Utilizing the DNA Aptamer to Determine Lethal α-Amanitin in Mushroom Samples and Urine by Magnetic Bead-ELISA (MELISA)

Amanita poisoning is one of the most deadly types of mushroom poisoning. α-Amanitin is the main lethal toxin in amanita, and the human-lethal dose is about 0.1 mg/kg. Most of the commonly used detection techniques for α-amanitin require expensive instruments. In this study, the α-amanitin aptamer was selected as the research object, and the stem-loop structure of the original aptamer was not damaged by truncating the redundant bases, in order to improve the affinity and specificity of the aptamer. The specificity and affinity of the truncated aptamers were determined using isothermal titration calorimetry (ITC) and gold nanoparticles (AuNPs), and the affinity and specificity of the aptamers decreased after truncation. Therefore, the original aptamer was selected to establish a simple and specific magnetic bead-based enzyme linked immunoassay (MELISA) method for α-amanitin. The detection limit was 0.369 μg/mL, while, in mushroom it was 0.372 μg/mL and in urine 0.337 μg/mL. Recovery studies were performed by spiking urine and mushroom samples with α-amanitin, and these confirmed the desirable accuracy and practical applicability of our method. The α-amanitin and aptamer recognition sites and binding pockets were investigated in an in vitro molecular docking environment, and the main binding bases of both were T3, G4, C5, T6, T7, C67, and A68. This study truncated the α-amanitin aptamer and proposes a method of detecting α-amanitin.

A UHM - ULM interface contributes to U2AF2 and SF3B1 association for pre-mRNA splicing

In the early stages of spliceosome assembly, the 3' splice site is recognized by sequential complexes of U2AF2 with SF1 followed by the SF3B1 subunit of the U2 small nuclear ribonucleoprotein particle. The U2AF2 - SF1 interface comprises a U2AF homology motif (UHM) of U2AF2 and a well-characterized U2AF ligand motif (ULM)/coiled coil region of SF1. However, the structure of the U2AF2 - SF3B1 interface and its importance for pre-mRNA splicing is unknown. To address this knowledge gap, we determined the crystal structure of the U2AF2 UHM bound to a SF3B1 ULM site at 1.8 Å resolution. The trajectory of the SF3B1 ULM across the U2AF2 UHM surface differed from prior UHM/ULM structures. This distinctive structure is expected to modulate the orientations of the full-length proteins. Using isothermal titration calorimetry, we established similar binding affinities of a minimal U2AF2 UHM - SF3B1 ULM complex and a nearly full-length U2AF2 protein binding the N-terminal SF3B1 region, with or without an auxiliary SF3B6 subunit. We showed that key residues at the U2AF2 UHM - SF3B1 ULM interface are required for high affinity association and co-immunoprecipitation of the splicing factors. Moreover, disrupting the U2AF2 - SF3B1 interface altered splicing of representative human transcripts. Further analysis of these transcripts and genome-wide data sets indicated that the subset of splice sites co-regulated by U2AF2 and SF3B1 are largely distinct from those co-regulated by U2AF2 and SF1. Altogether, these findings support distinct structural and functional roles for the sequential SF1 and SF3B1 complexes with U2AF2 during the pre-mRNA splicing process.

DNA binding by the antimalarial compound artemisinin

Artemisinin (ART) is a vital medicinal compound that is used alone or as part of a combination therapy against malaria. ART is thought to function by attaching to heme covalently and alkylating a range of proteins. Using a combination of biophysical methods, we demonstrate that ART is bound by three-way junction and duplex containing DNA molecules. Binding of ART by DNA is first shown for the cocaine-binding DNA aptamer and extensively studied using this DNA molecule. Isothermal titration calorimetry methods show that the binding of ART is both entropically and enthalpically driven at physiological NaCl concentration. Native mass spectrometry methods confirm DNA binding and show that a non-covalent complex is formed. Nuclear magnetic resonance spectroscopy shows that ART binds at the three-way junction of the cocaine-binding aptamer, and that binding results in the folding of the structure-switching variant of this aptamer. This structure-switching ability was exploited using the photochrome aptamer switch assay to demonstrate that ART can be detected using this biosensing assay. This study is the first to demonstrate the DNA binding ability of ART and should lay the foundation for further work to study implications of DNA binding for the antimalarial activity of ART.

Structure, dynamics, and inhibition of Staphylococcus aureus m1A22-tRNA methyltransferase

The enzyme m1A22-tRNA methyltransferase (TrmK) catalyses the transfer of a methyl group from SAM to the N1 of adenine 22 in tRNAs. TrmK is essential for Staphylococcus aureus survival during infection, but has no homologue in mammals, making it a promising target for antibiotic development. Here we describe the structural and functional characterisation of S. aureus TrmK. Crystal structures are reported for S. aureus TrmK apoenzyme and in complexes with SAM and SAH. Isothermal titration calorimetry showed that SAM binds to the enzyme with favourable but modest enthalpic and entropic contributions, whereas SAH binding leads to an entropic penalty compensated by a large favourable enthalpic contribution. Molecular dynamics simulations point to specific motions of the C-terminal domain being altered by SAM binding, which might have implications for tRNA recruitment. Activity assays for S. aureus TrmK-catalysed methylation of WT and position 22 mutants of tRNALeu demonstrate that the enzyme requires an adenine at position 22 of the tRNA. Intriguingly, a small RNA hairpin of 18 nucleotides is methylated by TrmK depending on the position of the adenine. In-silico screening of compounds suggested plumbagin as a potential inhibitor of TrmK, which was confirmed by activity measurements. Furthermore, LC-MS indicated the protein was covalently modified by one equivalent of the inhibitor, and proteolytic digestion coupled with LC-MS identified Cys92, in the vicinity of the SAM-binding site, as the sole residue modified. These results these results identify a cryptic binding pocket of S. aureus TrmK and lay the foundation for future structure-based drug discovery.

