Potent transmission-blocking monoclonal antibodies from naturally exposed individuals target a conserved epitope on Plasmodium falciparum Pfs230.

Pfs230 is essential for Plasmodium falciparum transmission to mosquitoes and is the protein targeted by the most advanced malaria-transmission-blocking vaccine candidate. Prior understanding of functional epitopes on Pfs230 is based on two monoclonal antibodies (mAbs) with moderate transmission-reducing activity (TRA), elicited from subunit immunization. Here, we screened the B cell repertoire of two naturally exposed individuals possessing serum TRA and identified five potent mAbs from sixteen Pfs230 domain-1-specific mAbs. Structures of three potent and three low-activity antibodies bound to Pfs230 domain 1 revealed four distinct epitopes. Highly potent mAbs from natural infection recognized a common conformational epitope that is highly conserved across P. falciparum field isolates, while antibodies with negligible TRA derived from natural infection or immunization recognized three distinct sites. Our study provides molecular blueprints describing P. falciparum TRA, informed by contrasting potent and non-functional epitopes elicited by natural exposure and vaccination.

Highly potent, naturally acquired human monoclonal antibodies against Pfs48/45 block Plasmodium falciparum transmission to mosquitoes.

Malaria transmission-blocking vaccines (TBVs) aim to induce antibodies that interrupt malaria parasite development in the mosquito, thereby blocking onward transmission, and provide a much-needed tool for malaria control and elimination. The parasite surface protein Pfs48/45 is a leading TBV candidate. Here, we isolated and characterized a panel of 81 human Pfs48/45-specific monoclonal antibodies (mAbs) from donors naturally exposed to Plasmodium parasites. Genetically diverse mAbs against each of the three domains (D1-D3) of Pfs48/45 were identified. The most potent mAbs targeted D1 and D3 and achieved >80% transmission-reducing activity in standard membrane-feeding assays, at 10 and 2 µg/mL, respectively. Co-crystal structures of D3 in complex with four different mAbs delineated two conserved protective epitopes. Altogether, these Pfs48/45-specific human mAbs provide important insight into protective and non-protective epitopes that can further our understanding of transmission and inform the design of refined malaria transmission-blocking vaccine candidates.

Vaccination with a structure-based stabilized version of malarial antigen Pfs48/45 elicits ultra-potent transmission-blocking antibody responses.

Malaria transmission-blocking vaccines (TBVs) aim to elicit human antibodies that inhibit sporogonic development of Plasmodium falciparum in mosquitoes, thereby preventing onward transmission. Pfs48/45 is a leading clinical TBV candidate antigen and is recognized by the most potent transmission-blocking monoclonal antibody (mAb) yet described; still, clinical development of Pfs48/45 antigens has been hindered, largely by its poor biochemical characteristics. Here, we used structure-based computational approaches to design Pfs48/45 antigens stabilized in the conformation recognized by the most potently inhibitory mAb, achieving >25°C higher thermostability compared with the wild-type protein. Antibodies elicited in mice immunized with these engineered antigens displayed on liposome-based or protein nanoparticle-based vaccine platforms exhibited 1-2 orders of magnitude superior transmission-reducing activity, compared with immunogens bearing the wild-type antigen, driven by improved antibody quality. Our data provide the founding principles for using molecular stabilization solely from antibody structure-function information to drive improved immune responses against a parasitic vaccine target.

Engineering pan-HIV-1 neutralization potency through multispecific antibody avidity.

