Specific RNA-binding antibodies with a four-amino-acid code

Isolation and Characterization of scFv Antibody against Internal Ribosomal Entry Site of Enterovirus A71

Enterovirus A71 (EV-A71) is one of the causative agents of hand-foot-mouth disease, which can be associated with neurocomplications of the central nervous system. A limited understanding of the virus's biology and pathogenesis has led to the unavailability of effective anti-viral treatments. The EV-A71 RNA genome carries type I internal ribosomal entry site (IRES) at 5' UTR that plays an essential role in the viral genomic translation. However, the detailed mechanism of IRES-mediated translation has not been elucidated. In this study, sequence analysis revealed that the domains IV, V, and VI of EV-A71 IRES contained the structurally conserved regions. The selected region was transcribed in vitro and labeled with biotin to use as an antigen for selecting the single-chain variable fragment (scFv) antibody from the naïve phage display library. The so-obtained scFv, namely, scFv #16-3, binds specifically to EV-A71 IRES. The molecular docking showed that the interaction between scFv #16-3 and EV-A71 IRES was mediated by the preferences of amino acid residues, including serine, tyrosine, glycine, lysine, and arginine on the antigen-binding sites contacted the nucleotides on the IRES domains IV and V. The so-produced scFv has the potential to develop as a structural biology tool to study the biology of the EV-A71 RNA genome.

Fab Chaperone-Assisted RNA Crystallography (Fab CARC)

Recent discovery of structured RNAs such as ribozymes and riboswitches shows that there is still much to learn about the structure and function of RNAs. Knowledge learned can be employed in both biochemical research and clinical applications. X-ray crystallography gives unparalleled atomic-level structural detail from which functional inferences can be deduced. However, the difficulty in obtaining high-quality crystals and their phasing information make it a very challenging task. RNA crystallography is particularly arduous due to several factors such as RNA's paucity of surface chemical diversity, lability, repetitive anionic backbone, and flexibility, all of which are counterproductive to crystal packing. Here we describe Fab chaperone assisted RNA crystallography (CARC), a systematic technique to increase RNA crystallography success by facilitating crystal packing as well as expediting phase determination through molecular replacement of conserved Fab domains. Major steps described in this chapter include selection of a synthetic Fab library displayed on M13 phage against a structured RNA crystallization target, ELISA for initial choice of binding Fabs, Fab expression followed by protein A affinity then cation exchange chromatography purification, final choice of Fab by binding specificity and affinity as determined by a dot blot assay, and lastly gel filtration purification of a large quantity of chosen Fabs for crystallization.

DNA-rescuable allosteric inhibition of aptamer II ligand affinity by aptamer I element in the shortened Vibrio cholerae glycine riboswitch

Glycine riboswitches contain two aptamers and turn on the expression of downstream genes in bacteria. Although full-length glycine riboswitches were shown to exhibit no glycine-binding cooperativity, the truncated glycine riboswitches were confirmed to bind two glycine molecules cooperatively. Thorough understanding of the ligand-binding cooperativity may shed light on the molecular basis of the cooperativity and help design novel intricate biosensing genetic circuits for application in synthetic biology. A previously proposed sequential model does not readily provide explanation for published data showing a deleterious mutation in the first aptamer inhibiting the glycine binding of the second one. Using the glycine riboswitch from Vibrio cholerae as a model system, we have identified a region in the first aptamer that modulates the second aptamer function especially in the shortened glycine riboswitch. Importantly, this modulation can be rescued by the addition of a complementary oligodeoxynucleotide, demonstrating the feasibility of developing this system into novel genetic circuits that sense both glycine and a DNA signal.

Characterizing the dynamics of the leader-linker interaction in the glycine riboswitch with site-directed spin labeling

Site-directed spin labeling with continuous wave electron paramagnetic resonance (EPR) spectroscopy was utilized to characterize dynamic features of the kink–turn motif formed through a leader–linker interaction in the Vibrio cholerae glycine riboswitch. Efficient incorporation of spin-labels into select sites within the phosphate backbone of the leader–linker region proceeded via splinted ligation of chemically synthesized spin-labeled oligonucleotides to in vitro transcribed larger RNA fragments. The resultant nitroxide EPR line shapes have spectral characteristics consistent with a kink–turn motif and reveal differential backbone dynamics that are modulated by the presence of magnesium, potassium, and glycine.

Site-directed spin-labeling strategies and electron paramagnetic resonance spectroscopy for large riboswitches

Genetic regulation effected by RNA riboswitches is governed by ligand-induced structural reorganization with modulation of RNA conformation and dynamics. Characterization of the conformational states of riboswitches in the presence or absence of salts and ligands is important for understanding how interconversion of riboswitch RNA folding states influences function. The methodology of site-directed spin labeling (SDSL) coupled with electron paramagnetic resonance (EPR) spectroscopy is suitable for such studies, wherein site-specific incorporation of a nitroxide radical spin probe allows for local dynamics and conformational changes to be investigated. This chapter reviews a strategy for SDSL-EPR studies of large riboswitches and uses the full length 232 nucleotide (nt) kink-turn motif-containing Vibrio cholerae (VC) glycine riboswitch as an example. Spin-labeling strategies and the challenges of incorporating spin labels into large riboswitches are reviewed and the approach to overcome these challenges is described. Results are subsequently presented illustrating changes in dynamics within the labeled region of the VC glycine riboswitch as observed using SDSL-EPR.