LanM Peptides – Unravelling the Binding Properties of the EF-Hand Loop Sequences Stripped from the Structural Corset

Since the discovery of the biological relevance of lanthanides (Lns) for methylotrophic bacteria in the last decade, the field has seen a steady rise in discoveries of bacteria using Lns. The major role of lanthanides here is in the active sites of enzymes: methanol dehydrogenases. Additionally, lanthanide binding proteins have also been identified. One such protein is lanmodulin (LanM) and, with a remarkable selectivity for Lns over Ca(II) and affinities in the picomolar range, it makes an attractive target to address challenges in lanthanide separation. Why LanM has such a high selectivity is currently not entirely understood, both the specific amino acid sequences of the EF-hand loops, together with cooperativity effects have been suggested. Consequently, we decided to remove the effect of cooperativity by focusing on the amino acid level. Thus, we synthesized all four 12-amino acid EF-Hand loop peptides of LanM using solid phase peptide synthesis and investigated their affinity for Lns (Eu(III), Tb(III)), the actinide Cm(III) and Ca(II). Using isothermal titration calorimetry and time resolved laser fluorescence spectroscopy combined with parallel factor analysis, we show that in the absence of cooperativity the short EF-Hand loop peptides have all similar affinities for lanthanides and that these are all in the micromolar range. Furthermore, calcium was shown not to bind to the peptides which was verified with circular dichroism spectroscopy. This technique also revealed that the peptides undergo a change to a more ordered state when lanthanides are added. These experimental observations were further supported by molecular dynamics simulations. Lastly, we put Eu(III) and Cm(III) in direct competition using TRLFS. Remarkably, a slightly higher affinity for the actinide, as was also observed for LanM, was found. Our results demonstrate that the picomolar affinities in LanM are largely an effect of pre-structuring in the full protein and therefore reduction of flexibility in combination with cooperative effects, and that all EF-Hand loops possess similar affinities when detached from the protein backbone, albeit still retaining the high selectivity for lanthanides and actinides over calcium.

Gold Nanoparticles Augment N-Terminal Cleavage and Splicing Reactions in Mycobacterium tuberculosis SufB

Protein splicing is a self-catalyzed event where the intervening sequence intein cleaves off, joining the flanking exteins together to generate a functional protein. Attempts have been made to regulate the splicing rate through variations in temperature, pH, and metals. Although metal-regulated protein splicing has been more captivating to researchers, metals were shown to only inhibit splicing reactions that confine their application. This is the first study to show the effect of nanoparticles (NPs) on protein splicing. We found that gold nanoparticles (AuNPs) of various sizes can increase the splicing efficiency by more than 50% and the N-terminal cleavage efficiency by more than 45% in Mycobacterium tuberculosis SufB precursor protein. This study provides an effective strategy for engineering splicing-enhanced intein platforms. UV-vis absorption spectroscopy, isothermal titration calorimetry (ITC), and transmission electron microscopy (TEM) confirmed AuNP interaction with the native protein. Quantum mechanics/molecular mechanics (QM/MM) analysis suggested a significant reduction in the energy barrier at the N-terminal cleavage site in the presence of gold atom, strengthening our experimental evidence on heightened the N-terminal cleavage reaction. The encouraging observation of enhanced N-terminal cleavage and splicing reaction can have potential implementations from developing a rapid drug delivery system to designing a contemporary protein purification system.

The human gut symbiont Ruminococcus gnavus shows specificity to blood group A antigen during mucin glycan foraging: Implication for niche colonisation in the gastrointestinal tract

The human gut symbiont Ruminococcus gnavus displays strain-specific repertoires of glycoside hydrolases (GHs) contributing to its spatial location in the gut. Sequence similarity network analysis identified strain-specific differences in blood-group endo-β-1,4-galactosidase belonging to the GH98 family. We determined the substrate and linkage specificities of GH98 from R. gnavus ATCC 29149, RgGH98, against a range of defined oligosaccharides and glycoconjugates including mucin. We showed by HPAEC-PAD and LC-FD-MS/MS that RgGH98 is specific for blood group A tetrasaccharide type II (BgA II). Isothermal titration calorimetry (ITC) and saturation transfer difference (STD) NMR confirmed RgGH98 affinity for blood group A over blood group B and H antigens. The molecular basis of RgGH98 strict specificity was further investigated using a combination of glycan microarrays, site-directed mutagenesis, and X-ray crystallography. The crystal structures of RgGH98 in complex with BgA trisaccharide (BgAtri) and of RgGH98 E411A with BgA II revealed a dedicated hydrogen network of residues, which were shown by site-directed mutagenesis to be critical to the recognition of the BgA epitope. We demonstrated experimentally that RgGH98 is part of an operon of 10 genes that is overexpresssed in vitro when R. gnavus ATCC 29149 is grown on mucin as sole carbon source as shown by RNAseq analysis and RT-qPCR confirmed RgGH98 expression on BgA II growth. Using MALDI-ToF MS, we showed that RgGH98 releases BgAtri from mucin and that pretreatment of mucin with RgGH98 confered R. gnavus E1 the ability to grow, by enabling the E1 strain to metabolise BgAtri and access the underlying mucin glycan chain. These data further support that the GH repertoire of R. gnavus strains enable them to colonise different nutritional niches in the human gut and has potential applications in diagnostic and therapeutics against infection.