Publications by authors named "Nadine Czudnochowski"

21 Publications

  • Page 1 of 1

Broad sarbecovirus neutralization by a human monoclonal antibody.

Nature 2021 09 19;597(7874):103-108. Epub 2021 Jul 19.

Humabs Biomed SA, a subsidiary of Vir Biotechnology, Bellinzona, Switzerland.

The recent emergence of SARS-CoV-2 variants of concern and the recurrent spillovers of coronaviruses into the human population highlight the need for broadly neutralizing antibodies that are not affected by the ongoing antigenic drift and that can prevent or treat future zoonotic infections. Here we describe a human monoclonal antibody designated S2X259, which recognizes a highly conserved cryptic epitope of the receptor-binding domain and cross-reacts with spikes from all clades of sarbecovirus. S2X259 broadly neutralizes spike-mediated cell entry of SARS-CoV-2, including variants of concern (B.1.1.7, B.1.351, P.1, and B.1.427/B.1.429), as well as a wide spectrum of human and potentially zoonotic sarbecoviruses through inhibition of angiotensin-converting enzyme 2 (ACE2) binding to the receptor-binding domain. Furthermore, deep-mutational scanning and in vitro escape selection experiments demonstrate that S2X259 possesses an escape profile that is limited to a single substitution, G504D. We show that prophylactic and therapeutic administration of S2X259 protects Syrian hamsters (Mesocricetus auratus) against challenge with the prototypic SARS-CoV-2 and the B.1.351 variant of concern, which suggests that this monoclonal antibody is a promising candidate for the prevention and treatment of emergent variants and zoonotic infections. Our data reveal a key antigenic site that is targeted by broadly neutralizing antibodies and will guide the design of vaccines that are effective against all sarbecoviruses.
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http://dx.doi.org/10.1038/s41586-021-03817-4DOI Listing
September 2021

SARS-CoV-2 RBD antibodies that maximize breadth and resistance to escape.

Nature 2021 09 14;597(7874):97-102. Epub 2021 Jul 14.

Humabs BioMed SA, a subsidiary of Vir Biotechnology, Bellinzona, Switzerland.

An ideal therapeutic anti-SARS-CoV-2 antibody would resist viral escape, have activity against diverse sarbecoviruses, and be highly protective through viral neutralization and effector functions. Understanding how these properties relate to each other and vary across epitopes would aid the development of therapeutic antibodies and guide vaccine design. Here we comprehensively characterize escape, breadth and potency across a panel of SARS-CoV-2 antibodies targeting the receptor-binding domain (RBD). Despite a trade-off between in vitro neutralization potency and breadth of sarbecovirus binding, we identify neutralizing antibodies with exceptional sarbecovirus breadth and a corresponding resistance to SARS-CoV-2 escape. One of these antibodies, S2H97, binds with high affinity across all sarbecovirus clades to a cryptic epitope and prophylactically protects hamsters from viral challenge. Antibodies that target the angiotensin-converting enzyme 2 (ACE2) receptor-binding motif (RBM) typically have poor breadth and are readily escaped by mutations despite high neutralization potency. Nevertheless, we also characterize a potent RBM antibody (S2E12) with breadth across sarbecoviruses related to SARS-CoV-2 and a high barrier to viral escape. These data highlight principles underlying variation in escape, breadth and potency among antibodies that target the RBD, and identify epitopes and features to prioritize for therapeutic development against the current and potential future pandemics.
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http://dx.doi.org/10.1038/s41586-021-03807-6DOI Listing
September 2021

Structural basis for broad sarbecovirus neutralization by a human monoclonal antibody.

bioRxiv 2021 Apr 8. Epub 2021 Apr 8.

Humabs Biomed SA, a subsidiary of Vir Biotechnology, 6500 Bellinzona, Switzerland.

