Zentrum für Biomolekulare Magnetische Resonanz (BMRZ)
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The β-barrel assembly machinery (BAM) consisting of the central β-barrel BamA and four other lipoproteins mediates the folding of the majority of the outer membrane proteins. BamA is placed in an asymmetric bilayer and its lateral gate is suggested to be the functional hotspot. Here we used in situ pulsed electron-electron double resonance spectroscopy to characterize BamA in the native outer membrane. In the detergent micelles, the data is consistent with mainly an inward-open conformation of BamA. The native membrane considerably enhanced the conformational heterogeneity. The lateral gate and the extracellular loop 3 exist in an equilibrium between different conformations. The outer membrane provides a favorable environment for occupying multiple conformational states independent of the lipoproteins. Our results reveal a highly dynamic behavior of the lateral gate and other key structural elements and provide direct evidence for the conformational modulation of a membrane protein in situ.
1H, 13C and 15N chemical shift assignment of the stem-loops 5b + c from the 5′-UTR of SARS-CoV-2
(2022)
The ongoing pandemic of the respiratory disease COVID-19 is caused by the SARS-CoV-2 (SCoV2) virus. SCoV2 is a member of the Betacoronavirus genus. The 30 kb positive sense, single stranded RNA genome of SCoV2 features 5′- and 3′-genomic ends that are highly conserved among Betacoronaviruses. These genomic ends contain structured cis-acting RNA elements, which are involved in the regulation of viral replication and translation. Structural information about these potential antiviral drug targets supports the development of novel classes of therapeutics against COVID-19. The highly conserved branched stem-loop 5 (SL5) found within the 5′-untranslated region (5′-UTR) consists of a basal stem and three stem-loops, namely SL5a, SL5b and SL5c. Both, SL5a and SL5b feature a 5′-UUUCGU-3′ hexaloop that is also found among Alphacoronaviruses. Here, we report the extensive 1H, 13C and 15N resonance assignment of the 37 nucleotides (nts) long sequence spanning SL5b and SL5c (SL5b + c), as basis for further in-depth structural studies by solution NMR spectroscopy.
Herein, we present a multi-cycle chemoenzymatic synthesis of modified RNA with simplified solid-phase handling to overcome size limitations of RNA synthesis. It combines the advantages of classical chemical solid-phase synthesis and enzymatic synthesis using magnetic streptavidin beads and biotinylated RNA. Successful introduction of light-controllable RNA nucleotides into the tRNAMet sequence was confirmed by gel electrophoresis and mass spectrometry. The methods tolerate modifications in the RNA phosphodiester backbone and allow introductions of photocaged and photoswitchable nucleotides as well as photocleavable strand breaks and fluorophores.
The family of scaffold attachment factor B (SAFB) proteins comprises three members and was first identified as binders of the nuclear matrix/scaffold. Over the past two decades, SAFBs were shown to act in DNA repair, mRNA/(l)ncRNA processing, and as part of protein complexes with chromatin-modifying enzymes. SAFB proteins are approximately-100-kDa-sized dual nucleic acid-binding proteins with dedicated domains in an otherwise largely unstructured context, but whether and how they discriminate DNA- and RNA-binding has remained enigmatic. We here provide the SAFB2 DNA- and RNA-binding SAP and RRM domains in their functional boundaries and use solution NMR spectroscopy to ascribe DNA- and RNA-binding functions. We give insight into their target nucleic acid preferences and map the interfaces with respective nucleic acids on sparse data-derived SAP and RRM domain structures. Further, we provide evidence that the SAP domain exhibits intra-domain dynamics and a potential tendency to dimerise, which may expand its specifically targeted DNA sequence range. Our data provide a first molecular basis of and a starting point towards deciphering DNA- and RNA-binding functions of SAFB2 on the molecular level and serve a basis for understanding its localization to specific regions of chromatin and its involvement in the processing of specific RNA species.
G-quadruplexes (G4), found in numerous places within the human genome, are involved in essential processes of cell regulation. Chromosomal DNA G4s are involved for example, in replication and transcription as first steps of gene expression. Hence, they influence a plethora of downstream processes. G4s possess an intricate structure that differs from canonical B-form DNA. Identical DNA G4 sequences can adopt multiple long-lived conformations, a phenomenon known as G4 polymorphism. A detailed understanding of the molecular mechanisms that drive G4 folding is essential to understand their ambivalent regulatory roles. Disentangling the inherent dynamic and polymorphic nature of G4 structures thus is key to unravel their biological functions and make them amenable as molecular targets in novel therapeutic approaches. We here review recent experimental approaches to monitor G4 folding and discuss structural aspects for possible folding pathways. Substantial progress in the understanding of G4 folding within the recent years now allows drawing comprehensive models of the complex folding energy landscape of G4s that we herein evaluate based on computational and experimental evidence.
