Solution structure and RNA-binding of a minimal ProQ-homolog from Legionella pneumophila (Lpp1663).

Small regulatory RNAs (sRNAs) play an important role for posttranscriptional gene regulation in bacteria. sRNAs recognize their target messenger RNAs (mRNAs) by base-pairing, which is often facilitated by interactions with the bacterial RNA-binding proteins Hfq or ProQ. The FinO/ProQ RNA-binding protein domain was first discovered in the bacterial repressor of conjugation, FinO. Since then, the functional role of FinO/ProQ-like proteins in posttranscriptional gene regulation was extensively studied in particular in the enterobacteria E. coli and Salmonella enterica and a wide range of sRNA-targets was identified for these proteins. In addition, enterobacterial ProQ homologs also recognize and protect the 3'-ends of a number of mRNAs from exonucleolytic degradation. However, the RNA-binding properties of FinO/ProQ proteins with regard to the recognition of different RNA targets are not yet fully understood. Here, we present the solution NMR structure of the so far functionally uncharacterized ProQ homolog Lpp1663 from Legionella pneumophila as a newly confirmed member and a minimal model system of the FinO/ProQ protein family. In addition, we characterize the RNA-binding preferences of Lpp1663 with high resolution NMR spectroscopy and isothermal titration calorimetry (ITC). Our results suggest a binding preference for single-stranded uridine-rich RNAs in the vicinity of stable stem-loop structures. According to chemical shift perturbation experiments, the single-stranded U-rich RNAs interact mainly with a conserved RNA-binding surface on the concave site of Lpp1663.

Secondary structure determination of conserved SARS-CoV-2 RNA elements by NMR spectroscopy.

The current pandemic situation caused by the Betacoronavirus SARS-CoV-2 (SCoV2) highlights the need for coordinated research to combat COVID-19. A particularly important aspect is the development of medication. In addition to viral proteins, structured RNA elements represent a potent alternative as drug targets. The search for drugs that target RNA requires their high-resolution structural characterization. Using nuclear magnetic resonance (NMR) spectroscopy, a worldwide consortium of NMR researchers aims to characterize potential RNA drug targets of SCoV2. Here, we report the characterization of 15 conserved RNA elements located at the 5' end, the ribosomal frameshift segment and the 3'-untranslated region (3'-UTR) of the SCoV2 genome, their large-scale production and NMR-based secondary structure determination. The NMR data are corroborated with secondary structure probing by DMS footprinting experiments. The close agreement of NMR secondary structure determination of isolated RNA elements with DMS footprinting and NMR performed on larger RNA regions shows that the secondary structure elements fold independently. The NMR data reported here provide the basis for NMR investigations of RNA function, RNA interactions with viral and host proteins and screening campaigns to identify potential RNA binders for pharmaceutical intervention.

The Solution Structure of the Lantibiotic Immunity Protein NisI and Its Interactions with Nisin.

Many Gram-positive bacteria produce lantibiotics, genetically encoded and posttranslationally modified peptide antibiotics, which inhibit the growth of other Gram-positive bacteria. To protect themselves against their own lantibiotics these bacteria express a variety of immunity proteins including the LanI lipoproteins. The structural and mechanistic basis for LanI-mediated lantibiotic immunity is not yet understood. Lactococcus lactis produces the lantibiotic nisin, which is widely used as a food preservative. Its LanI protein NisI provides immunity against nisin but not against structurally very similar lantibiotics from other species such as subtilin from Bacillus subtilis. To understand the structural basis for LanI-mediated immunity and their specificity we investigated the structure of NisI. We found that NisI is a two-domain protein. Surprisingly, each of the two NisI domains has the same structure as the LanI protein from B. subtilis, SpaI, despite the lack of significant sequence homology. The two NisI domains and SpaI differ strongly in their surface properties and function. Additionally, SpaI-mediated lantibiotic immunity depends on the presence of a basic unstructured N-terminal region that tethers SpaI to the membrane. Such a region is absent from NisI. Instead, the N-terminal domain of NisI interacts with membranes but not with nisin. In contrast, the C-terminal domain specifically binds nisin and modulates the membrane affinity of the N-terminal domain. Thus, our results reveal an unexpected structural relationship between NisI and SpaI and shed light on the structural basis for LanI mediated lantibiotic immunity.

Large-Scale Recombinant Production of the SARS-CoV-2 Proteome for High-Throughput and Structural Biology Applications.

The highly infectious disease COVID-19 caused by the Betacoronavirus SARS-CoV-2 poses a severe threat to humanity and demands the redirection of scientific efforts and criteria to organized research projects. The international COVID19-NMR consortium seeks to provide such new approaches by gathering scientific expertise worldwide. In particular, making available viral proteins and RNAs will pave the way to understanding the SARS-CoV-2 molecular components in detail. The research in COVID19-NMR and the resources provided through the consortium are fully disclosed to accelerate access and exploitation. NMR investigations of the viral molecular components are designated to provide the essential basis for further work, including macromolecular interaction studies and high-throughput drug screening. Here, we present the extensive catalog of a holistic SARS-CoV-2 protein preparation approach based on the consortium's collective efforts. We provide protocols for the large-scale production of more than 80% of all SARS-CoV-2 proteins or essential parts of them. Several of the proteins were produced in more than one laboratory, demonstrating the high interoperability between NMR groups worldwide. For the majority of proteins, we can produce isotope-labeled samples of HSQC-grade. Together with several NMR chemical shift assignments made publicly available on covid19-nmr.com, we here provide highly valuable resources for the production of SARS-CoV-2 proteins in isotope-labeled form.