The recent emergence of SARS-CoV-2 variants of concern (VOC) and the recurrent spillovers of coronaviruses in the human population highlight the need for broadly neutralizing antibodies that are not affected by the ongoing antigenic drift and that can prevent or treat future zoonotic infections. Here, we describe a human monoclonal antibody (mAb), designated S2×259, recognizing a highly conserved cryptic receptor-binding domain (RBD) epitope and cross-reacting with spikes from all sarbecovirus clades. S2×259 broadly neutralizes spike-mediated entry of SARS-CoV-2 including the B.1.1.7, B.1.351, P.1 and B.1.427/B.1.429 VOC, as well as a wide spectrum of human and zoonotic sarbecoviruses through inhibition of ACE2 binding to the RBD. Furthermore, deep-mutational scanning and escape selection experiments demonstrate that S2×259 possesses a remarkably high barrier to the emergence of resistance mutants. We show that prophylactic administration of S2×259 protects Syrian hamsters against challenges with the prototypic SARS-CoV-2 and the B.1.351 variant, suggesting this mAb is a promising candidate for the prevention and treatment of emergent VOC and zoonotic infections. Our data unveil a key antigenic site targeted by broadly-neutralizing antibodies and will guide the design of pan-sarbecovirus vaccines.
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http://dx.doi.org/10.1101/2021.04.07.438818DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8043460PMC
April 2021

Antibodies to the SARS-CoV-2 receptor-binding domain that maximize breadth and resistance to viral escape.

bioRxiv 2021 Apr 8. Epub 2021 Apr 8.

Vir Biotechnology, San Francisco, CA 94158, USA.

An ideal anti-SARS-CoV-2 antibody would resist viral escape , have activity against diverse SARS-related coronaviruses , and be highly protective through viral neutralization and effector functions . Understanding how these properties relate to each other and vary across epitopes would aid development of antibody therapeutics and guide vaccine design. Here, we comprehensively characterize escape, breadth, and potency across a panel of SARS-CoV-2 antibodies targeting the receptor-binding domain (RBD), including S309 , the parental antibody of the late-stage clinical antibody VIR-7831. We observe a tradeoff between SARS-CoV-2 neutralization potency and breadth of binding across SARS-related coronaviruses. Nevertheless, we identify several neutralizing antibodies with exceptional breadth and resistance to escape, including a new antibody (S2H97) that binds with high affinity to all SARS-related coronavirus clades via a unique RBD epitope centered on residue E516. S2H97 and other escape-resistant antibodies have high binding affinity and target functionally constrained RBD residues. We find that antibodies targeting the ACE2 receptor binding motif (RBM) typically have poor breadth and are readily escaped by mutations despite high neutralization potency, but we identify one potent RBM antibody (S2E12) with breadth across sarbecoviruses closely related to SARS-CoV-2 and with a high barrier to viral escape. These data highlight functional diversity among antibodies targeting the RBD and identify epitopes and features to prioritize for antibody and vaccine development against the current and potential future pandemics.
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http://dx.doi.org/10.1101/2021.04.06.438709DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8043444PMC
April 2021

Circulating SARS-CoV-2 spike N439K variants maintain fitness while evading antibody-mediated immunity.

Cell 2021 03 28;184(5):1171-1187.e20. Epub 2021 Jan 28.

MRC-University of Glasgow Centre for Virus Research, University of Glasgow, Glasgow G61 1QH, UK.

SARS-CoV-2 can mutate and evade immunity, with consequences for efficacy of emerging vaccines and antibody therapeutics. Here, we demonstrate that the immunodominant SARS-CoV-2 spike (S) receptor binding motif (RBM) is a highly variable region of S and provide epidemiological, clinical, and molecular characterization of a prevalent, sentinel RBM mutation, N439K. We demonstrate N439K S protein has enhanced binding affinity to the hACE2 receptor, and N439K viruses have similar in vitro replication fitness and cause infections with similar clinical outcomes as compared to wild type. We show the N439K mutation confers resistance against several neutralizing monoclonal antibodies, including one authorized for emergency use by the US Food and Drug Administration (FDA), and reduces the activity of some polyclonal sera from persons recovered from infection. Immune evasion mutations that maintain virulence and fitness such as N439K can emerge within SARS-CoV-2 S, highlighting the need for ongoing molecular surveillance to guide development and usage of vaccines and therapeutics.
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http://dx.doi.org/10.1016/j.cell.2021.01.037DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7843029PMC
March 2021

Mapping Neutralizing and Immunodominant Sites on the SARS-CoV-2 Spike Receptor-Binding Domain by Structure-Guided High-Resolution Serology.