Deep mining of B cell repertoires of HIV-1-infected individuals has resulted in the isolation of dozens of HIV-1 broadly neutralizing antibodies (bNAbs). Yet, it remains uncertain whether any such bNAbs alone are sufficiently broad and potent to deploy therapeutically. Here, we engineered HIV-1 bNAbs for their combination on a single multispecific and avid molecule via direct genetic fusion of their Fab fragments to the human apoferritin light chain. The resulting molecule demonstrated a remarkable median IC50 value of 0.0009 µg/mL and 100% neutralization coverage of a broad HIV-1 pseudovirus panel (118 isolates) at a 4 µg/mL cutoff-a 32-fold enhancement in viral neutralization potency compared to a mixture of the corresponding HIV-1 bNAbs. Importantly, Fc incorporation on the molecule and engineering to modulate Fc receptor binding resulted in IgG-like bioavailability in vivo. This robust plug-and-play antibody design is relevant against indications where multispecificity and avidity are leveraged simultaneously to mediate optimal biological activity.

Structural analysis of HopPmaL reveals the presence of a second adaptor domain common to the HopAB family of Pseudomonas syringae type III effectors.

HopPmaL is a member of the HopAB family of type III effectors present in the phytopathogen Pseudomonas syringae. Using both X-ray crystallography and solution nuclear magnetic resonance, we demonstrate that HopPmaL contains two structurally homologous yet functionally distinct domains. The N-terminal domain corresponds to the previously described Pto-binding domain, while the previously uncharacterised C-terminal domain spans residues 308-385. While structurally similar, these domains do not share significant sequence similarity and most importantly demonstrate significant differences in key residues involved in host protein recognition, suggesting that each of them targets a different host protein.

NleG Type 3 effectors from enterohaemorrhagic Escherichia coli are U-Box E3 ubiquitin ligases.

NleG homologues constitute the largest family of Type 3 effectors delivered by pathogenic E. coli, with fourteen members in the enterohaemorrhagic (EHEC) O157:H7 strain alone. Identified recently as part of the non-LEE-encoded (Nle) effector set, this family remained uncharacterised and shared no sequence homology to other proteins including those of known function. The C-terminal domain of NleG2-3 (residues 90 to 191) is the most conserved region in NleG proteins and was solved by NMR. Structural analysis of this structure revealed the presence of a RING finger/U-box motif. Functional assays demonstrated that NleG2-3 as well as NleG5-1, NleG6-2 and NleG9' family members exhibited a strong autoubiquitination activity in vitro; a characteristic usually expressed by eukaryotic ubiquitin E3 ligases. When screened for activity against a panel of 30 human E2 enzymes, the NleG2-3 and NleG5-1 homologues showed an identical profile with only UBE2E2, UBE2E3 and UBE2D2 enzymes supporting NleG activity. Fluorescence polarization analysis yielded a binding affinity constant of 56+/-2 microM for the UBE2D2/NleG5-1 interaction, a value comparable with previous studies on E2/E3 affinities. The UBE2D2 interaction interface on NleG2-3 defined by NMR chemical shift perturbation and mutagenesis was shown to be generally similar to that characterised for human RING finger ubiquitin ligases. The alanine substitutions of UBE2D2 residues Arg5 and Lys63, critical for activation of eukaryotic E3 ligases, also significantly decreased both NleG binding and autoubiquitination activity. These results demonstrate that bacteria-encoded NleG effectors are E3 ubiquitin ligases analogous to RING finger and U-box enzymes in eukaryotes.

Multivalency transforms SARS-CoV-2 antibodies into ultrapotent neutralizers.

SARS-CoV-2, the virus responsible for COVID-19, has caused a global pandemic. Antibodies can be powerful biotherapeutics to fight viral infections. Here, we use the human apoferritin protomer as a modular subunit to drive oligomerization of antibody fragments and transform antibodies targeting SARS-CoV-2 into exceptionally potent neutralizers. Using this platform, half-maximal inhibitory concentration (IC50) values as low as 9 × 10-14 M are achieved as a result of up to 10,000-fold potency enhancements compared to corresponding IgGs. Combination of three different antibody specificities and the fragment crystallizable (Fc) domain on a single multivalent molecule conferred the ability to overcome viral sequence variability together with outstanding potency and IgG-like bioavailability. The MULTi-specific, multi-Affinity antiBODY (Multabody or MB) platform thus uniquely leverages binding avidity together with multi-specificity to deliver ultrapotent and broad neutralizers against SARS-CoV-2. The modularity of the platform also makes it relevant for rapid evaluation against other infectious diseases of global health importance. Neutralizing antibodies are a promising therapeutic for SARS-CoV-2.