NMR assignments of a dynamically perturbed and dimerization inhibited N-terminal domain variant of a spider silk protein from E. australis.

Web spiders use specialized glands to produce silk proteins, so-called spidroins, which assemble into extraordinarily tough silk fibers through tightly regulated phase and structural transitions. A crucial step in the polymerization of spidroins is the pH-triggered assembly of their N-terminal domains (NTDs) into tight dimers. Major ampullate spidroin NTDs contain an unusually high content of the amino acid methionine. We previously showed that the simultaneous mutation of the six hydrophobic core methionine residues to leucine in the NTD of the major ampullate spidroin 1 from Euprosthenops australis, a nursery web spider, yields a protein (L6-NTD) retaining a three-dimensional fold identical to the wildtype (WT) domain, yet with a significantly increased stability. Further, the dynamics of the L6-NTD are significantly reduced and the ability to dimerize is severely impaired compared to the WT domain. These properties lead to significant changes in the NMR spectra between WT and L6-NTD so that the previously available WT-NTD assignments cannot be transferred to the mutant protein. Here, we thus report the de novo NMR backbone and side chain assignments of the major ampullate spidroin 1 L6-NTD variant from E. australis as a prerequisite for obtaining further insights into protein structure and dynamics.

A New Docking Domain Type in the Peptide-Antimicrobial-Xenorhabdus Peptide Producing Nonribosomal Peptide Synthetase from Xenorhabdus bovienii.

Nonribosomal peptide synthetases (NRPSs) produce a wide variety of different natural products from amino acid precursors. In contrast to single protein NRPS, the NRPS of the bacterium Xenorhabdus bovienii producing the peptide-antimicrobial-Xenorhabdus (PAX) peptide consists of three individual proteins (PaxA/B/C), which interact with each other noncovalently in a linear fashion. The specific interactions between the three different proteins in this NRPS system are mediated by short C- and N-terminal docking domains (C/NDDs). Here, we investigate the structural basis for the specific interaction between the CDD from the protein PaxB and the NDD from PaxC. The isolated DD peptides feature transient α-helical conformations in the absence of the respective DD partner. Isothermal titration calorimetry (ITC) and nuclear magnetic resonance (NMR) titration experiments showed that the two isolated DDs bind to each other and form a structurally well-defined complex with a dissociation constant in the micromolar range as is typical for many DD interactions. Artificial linking of this DD pair via a flexible glycine-serine (GS) linker enabled us to solve the structure of the DD complex by NMR spectroscopy. In the complex, the two DDs interact with each other by forming a three helix bundle arranged in an overall coiled-coil motif. Key interacting residues were identified in mutagenesis experiments. Overall, our structure of the PaxB CDD/PaxC NDD complex represents an architecturally new type of DD interaction motif.

1H, 13C, 15N backbone NMR resonance assignments for the rRNA methyltransferase Dim1 from the hyperthermophilic archaeon Pyrococcus horikoshii.

The protein dimethyladenosine transferase 1 (Dim1) is a highly conserved protein occurring in organisms ranging from bacteria such as E. coli where it is named KsgA to humans. Since Dim1 is involved in the biogenesis of the small ribosomal subunit it is an essential protein. During ribosome biogenesis Dim1 acts as an rRNA modification enzyme and dimethylates two adjacent adenosine residues of the small ribosomal subunit rRNA. In eukaryotes it is also required to ensure the proper endonucleolytic processing of the small ribosomal subunit rRNA precursor. Recently, a third function was proposed for eukaryotic Dim1. Karbstein and coworkers suggested that Dim1 interacts with the essential ribosome assembly factor Fap7 and that Fap7 is responsible for the dissociation of Dim1 from the nascent small ribosomal subunit. Here, we report the backbone 1H, 13C and 15N NMR resonance assignments for the 30.9 kDa Dim1 homologue from the hyperthermophilic archaeon Pyrococcus horikoshii (PhDim1) as a prerequisite for a detailed structural investigation of the PhDim1/PhFap7 interaction.

NMR resonance assignments for the GSPII-B domain of the traffic ATPase PilF from Thermus thermophilus in the apo and the c-di-GMP-bound state.