Cell 2020 11 16;183(4):1024-1042.e21. Epub 2020 Sep 16.

III Division of Infectious Diseases, ASST Fatebenefratelli Sacco, Luigi Sacco Hospital, 20157 Milan, Italy.

Analysis of the specificity and kinetics of neutralizing antibodies (nAbs) elicited by SARS-CoV-2 infection is crucial for understanding immune protection and identifying targets for vaccine design. In a cohort of 647 SARS-CoV-2-infected subjects, we found that both the magnitude of Ab responses to SARS-CoV-2 spike (S) and nucleoprotein and nAb titers correlate with clinical scores. The receptor-binding domain (RBD) is immunodominant and the target of 90% of the neutralizing activity present in SARS-CoV-2 immune sera. Whereas overall RBD-specific serum IgG titers waned with a half-life of 49 days, nAb titers and avidity increased over time for some individuals, consistent with affinity maturation. We structurally defined an RBD antigenic map and serologically quantified serum Abs specific for distinct RBD epitopes leading to the identification of two major receptor-binding motif antigenic sites. Our results explain the immunodominance of the receptor-binding motif and will guide the design of COVID-19 vaccines and therapeutics.
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http://dx.doi.org/10.1016/j.cell.2020.09.037DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7494283PMC
November 2020

Ultrapotent human antibodies protect against SARS-CoV-2 challenge via multiple mechanisms.

Science 2020 11 24;370(6519):950-957. Epub 2020 Sep 24.

Humabs BioMed SA, a subsidiary of Vir Biotechnology, Bellinzona, Switzerland.

Efficient therapeutic options are needed to control the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that has caused more than 922,000 fatalities as of 13 September 2020. We report the isolation and characterization of two ultrapotent SARS-CoV-2 human neutralizing antibodies (S2E12 and S2M11) that protect hamsters against SARS-CoV-2 challenge. Cryo-electron microscopy structures show that S2E12 and S2M11 competitively block angiotensin-converting enzyme 2 (ACE2) attachment and that S2M11 also locks the spike in a closed conformation by recognition of a quaternary epitope spanning two adjacent receptor-binding domains. Antibody cocktails that include S2M11, S2E12, or the previously identified S309 antibody broadly neutralize a panel of circulating SARS-CoV-2 isolates and activate effector functions. Our results pave the way to implement antibody cocktails for prophylaxis or therapy, circumventing or limiting the emergence of viral escape mutants.
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http://dx.doi.org/10.1126/science.abe3354DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7857395PMC
November 2020

The structure of the endogenous ESX-3 secretion system.

Elife 2019 12 30;8. Epub 2019 Dec 30.

Department of Medicine, Division of Infectious Diseases, University of California, San Francisco, San Francisco, United States.

The ESX (or Type VII) secretion systems are protein export systems in mycobacteria and many Gram-positive bacteria that mediate a broad range of functions including virulence, conjugation, and metabolic regulation. These systems translocate folded dimers of WXG100-superfamily protein substrates across the cytoplasmic membrane. We report the cryo-electron microscopy structure of an ESX-3 system, purified using an epitope tag inserted with recombineering into the chromosome of the model organism . The structure reveals a stacked architecture that extends above and below the inner membrane of the bacterium. The ESX-3 protomer complex is assembled from a single copy of the EccB, EccC, and EccE and two copies of the EccD protein. In the structure, the protomers form a stable dimer that is consistent with assembly into a larger oligomer. The ESX-3 structure provides a framework for further study of these important bacterial transporters.
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http://dx.doi.org/10.7554/eLife.52983DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6986878PMC
December 2019

Two Accessory Proteins Govern MmpL3 Mycolic Acid Transport in Mycobacteria.

mBio 2019 06 25;10(3). Epub 2019 Jun 25.

