Dissecting RNA Selectivity Mediated by Tandem RNA-Binding Domains
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ABSTRACT: RNA-protein interactions are pivotal to proper gene regulation. Many RNA-binding proteins possess multiple RNA-binding domains; however, how these domains interplay to specify and regulate RNA targets remains poorly understood. Here, we investigate three multi-domain proteins, Musashi-1, Musashi-2, and Unkempt, three factors which share a high degree of RNA specificity. We use a combination of massively parallel in vitro assays with random or naturally derived RNA sequences and find that individual domains within a protein can have differing affinities, specificities, and spacing preferences. Further, we emphasize that while all three proteins have overlapping motif specificities, non-overlapping sequences may allow for target discrimination. We carry out large scale competition assays between these proteins and determine how individual protein specificities and affinities influence competitive binding. Integration of in vivo binding and regulation with in vitro specificities shows that target selection involves a combination of the protein intrinsic specificities described here, but cellular context is critical to drive these proteins to motifs in specific transcript regions. Finally, evolutionarily conserved RNA regions display evidence of binding multiple RBPs in vivo, and these RNA regions recapitulate this trend with the highest affinity in vitro. We highlight the importance of understanding features of complex RNA-protein interactions and how protein-target discrimination can be established.
ORGANISM(S): synthetic construct Mus musculus Homo sapiens
Project description:RNA recognition motifs (RRMs) are widespread RNA-binding protein domains in eukaryotes, which represent promising synthetic biology tools due to their compact structure and efficient activity. Yet, their use in prokaryotes is limited and their functionality poorly characterized. Recently, we repurposed a mammalian Musashi protein containing two RRMs as a translation regulator in Escherichia coli. Here, employing high-throughput RNA sequencing, we explored the impact of Musashi expression on the transcriptomic and translatomic profiles of E. coli, revealing certain metabolic interference, induction of post-transcriptional regulatory processes, and spurious protein-RNA interactions. Engineered Musashi protein mutants displayed compromised regulatory activity, emphasizing the importance of both RRMs for specific and sensitive RNA binding. We found that a mutation known to impede allosteric regulation led to similar translation control activity. Evolutionary experiments disclosed a loss of function of the synthetic circuit in about 40 generations, with the gene coding for the Musashi protein showing a stability comparable to other heterologous genes. Overall, this work expands our understanding of RRMs for post-transcriptional regulation in prokaryotes and highlight their potential for biotechnological and biomedical applications.
Project description:RNA recognition motifs (RRMs) are widespread RNA-binding protein domains in eukaryotes, which represent promising synthetic biology tools due to their compact structure and efficient activity. Yet, their use in prokaryotes is limited and their functionality poorly characterized. Recently, we repurposed a mammalian Musashi protein containing two RRMs as a translation regulator in Escherichia coli. Here, employing high-throughput RNA sequencing, we explored the impact of Musashi expression on the transcriptomic and translatomic profiles of E. coli, revealing certain metabolic interference, induction of post-transcriptional regulatory processes, and spurious protein-RNA interactions. Engineered Musashi protein mutants displayed compromised regulatory activity, emphasizing the importance of both RRMs for specific and sensitive RNA binding. We found that a mutation known to impede allosteric regulation led to similar translation control activity. Evolutionary experiments disclosed a loss of function of the synthetic circuit in about 40 generations, with the gene coding for the Musashi protein showing a stability comparable to other heterologous genes. Overall, this work expands our understanding of RRMs for post-transcriptional regulation in prokaryotes and highlight their potential for biotechnological and biomedical applications.
Project description:The anterior pituitary is comprised of distinct cell types that each secrete specific hormones to control a variety of biological processes including growth, metabolism, reproduction and stress responses. The anterior pituitary shows a remarkable level of cell type plasticity that allows shifts in hormone producing populations to meet organismal demands. Pituitary cell plasticity is tightly regulated and both deficiency in cell plasticity and excessive cell plasticity are associated with common pituitary pathologies. The molecular mechanisms underlying this plasticity are not well characterized but recent work has implicated the stem cell determinants and sequence-specific mRNA binding proteins of the Musashi family as regulators of adult pituitary hormone production. In this study we have sought to identify the full range of Musashi target mRNAs in the adult mouse pituitary. Using Musashi RNA immunoprecipitation we identify a cohort of 1192 mRNAs that show specific Musashi binding. These include mRNAs restricted to discrete hormone-producing cell lineages as well as mRNAs associated with stem and progenitor cells. The processes influenced by the proteins encoded by the Musashi-associated mRNAs include cellular homeostasis, protein trafficking and secretion, unfolded protein response, endocrine processes and female pregnancy. Functional analysis of validated mRNA regulatory 3' untranslated regions (3’ UTRs) reveals UTR-specific positive or negative control by the Musashi proteins within the same cellular context. Together, our findings indicate a broad role for Musashi proteins in the control of pituitary function.
