalent inheritable mental retardation, is caused by the loss of fragile X mental retardation protein (FMRP) expression. FMRP is an RNA-binding protein with nucleo-cytoplasmic shuttle activity, proposed to act as a translation regulator of specific mRNAs in the brain. It has been shown that FMRP uses its arginine-glycine-glycine (RGG) box domain to bind a subset of mRNA targets that form a G-quadruplex structure. FMRP has also been shown to undergo the post-translational modifications of arginine methylation and phosphorylation, as well as alternative splicing, resulting in multiple isoforms. The alternative splice isoforms investigated in this study, isoform 1 (ISO1), isoform 2 (ISO2), and isoform 3 (ISO3), are created by the alternative splicing acceptor site at exon 15. FMRP ISO2 and ISO3 are truncated by 12 and 13 residues, respectively, relative to the longest FMRP isoform ISO1. These truncations, which are in the close proximity of the RGG box domain, preserve the integrity of the RGG box in all three isoforms, but eliminate the in vivo phosphorylation sites, present only on FMRP ISO1. We have expressed and purified recombinant FMRP ISO1, ISO2 and ISO3 in Escherichia coli, free of post-translational modifications, and by using fluorescence spectroscopy, we show that each FMRP isoform binds G-quadruplex RNA, albeit with different binding affinities, suggesting that naturally occurring sequence modifications in the proximity of the RGG box modulate its G-quadruplex RNA binding ability.
Assuntos
Proteína do X Frágil da Deficiência Intelectual/metabolismo , Quadruplex G , Proteínas de Membrana/genética , Proteínas do Tecido Nervoso/genética , Isoformas de Proteínas/metabolismo , Sequência de Aminoácidos , Feminino , Proteína do X Frágil da Deficiência Intelectual/genética , Humanos , Masculino , Proteínas de Membrana/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Conformação de Ácido Nucleico , Isoformas de Proteínas/análise , Isoformas de Proteínas/genética , Processamento de Proteína Pós-Traducional , RNA Mensageiro/genética , RNA Mensageiro/metabolismoRESUMO
The loss of expression of the fragile X mental retardation protein (FMRP) leads to fragile X syndrome. FMRP has two types of RNA binding domains, two K-homology domains and an arginine-glycine-glycine box domain, and it is proposed to act as a translation regulator of specific messenger RNA. The interest to produce sufficient quantities of pure recombinant FMRP for biochemical and biophysical studies is high. However, the recombinant bacterial expression of FMRP has had limited success, and subsequent recombinant eukaryotic and in vitro expression has also resulted in limited success. In addition, the in vitro and eukaryotic expression systems may produce FMRP which is posttranslationally modified, as phosphorylation and arginine methylation have been shown to occur on FMRP. In this study, we have successfully isolated the conditions for recombinant expression, purification and long-term storage of FMRP using Escherichia coli, with a high yield. The expression of FMRP using E. coli renders the protein devoid of the posttranslational modifications of phosphorylation and arginine methylation, allowing the study of the direct effects of these modifications individually and simultaneously. In order to assure that FMRP retained activity throughout the process, we used fluorescence spectroscopy to assay the binding activity of the FMRP arginine-glycine-glycine box for the semaphorin 3F mRNA and confirmed that FMRP remained active.