Immunology Program, Sloan Kettering Institute, New York, New York, USA

Mycolic acids are the signature lipid of mycobacteria and constitute an important physical component of the cell wall, a target of mycobacterium-specific antibiotics and a mediator of pathogenesis. Mycolic acids are synthesized in the cytoplasm and are thought to be transported to the cell wall as a trehalose ester by the MmpL3 transporter, an antibiotic target for However, the mechanism by which mycolate synthesis is coupled to transport, and the full MmpL3 transport machinery, is unknown. Here, we identify two new components of the MmpL3 transport machinery in mycobacteria. The protein encoded by / is essential for growth of and and is anchored to the cytoplasmic membrane, physically interacts with and colocalizes with MmpL3 in growing cells, and is required for trehalose monomycolate (TMM) transport to the cell wall. In light of these findings, we propose MSMEG_0736/Rv0383c be named "TMM transport factor A", TtfA. The protein encoded by also interacts with the MmpL3 complex but is nonessential for growth or TMM transport. However, MSMEG_5308 accumulates with inhibition of MmpL3-mediated TMM transport and stabilizes the MmpL3/TtfA complex, indicating that it may stabilize the transport system during stress. These studies identify two new components of the mycobacterial mycolate transport machinery, an emerging antibiotic target in The cell envelope of , the bacterium that causes the disease tuberculosis, is a complex structure composed of abundant lipids and glycolipids, including the signature lipid of these bacteria, mycolic acids. In this study, we identified two new components of the transport machinery that constructs this complex cell wall. These two accessory proteins are in a complex with the MmpL3 transporter. One of these proteins, TtfA, is required for mycolic acid transport and cell viability, whereas the other stabilizes the MmpL3 complex. These studies identify two new components of the essential cell envelope biosynthetic machinery in mycobacteria.
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http://dx.doi.org/10.1128/mBio.00850-19DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6593404PMC
June 2019

interfere with an interaction between the mannose-6-phosphate receptor and sorting nexins to counteract host restriction.

Elife 2017 03 2;6. Epub 2017 Mar 2.

Department of Medicine, University of California, San Francisco, San Francisco, United States.

is an obligate intracellular pathogen that resides in a membrane-bound compartment, the inclusion. The bacteria secrete a unique class of proteins, Incs, which insert into the inclusion membrane and modulate the host-bacterium interface. We previously reported that IncE binds specifically to the Sorting Nexin 5 Phox domain (SNX5-PX) and disrupts retromer trafficking. Here, we present the crystal structure of the SNX5-PX:IncE complex, showing IncE bound to a unique and highly conserved hydrophobic groove on SNX5. Mutagenesis of the SNX5-PX:IncE binding surface disrupts a previously unsuspected interaction between SNX5 and the cation-independent mannose-6-phosphate receptor (CI-MPR). Addition of IncE peptide inhibits the interaction of CI-MPR with SNX5. Finally, infection interferes with the SNX5:CI-MPR interaction, suggesting that IncE and CI-MPR are dependent on the same binding surface on SNX5. Our results provide new insights into retromer assembly and underscore the power of using pathogens to discover disease-related cell biology.
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http://dx.doi.org/10.7554/eLife.22709DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5364026PMC
March 2017

Crystal Structure of the Human tRNA m(1)A58 Methyltransferase-tRNA(3)(Lys) Complex: Refolding of Substrate tRNA Allows Access to the Methylation Target.

J Mol Biol 2015 Dec 22;427(24):3862-76. Epub 2015 Oct 22.

Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, CA 94143, USA.