Project description:Interactions between RNAs and RNA binding proteins (RBPs) regulate gene expression in eukaryotic cells. RNA-RBP affinities measured in vitro reveal diverse binding specificities, yet approaches to directly compare specificities across RBPs are lacking. Here, we introduce two quantitative metrics: inherent specificity, which measures how selectively an RBP distinguishes its strongest binding motif from all possible motifs, and mutational sensitivity, which assesses tolerance to single nucleotide variations within preferred motifs. Analyzing high-throughput sequencing datasets, we compared these metrics across 100 RBPs in vitro and 27 RBPs in cells, finding strong correlation between in vitro and cellular measurements for RBPs that bind RNA independently of a local structural context. Through CLIP experiments with swapped RNA recognition motifs between a low-specificity RBP (RBM25) and a high-specificity RBP (HNRNPC), we demonstrated that sequence specificity can be transferred between protein contexts. Using these insights, we developed mathematical models showing how RBPs with different specificity profiles compete for binding sites, revealing how variations in inherent specificity and mutational sensitivity influence target selection. Together, our results provide a quantitative framework for modeling RNA-RBP interactions and designing RBPs with targeted specificity.
Project description:To illuminate the extent and roles of exonic sequences in the splicing of human RNA transcripts we conducted saturation mutagenesis of a 51 nt internal exon in a 3-exon minigene. All possible single and tandem dinucleotide substitutions were surveyed. Using high throughput genetics, 5560 minigene molecules were assayed for splicing in HEK293 cells. Over 70% of mutations produced substantial (>2X) phenotypes of either increased or decreased splicing. Of all predicted secondary structural elements only a single 15 nt stem-loop, showed a strong correlation with splicing, acting negatively. The in vitro formation of exon-protein complexes between the mutant molecules and proteins associated with spliceosome formation (U2AF35, U2AF65, U1Aa, and U1-70K) correlated with splicing efficiencies, suggesting exon definition as the step affected by most mutations. The measured relative binding affinities of dozens of human RNA binding protein domains as reported in the CISBP-RNA database were found to correlate either positively or negatively with splicing efficiency, more than could fit on the 51 nt test exon simultaneously. Surprisingly, such correlations extended to weak relative protein-sequence affinities. These myriad protein binding correlations point to a dynamic and heterogeneous population of pre-mRNA molecules, each responding to a particular collection of binding proteins.
Project description:Tristetraprolin/ZFP36/TTP and ELAVL1/HuR are two disease-relevant RNA-binding proteins (RBPs) that both interact with AU-rich sequences but have antagonistic roles. While ELAVL1 binding has been profiled in several studies, the precise in vivo binding specificity of ZFP36 has not been investigated on a global scale. We determined ZFP36 binding preferences using cross-linking and immunoprecipitation in human embyonic kidney cells and examined combinatorial regulation of AU-rich elements by ZFP36 and ELAVL1. Among the targets ZFP36 binds and negatively regulates the mRNA of genes encoding proteins necessary for immune function and cancer, and other RBPs. Using partial correlation analysis, we were able to quantify the association between ZFP36 binding sites and differential target RNA abundance from ZFP36 overexpression independent of effects from confounding features, such as 3M-bM-^@M-^Y UTR length. We identified thousands of overlapping ZFP36 and ELAVL1 binding sites, in 1,313 genes. ZFP36 preferentially interacts with and regulates AU-rich sequences while ELAVL1 prefers predominantly U- and CU-rich sequences. RNA target specificity identified by global in vivo ZFP36-mRNA interactions were quantitatively similar to previously reported in vitro binding affinities. ZFP36 and ELAVL1 both bind an overlapping spectrum of RNA sequences, yet with differential relative preferences that dictate combinatorial regulatory potential. Our findings and methodology delineate an approach to untangle the in vivo combinatorial regulation by RNA-binding proteins. FLAG-HA ZFP36
Project description:Thousands of RNA-binding proteins (RBPs) crosslink to cellular mRNA. Among these are numerous unconventional RBPs (ucRBPs)—proteins that associate with RNA but lack known RNA-binding domains (RBDs). The vast majority of ucRBPs have uncharacterized RNA-binding specificities. We analyzed 492 human ucRBPs for intrinsic RNA-binding in vitro and identified 23 that bind specific RNA sequences. Most (17/23), including 8 ribosomal proteins, were previously associated with RNA-related function. We identified the RBDs responsible for sequence-specific RNA-binding for several of these 23 ucRBPs and surveyed whether corresponding domains from homologous proteins also display RNA sequence specificity. CCHC-zf domains from seven human proteins recognized specific RNA motifs, indicating that this is a major class of RBD. For Nudix, HABP4, TPR, RanBP2-zf, and L7Ae domains, however, only isolated members or closely related homologs yielded motifs, consistent with RNA-binding as a derived function. The lack of sequence specificity for most ucRBPs is striking, and we suggest that many may function analogously to chromatin factors, which often crosslink efficiently to cellular DNA, presumably via indirect recruitment. Finally, we show that ucRBPs tend to be highly abundant proteins and suggest their identification in RNA interactome capture studies could also result from weak nonspecific interactions with RNA.
Project description:Deciphering the mechanism of secondary cell wall/SCW formation in vascular plants is key to understanding their development and the molecular basis of biomass recalcitrance. Although a sophisticated network of transcription factors has been implicated in SCW synthesis in plants, little is known about the implication of RNA-binding proteins in this process. Here we report that two RNA-binding proteins homologous to the animal translational regulator Musashi, Musashi-Like2/MSIL2 and Musashi-Like4/MSIL4, function redundantly to control SCW formation in interfascicular fibers and the setting of biomass recalcitrance. We show that the disruption of MSIL2/4 (msil2-4 mutant) decreases the abundance of lignin in fibers and triggers an hypermethylation of glucuronoxylan that is linked to an over-accumulation of GlucuronoXylan Methyltransferase1/3 (GXM1/3) proteins.