Structural characterization of the ICOS/ICOS-L immune complex reveals high molecular mimicry by therapeutic antibodies.

The inducible co-stimulator (ICOS) is a member of the CD28/B7 superfamily, and delivers a positive co-stimulatory signal to activated T cells upon binding to its ligand (ICOS-L). Dysregulation of this pathway has been implicated in autoimmune diseases and cancer, and is currently under clinical investigation as an immune checkpoint blockade. Here, we describe the molecular interactions of the ICOS/ICOS-L immune complex at 3.3 Å resolution. A central FDPPPF motif and residues within the CC' loop of ICOS are responsible for the specificity of the interaction with ICOS-L, with a distinct receptor binding orientation in comparison to other family members. Furthermore, our structure and binding data reveal that the ICOS N110 N-linked glycan participates in ICOS-L binding. In addition, we report crystal structures of ICOS and ICOS-L in complex with monoclonal antibodies under clinical evaluation in immunotherapy. Strikingly, antibody paratopes closely mimic receptor-ligand binding core interactions, in addition to contacting peripheral residues to confer high binding affinities. Our results uncover key molecular interactions of an immune complex central to human adaptive immunity and have direct implications for the ongoing development of therapeutic interventions targeting immune checkpoint receptors.

Solution structure of ribosomal protein L40E, a unique C4 zinc finger protein encoded by archaeon Sulfolobus solfataricus.

The ribosomal protein L40E from archaeon Sulfolobus solfataricus is a component of the 50S ribosomal subunit. L40E is a 56-residue, highly basic protein that contains a C4 zinc finger motif, CRKC_X(10)_CRRC. Homologs are found in both archaea and eukaryotes but are not present in bacteria. Eukaryotic genomes encode L40E as a ubiquitin-fusion protein. L40E was absent from the crystal structure of euryarchaeota 50S ribosomal subunit. Here we report the three-dimensional solution structure of L40E by NMR spectroscopy. The structure of L40E is a three-stranded beta-sheet with a simple beta2beta1beta3 topology. There are two unique characteristics revealed by the structure. First, a large and ordered beta2-beta3 loop twists to pack across the one side of the protein. L40E contains a buried polar cluster comprising Lys19, Lys20, Cys22, Asn29, and Cys36. Second, the surface of L40E is almost entirely positively charged. Ten conserved basic residues are positioned on the two sides of the surface. It is likely that binding of zinc is essential in stabilizing the tertiary structure of L40E to act as a scaffold to create a broad positively charged surface for RNA and/or protein recognition.

The hypothetical protein Atu4866 from Agrobacterium tumefaciens adopts a streptavidin-like fold.

Atu4866 is a 79-residue conserved hypothetical protein of unknown function from Agrobacterium tumefaciens. Protein sequence alignments show that it shares > or =60% sequence identity with 20 other hypothetical proteins of bacterial origin. However, the structures and functions of these proteins remain unknown so far. To gain insight into the function of this family of proteins, we have determined the structure of Atu4866 as a target of a structural genomics project using solution NMR spectroscopy. Our results reveal that Atu4866 adopts a streptavidin-like fold featuring a beta-barrel/sandwich formed by eight antiparallel beta-strands. Further structural analysis identified a continuous patch of conserved residues on the surface of Atu4866 that may constitute a potential ligand-binding site.

The solution structure of ribosomal protein S17E from Methanobacterium thermoautotrophicum: a structural homolog of the FF domain.