Human tRNA3(Lys) is the primer for reverse transcription of HIV; the 3' end is complementary to the primer-binding site on HIV RNA. The complementarity ends at the 18th base, A58, which in tRNA3(Lys) is modified to remove Watson-Crick pairing. Motivated to test the role of the modification in terminating the primer-binding sequence and thus limiting run-on transcription, we asked how the modification of RNA could be accomplished. tRNA m(1)A58 methyltransferase (m(1)A58 MTase) methylates N1 of A58, which is buried in the TΨC-loop of tRNA, from cofactor S-adenosyl-L-methionine. This conserved tRNA modification is essential for stability of initiator tRNA in Saccharomyces cerevisiae. Reported here, three structures of human tRNA m(1)A58 MTase in complex with human tRNA3(Lys) and the product S-adenosyl-L-homocysteine show a dimer of heterodimers in which each heterodimer comprises a catalytic chain, Trm61, and a homologous but noncatalytic chain, Trm6, repurposed as a tRNA-binding subunit that acts in trans; tRNAs bind across the dimer interface such that Trm6 from the opposing heterodimer brings A58 into the active site of Trm61. T-loop and D-loop are splayed apart showing how A58, normally buried in tRNA, becomes accessible for modification. This result has broad impact on our understanding of the mechanisms of modifying internal sites in folded tRNA. The structures serve as templates for design of inhibitors that could be used to test tRNA m(1)A58 MTase's impact on retroviral priming and transcription.
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http://dx.doi.org/10.1016/j.jmb.2015.10.005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4663122PMC
December 2015

The mechanism of pseudouridine synthases from a covalent complex with RNA, and alternate specificity for U2605 versus U2604 between close homologs.

Nucleic Acids Res 2014 Feb 7;42(3):2037-48. Epub 2013 Nov 7.

Department of Biochemistry and Biophysics, University of California, San Francisco, 600 16th Street, San Francisco, CA 94158, USA, ProLynx, 455 Mission Bay Blvd., Suite 145, San Francisco, CA 94158, USA and Faculty of Biology, Technion-Israel Institute of Technology, Technion City, Haifa 320003, Israel.

RluB catalyses the modification of U2605 to pseudouridine (Ψ) in a stem-loop at the peptidyl transferase center of Escherichia coli 23S rRNA. The homolog RluF is specific to the adjacent nucleotide in the stem, U2604. The 1.3 Å resolution crystal structure of the complex between the catalytic domain of RluB and the isolated substrate stem-loop, in which the target uridine is substituted by 5-fluorouridine (5-FU), reveals a covalent bond between the isomerized target base and tyrosine 140. The structure is compared with the catalytic domain alone determined at 2.5 Å resolution. The RluB-bound stem-loop has essentially the same secondary structure as in the ribosome, with a bulge at A2602, but with 5-FU2605 flipped into the active site. We showed earlier that RluF induced a frame-shift of the RNA, moving A2602 into the stem and translating its target, U2604, into the active site. A hydrogen-bonding network stabilizes the bulge in the RluB-RNA but is not conserved in RluF and so RluF cannot stabilize the bulge. On the basis of the covalent bond between enzyme and isomerized 5-FU we propose a Michael addition mechanism for pseudouridine formation that is consistent with all experimental data.
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http://dx.doi.org/10.1093/nar/gkt1050DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3919597PMC
February 2014

In human pseudouridine synthase 1 (hPus1), a C-terminal helical insert blocks tRNA from binding in the same orientation as in the Pus1 bacterial homologue TruA, consistent with their different target selectivities.

J Mol Biol 2013 Oct 23;425(20):3875-87. Epub 2013 May 23.

Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, CA 94158, USA.

Human pseudouridine (Ψ) synthase Pus1 (hPus1) modifies specific uridine residues in several non-coding RNAs: tRNA, U2 spliceosomal RNA, and steroid receptor activator RNA. We report three structures of the catalytic core domain of hPus1 from two crystal forms, at 1.8Å resolution. The structures are the first of a mammalian Ψ synthase from the set of five Ψ synthase families common to all kingdoms of life. hPus1 adopts a fold similar to bacterial Ψ synthases, with a central antiparallel β-sheet flanked by helices and loops. A flexible hinge at the base of the sheet allows the enzyme to open and close around an electropositive active-site cleft. In one crystal form, a molecule of Mes [2-(N-morpholino)ethane sulfonic acid] mimics the target uridine of an RNA substrate. A positively charged electrostatic surface extends from the active site towards the N-terminus of the catalytic domain, suggesting an extensive binding site specific for target RNAs. Two α-helices C-terminal to the core domain, but unique to hPus1, extend along the back and top of the central β-sheet and form the walls of the RNA binding surface. Docking of tRNA to hPus1 in a productive orientation requires only minor conformational changes to enzyme and tRNA. The docked tRNA is bound by the electropositive surface of the protein employing a completely different binding mode than that seen for the tRNA complex of the Escherichia coli homologue TruA.
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http://dx.doi.org/10.1016/j.jmb.2013.05.014DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3900414PMC
October 2013