The PilF protein from the thermophilic bacterium Thermus thermophilus is a traffic ATPase powering the assembly of the DNA translocation machinery as well as of type 4 pili. Thereby PilF mediates the natural transformability of T. thermophilus. PilF contains a C-terminal ATPase domain and three N-terminal domains with partial homology to so-called general secretory pathway II (GSPII) domains. These three GSPII domains (GSPII-A, GSPII-B and GSPII-C) are essential for pilus assembly and twitching motility. They show varying degrees of sequence homology to the N-terminal domain of the ATPase MshE from Vibrio cholerae which binds the bacterial second messenger molecule c-di-GMP. NMR experiments demonstrate that the GSPII-B domain of PilF also binds c-di-GMP with high affinity and forms a 1:1 complex in slow exchange on the NMR time scale. As a prerequisite for structural studies of c-di-GMP binding to the GSPII-B domain of T. thermophilus PilF we present here the NMR resonance assignments for the apo and the c-di-GMP bound state of GSPII-B. In addition, we map the binding site for c-di-GMP on the GSPII-B domain using chemical shift perturbation data and compare the dynamics of the apo and the c-di-GMP-bound state of the GSPII-B domain based on {1H},15N-hetNOE data.

NMR resonance assignments for a ProQ homolog from Legionella pneumophila.

Regulation of gene expression on a post-transcriptional level by small non-coding regulatory RNAs (sRNAs) is very common in bacteria. sRNAs base pair with sequences in their target messenger RNAs (mRNAs) and thereby regulate translation initiation or mRNA stability. Specialized RNA-binding proteins (RBPs) facilitate these regulatory sRNA/mRNA interactions by acting as RNA chaperones. A well-known example for such an RNA chaperone which is widespread in bacteria is the Hfq protein. Recently, the ProQ/FinO protein family was identified as a new class of RNA chaperones involved in sRNA based regulation. Only a few members of this protein family have been structurally characterized so far. In particular, the structural basis for RNA-binding and recognition has not yet been established for this class of proteins. Here, we report the 1H, 13C and 15N NMR resonance assignments for a ProQ homolog (Lpp 1663) from the gram-negative pathogenic bacterium Legionella pneumophila which will facilitate further structural and dynamic studies of this protein and its interaction with RNA targets.

Structural Basis of TRPV4 N Terminus Interaction with Syndapin/PACSIN1-3 and PIP2.

Transient receptor potential (TRP) channels are polymodally regulated ion channels. TRPV4 (vanilloid 4) is sensitized by PIP2 and desensitized by Syndapin3/PACSIN3, which bind to the structurally uncharacterized TRPV4 N terminus. We determined the nuclear magnetic resonance structure of the Syndapin3/PACSIN3 SH3 domain in complex with the TRPV4 N-terminal proline-rich region (PRR), which binds as a class I polyproline II (PPII) helix. This PPII conformation is broken by a conserved proline in a cis conformation. Beyond the PPII, we find that the proximal TRPV4 N terminus is unstructured, a feature conserved across species thus explaining the difficulties in resolving it in previous structural studies. Syndapin/PACSIN SH3 domain binding leads to rigidification of both the PRR and the adjacent PIP2 binding site. We determined the affinities of the TRPV4 N terminus for PACSIN1, 2, and 3 SH3 domains and PIP2 and deduce a hierarchical interaction network where Syndapin/PACSIN binding influences the PIP2 binding site but not vice versa.

Structure-based redesign of docking domain interactions modulates the product spectrum of a rhabdopeptide-synthesizing NRPS.

Several peptides in clinical use are derived from non-ribosomal peptide synthetases (NRPS). In these systems multiple NRPS subunits interact with each other in a specific linear order mediated by specific docking domains (DDs), whose structures are not known yet, to synthesize well-defined peptide products. In contrast to classical NRPSs, single-module NRPS subunits responsible for the generation of rhabdopeptide/xenortide-like peptides (RXPs) can act in different order depending on subunit stoichiometry thereby producing peptide libraries. To define the basis for their unusual interaction patterns, we determine the structures of all N-terminal DDs (NDDs) as well as of an NDD-CDD complex and characterize all putative DD interactions thermodynamically for such a system. Key amino acid residues for DD interactions are identified that upon their exchange change the DD affinity and result in predictable changes in peptide production. Recognition rules for DD interactions are identified that also operate in other megasynthase complexes.

NMR resonance assignments of the lantibiotic immunity protein NisI from Lactococcus lactis.

The lantibiotic nisin is a small antimicrobial peptide which acts against a wide range of Gram-positive bacteria. Nisin-producing Lactococcus lactis strains express four genes for self-protection against their own antimicrobial compound. This immunity system consists of the lipoprotein NisI and the ABC transporter NisFEG. NisI is attached to the outside of the cytoplasmic membrane via a covalently linked diacylglycerol anchor. Both the lipoprotein and the ABC transporter are needed for full immunity but the exact immunity mechanism is still unclear. To gain insights into the highly specific immunity mechanism of nisin producing strains on a structural level we present here the backbone resonance assignment of NisI (25.8 kDa) as well as the virtually complete (1)H,(15)N,(13)C chemical shift assignments for the isolated 12.7 kDa N-terminal and 14.6 kDa C-terminal domains of NisI.