Specialized surveillance mechanisms are essential to maintain the genetic integrity of germ cells, which are not only the source of all somatic cells but also of the germ cells of the next generation. DNA damage and chromosomal aberrations are, therefore, not only detrimental for the individual but affect the entire species. In oocytes, the surveillance of the structural integrity of the DNA is maintained by the p53 family member TAp63α. The TAp63α protein is highly expressed in a closed and inactive state and gets activated to the open conformation upon the detection of DNA damage, in particular DNA double-strand breaks. To understand the cellular response to DNA damage that leads to the TAp63α triggered oocyte death we have investigated the RNA transcriptome of oocytes following irradiation at different time points. The analysis shows enhanced expression of pro-apoptotic and typical p53 target genes such as CDKn1a or Mdm2, concomitant with the activation of TAp63α. While DNA repair genes are not upregulated, inflammation-related genes become transcribed when apoptosis is initiated by activation of STAT transcription factors. Furthermore, comparison with the transcriptional profile of the ΔNp63α isoform from other studies shows only a minimal overlap, suggesting distinct regulatory programs of different p63 isoforms.
Rhodesain is the lysosomal cathepsin L-like cysteine protease of Trypanosoma brucei rhodesiense, the causative agent of Human African Trypanosomiasis. The enzyme is essential for the proliferation and pathogenicity of the parasite as well as its ability to overcome the blood–brain barrier of the host. Lysosomal cathepsins are expressed as zymogens with an inactivating prodomain that is cleaved under acidic conditions. A structure of the uncleaved maturation intermediate from a trypanosomal cathepsin L-like protease is currently not available. We thus established the heterologous expression of T. brucei rhodesiense pro-rhodesain in Escherichia coli and determined its crystal structure. The trypanosomal prodomain differs from nonparasitic pro-cathepsins by a unique, extended α-helix that blocks the active site and whose side-chain interactions resemble those of the antiprotozoal inhibitor K11777. Interdomain dynamics between pro- and core protease domain as observed by photoinduced electron transfer fluorescence correlation spectroscopy increase at low pH, where pro-rhodesain also undergoes autocleavage. Using the crystal structure, molecular dynamics simulations, and mutagenesis, we identify a conserved interdomain salt bridge that prevents premature intramolecular cleavage at higher pH values and may thus present a control switch for the observed pH sensitivity of proenzyme cleavage in (trypanosomal) CathL-like proteases.
Rhodesain is the lysosomal cathepsin L-like cysteine protease of T. brucei rhodesiense, the causative agent of Human African Trypanosomiasis. The enzyme is essential for the proliferation and pathogenicity of the parasite as well as its ability to overcome the blood-brain barrier of the host. Lysosomal cathepsins are expressed as zymogens with an inactivating pro-domain that is cleaved under acidic conditions. A structure of the uncleaved maturation intermediate from a trypanosomal cathepsin L-like protease is currently not available. We thus established the heterologous expression of T. brucei rhodesiense pro-rhodesain in E. coli and determined its crystal structure. The trypanosomal pro-domain differs from non-parasitic pro-cathepsins by a unique, extended α-helix that blocks the active site and whose interactions resemble that of the antiprotozoal inhibitor K11777. Interdomain dynamics between pro- and core protease domain as observed by photoinduced electron transfer fluorescence correlation spectroscopy increase at low pH, where pro-rhodesain also undergoes autocleavage. Using the crystal structure, molecular dynamics simulations and mutagenesis, we identify a conserved interdomain salt bridge that prevents premature intramolecular cleavage at higher pH values and may thus present a control switch for the observed pH-sensitivity of pro-enzyme cleavage in (trypanosomal) CathL-like proteases.
The function of the p53 transcription factor family is dependent on several folded domains. In addition to a DNA-binding domain, members of this family contain an oligomerization domain. p63 and p73 also contain a C-terminal Sterile α-motif domain. Inhibition of most transcription factors is difficult as most of them lack deep pockets that can be targeted by small organic molecules. Genetic knock-out procedures are powerful in identifying the overall function of a protein, but they do not easily allow one to investigate roles of individual domains. Here we describe the characterization of Designed Ankyrin Repeat Proteins (DARPins) that were selected as tight binders against all folded domains of p63. We determine binding affinities as well as specificities within the p53 protein family and show that DARPins can be used as intracellular inhibitors for the modulation of transcriptional activity. By selectively inhibiting DNA binding of the ΔNp63α isoform that competes with p53 for the same promoter sites, we show that p53 can be reactivated. We further show that inhibiting the DNA binding activity stabilizes p63, thus providing evidence for a transcriptionally regulated negative feedback loop. Furthermore, the ability of DARPins to bind to the DNA-binding domain and the Sterile α-motif domain within the dimeric-only and DNA-binding incompetent conformation of TAp63α suggests a high structural plasticity within this special conformation. In addition, the developed DARPins can also be used to specifically detect p63 in cell culture and in primary tissue and thus constitute a very versatile research tool for studying the function of p63.
The SARS-CoV-2 nucleocapsid (N) protein is crucial for the highly organized packaging and transcription of the genomic RNA. Studying atomic details of the role of its intrinsically disordered regions (IDRs) in RNA recognition is challenging due to the absence of structure and to the repetitive nature of their primary sequence. IDRs are known to act in concert with the folded domains of N and here we use NMR spectroscopy to identify the priming events of N interacting with a regulatory SARS-CoV-2 RNA element. 13C-detected NMR experiments, acquired simultaneously to 1H detected ones, provide information on the two IDRs flanking the N-terminal RNA binding domain (NTD) within the N-terminal region of the protein (NTR, 1–248). We identify specific tracts of the IDRs that most rapidly sense and engage with RNA, and thus provide an atom-resolved picture of the interplay between the folded and disordered regions of N during RNA interaction.