Serine-7 but not serine-5 phosphorylation primes RNA polymerase II CTD for P-TEFb recognition.

Nat Commun 2012 May 15;3:842. Epub 2012 May 15.

Max-Planck-Institut für molekulare Physiologie, Abt. Physikalische Biochemie, Otto-Hahn-Straße 11, 44227 Dortmund, Germany.

Phosphorylation of RNA polymerase II carboxy-terminal domain (CTD) in hepta-repeats YSPTSPS regulates eukaryotic transcription. Whereas Ser5 is phosphorylated in the initiation phase, Ser2 phosphorylation marks the elongation state. Here we show that the positive transcription elongation factor P-TEFb is a Ser5 CTD kinase that is unable to create Ser2/Ser5 double phosphorylations, while it exhibits fourfold higher activity on a CTD substrate pre-phosphorylated at Ser7 compared with the consensus hepta-repeat or the YSPTSPK variant. Mass spectrometry reveals an equal number of phosphorylations to the number of hepta-repeats provided, yet the mechanism of phosphorylation is distributive despite the repetitive nature of the substrate. Inhibition of P-TEFb activity is mediated by two regions in Hexim1 that act synergistically on Cdk9 and Cyclin T1. HIV-1 Tat/TAR abrogates Hexim1 inhibition to stimulate transcription of viral genes but does not change the substrate specificity. Together, these results provide insight into the multifaceted pattern of CTD phosphorylation.
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http://dx.doi.org/10.1038/ncomms1846DOI Listing
May 2012

Mechanistic insights into the translocation of full length HIV-1 Tat across lipid membranes.

Biochim Biophys Acta 2011 Nov 27;1808(11):2685-93. Epub 2011 Jul 27.

Institute of Organic and Biomolecular Chemistry, Tammannstr. 2, Göttingen, Germany.

The mechanism of how full length Tat (aa 1-86) crosses artificial lipid membranes was elucidated by means of fluorescence spectroscopy and fluorescence microscopy. It was shown that full length Tat (aa 1-86) neither forms pores in large unilamellar vesicles (LUVs) nor in giant unilamellar vesicles (GUVs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). In contrast, an N-terminally truncated Tat protein (aa 35-86) that lacks the structurally defined proline- and cysteine-rich region as well as the highly conserved tryptophan residue at position 11 generates pores in artificial POPC-membranes, through which a water-soluble dye up to a size of 10kDa can pass. By means of fluorescence microscopy, the transfer of fluorescently labeled full length Tat across POPC-bilayers was unambiguously visualized with a concomitant accumulation of the protein in the membrane interface. However, if the dye was attached to the protein, also pore formation was induced. The size of the pores was, however smaller than the protein size, i.e. the labeled protein with a mass of 11.6kDa passed the membrane, while a fluorescent dye with a mass of 10kDa was excluded from the vesicles' interior. The results demonstrate that pore formation is not the prime mechanism by which full length Tat crosses a membrane.
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http://dx.doi.org/10.1016/j.bbamem.2011.07.030DOI Listing
November 2011

Formation of Tat-TAR containing ribonucleoprotein complexes for biochemical and structural analyses.

Methods 2011 Jan 10;53(1):78-84. Epub 2010 Apr 10.

Max-Planck-Institut für molekulare Physiologie, Abteilung Physikalische Biochemie, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany.

Viruses manipulate multiple processes of the host cell machinery in order to replicate successfully in the infected cell. Among these, stimulation of transcription of the viral genes is crucial for lentiviruses such as HIV for increased protein expression levels and generation of escape mutants. The transactivation response (TAR) element at the 5'-end of HIV, SIV, BIV, EIAV or JDV retroviruses forms a unique RNA based promoter element that together with the transcription activator protein Tat stimulates viral gene expression at the level of transcription elongation. TAR is a double stranded non-coding RNA of typically 24-40 nucleotides length. Together with Tat it interacts with the Cyclin T subunit of the positive transcription elongation factor P-TEFb to recruit Cyclin T and its corresponding Cyclin-dependent kinase Cdk9 to the RNA polymerase II. In vitro formations of these Tat-TAR containing ribonucleoprotein complexes are a key requisite for biochemical characterizations and interaction studies that eventually will allow structural analyses. Here, we describe purification methods of the different factors employed and chromatography techniques that yield highly specific complex assemblies suitable for crystallization.
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http://dx.doi.org/10.1016/j.ymeth.2010.04.001DOI Listing
January 2011

Specificity of Hexim1 and Hexim2 complex formation with cyclin T1/T2, importin alpha and 7SK snRNA.

J Mol Biol 2010 Jan 31;395(1):28-41. Epub 2009 Oct 31.

Abteilung Physikalische Biochemie, Max-Planck-Institut für molekulare Physiologie, Otto-Hahn-Strasse 11, D-44227 Dortmund, Germany.

Positive transcription elongation factor b (P-TEFb) stimulates the transition from transcription initiation to productive elongation by phosphorylation of the C-terminal domain of RNA polymerase II. P-TEFb consists of the cyclin-dependent kinase Cdk9 and a T-type cyclin and is regulated by the small nuclear RNA 7SK and the coupling protein Hexim1 or Hexim2. In this study, we analyzed the tripartite protein-RNA complex formation between Hexim, Cyclin T and 7SK snRNA. Using isothermal titration calorimetry, we observed higher affinities for Cyclin T1-Hexim1 and Cyclin T2-Hexim2 complex formations compared with the interactions in reverse. Importin alpha, which is part of the Ran-mediated nuclear import pathway, bound Hexim1 and Hexim2 with dissociation constants of 2.0 and 0.5 muM, respectively. Furthermore, tripartite complex formations between Cyclin T, Hexim and Importin alpha showed the suitability of a collaborative nuclear import pathway for Cyclin T. Electrophoretic mobility shift assays using radioactively labelled full-length 7SK snRNA revealed a tight association of the RNA to Cyclin T1-Hexim1 with dissociation constants lower than 0.3 muM. Similar binding affinities were recorded for both Hexim orthologues to a 66-mer double-stranded 5' hairpin loop encompassing nucleotides 23-88 of 7SK, while a 39-mer fragment, resulting from different RNA folding predictions, did not bind as tightly. These results provide the molecular basis for the generation of a core complex for the inhibition of P-TEFb.
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http://dx.doi.org/10.1016/j.jmb.2009.10.055DOI Listing
January 2010

Tat competes with HEXIM1 to increase the active pool of P-TEFb for HIV-1 transcription.

Nucleic Acids Res 2007 6;35(6):2003-12. Epub 2007 Mar 6.

Department of Medicine, Rosalind Russell Medical Research Center, University of California at San Francisco, San Francisco, CA 94143-0703, USA.

Human immunodeficiency virus type 1 (HIV-1) transcriptional transactivator (Tat) recruits the positive transcription elongation factor b (P-TEFb) to the viral promoter. Consisting of cyclin dependent kinase 9 (Cdk9) and cyclin T1, P-TEFb phosphorylates RNA polymerase II and the negative transcription elongation factor to stimulate the elongation of HIV-1 genes. A major fraction of nuclear P-TEFb is sequestered into a transcriptionally inactive 7SK small nuclear ribonucleoprotein (snRNP) by the coordinated actions of the 7SK small nuclear RNA (snRNA) and hexamethylene bisacetamide (HMBA) induced protein 1 (HEXIM1). In this study, we demonstrate that Tat prevents the formation of and also releases P-TEFb from the 7SK snRNP in vitro and in vivo. This ability of Tat depends on the integrity of its N-terminal activation domain and stems from the high affinity interaction between Tat and cyclin T1, which allows Tat to directly displace HEXIM1 from cyclin T1. Furthermore, we find that in contrast to the Tat-independent activation of the HIV-1 promoter, Tat-dependent HIV-1 transcription is largely insensitive to the inhibition by HEXIM1. Finally, primary blood lymphocytes display a reduced amount of the endogenous 7SK snRNP upon HIV-1 infection. All these data are consistent with the model that Tat not only recruits but also increases the active pool of P-TEFb for efficient HIV-1 transcription.
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http://dx.doi.org/10.1093/nar/gkm063DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1874611PMC
May 2007

Structure of the hemolysin E (HlyE, ClyA, and SheA) channel in its membrane-bound form.

J Biol Chem 2006 Aug 5;281(32):23042-9. Epub 2006 Jun 5.

Department of Molecular Biology and Biotechnology, Krebs Institute for Biomolecular Research, The University of Sheffield, Sheffield S10 2TN, United Kingdom.

Hemolysin E (HlyE, ClyA, SheA) is a pore-forming protein toxin isolated from Escherichia coli. The three-dimensional structure of its water-soluble form is known, but that of the membrane-bound HlyE complex is not. We have used electron microscopy and image processing to show that the pores are predominantly octameric. Three-dimensional reconstructions of HlyE pores assembled in lipid/detergent micelles suggest a degree of conformational variability in the octameric complexes. The reconstructed pores were significantly longer than the maximum dimension of the water-soluble molecule, indicating that conformational changes occur on pore formation.
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http://dx.doi.org/10.1074/jbc.M602421200DOI Listing
August 2006

Identification of a cyclin T-binding domain in Hexim1 and biochemical analysis of its binding competition with HIV-1 Tat.

J Biol Chem 2005 Jul 25;280(26):24968-77. Epub 2005 Apr 25.

Max-Planck-Institut für Molekulare Physiologie, Abteilung Physikalische Biochemie, Otto-Hahn-Strasse 11, D-44227 Dortmund, Germany.

The active form of the positive transcription elongation factor b (P-TEFb) consists of cyclin T and the kinase Cdk9. P-TEFb stimulates transcription by phosphorylating the C-terminal domain of RNA polymerase II. It becomes inactivated when associated in a tetrameric complex with the abundant 7SK small nuclear RNA and the recently identified protein Hexim1. In this study, we identified a stable and soluble C-terminal domain (residues 255-359) in Hexim1 of 12.5-kDa size that binds the cyclin boxes of Cyclin T1. Functional assays in HeLa cells showed that this cyclin T-binding domain (TBD) is required for the binding of Hexim1 to P-TEFb and inhibition of transcriptional activity in vivo. Analytical gel filtration and GST pull-down experiments revealed that both full-length Hexim1 and the TBD are homodimers. Isothermal titration calorimetry yielded a weak multimer for the TBD with a multimerization constant of 1.3 x 10(3) m. The binding affinity between the TBD and cyclin T1 was analyzed with fluorescence spectroscopy methods, using a dansyl-based fluorescence label at position G257C. Equilibrium fluorescence titration and stopped flow fast kinetics yield a dissociation constant of 1.2 mum. Finally, we tested the effect of the HIV-1 Tat protein on the cyclin T1-TBD complex formation. GST pull-down experiments and size exclusion chromatography exhibit a mutually exclusive binding of the two effectors to cyclin T1. Our data suggest a model where HIV-1 Tat competes with Hexim1 for cyclin T1 binding, thus releasing P-TEFb from the inactive complex to stimulate the transcription of HIV-1 gene expression.
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http://dx.doi.org/10.1074/jbc.M501431200DOI Listing
July 2005
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