The ribosomal protein S17E from the archaeon Methanobacterium thermoautotrophicum is a component of the 30S ribosomal subunit. S17E is a 62-residue protein conserved in archaea and eukaryotes and has no counterparts in bacteria. Mammalian S17E is a phosphoprotein component of eukaryotic ribosomes. Archaeal S17E proteins range from 59 to 79 amino acids, and are about half the length of the eukaryotic homologs which have an additional C-terminal region. Here we report the three-dimensional solution structure of S17E. S17E folds into a small three-helix bundle strikingly similar to the FF domain of human HYPA/FBP11, a novel phosphopeptide-binding fold. S17E bears a conserved positively charged surface acting as a robust scaffold for molecular recognition. The structure of M. thermoautotrophicum S17E provides a template for homology modeling of eukaryotic S17E proteins in the family.

Structural delineation of potent transmission-blocking epitope I on malaria antigen Pfs48/45.

Interventions that can block the transmission of malaria-causing Plasmodium falciparum (Pf) between the human host and Anopheles vector have the potential to reduce the incidence of malaria. Pfs48/45 is a gametocyte surface protein critical for parasite development and transmission, and its targeting by monoclonal antibody (mAb) 85RF45.1 leads to the potent reduction of parasite transmission. Here, we reveal how the Pfs48/45 6C domain adopts a (SAG1)-related-sequence (SRS) fold. We structurally delineate potent epitope I and show how mAb 85RF45.1 recognizes an electronegative surface with nanomolar affinity. Analysis of Pfs48/45 sequences reveals that polymorphisms are rare for residues involved at the binding interface. Humanization of rat-derived mAb 85RF45.1 conserved the mode of recognition and activity of the parental antibody, while also improving its thermostability. Our work has implications for the development of transmission-blocking interventions, both through improving vaccine designs and the testing of passive delivery of mAbs in humans.

NMR structure and binding studies confirm that PA4608 from Pseudomonas aeruginosa is a PilZ domain and a c-di-GMP binding protein.

PA4608 is a 125 residue protein from Pseudomonas aeruginosa with a recent identification as a PilZ domain and putative bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) adaptor protein that plays a role in bacterial second-messenger regulated processes. The nuclear magnetic resonance (NMR) structure of PA4608 has been determined and c-di-GMP binding has been confirmed by NMR titration studies. The monomeric core structure of PA4608 contains a six-stranded anti-parallel beta barrel flanked by three helices. Conserved surface residues among PA4608 homologs suggest the c-di-GMP binding site is at one end of the barrel and includes residues in the helices as well as in the unstructured N-terminus. Chemical shift changes in PA4608 resonances upon titration with c-di-GMP confirm binding. This evidence supports the hypothesis that proteins containing PilZ domains are the long-sought c-di-GMP adaptor proteins.

Solution structure of the Escherichia coli protein ydhR: a putative mono-oxygenase.

YdhR is a 101-residue conserved protein from Escherichia coli. Sequence searches reveal that the protein has >50% identity to proteins found in a variety of other bacterial genomes. Using size exclusion chromatography and fluorescence spectroscopy, we determined that ydhR exists in a dimeric state with a dissociation constant of approximately 40 nM. The three-dimensional structure of dimeric ydhR was determined using NMR spectroscopy. A total of 3400 unambiguous NOEs, both manually and automatically assigned, were used for the structure calculation that was refined using an explicit hydration shell. A family of 20 structures was obtained with a backbone RMSD of 0.48 A for elements of secondary structure. The structure reveals a dimeric alpha,beta fold characteristic of the alpha+beta barrel superfamily of proteins. Bioinformatic approaches were used to show that ydhR likely belongs to a recently identified group of mono-oxygenase proteins that includes ActVA-Orf6 and YgiN and are involved in the oxygenation of polyaromatic ring compounds.

Solution-state NMR structure of the putative morphogene protein BolA (PFE0790c) from Plasmodium falciparum.

Protozoa of the genus Plasmodium are responsible for malaria, which is perhaps the most important parasitic disease to infect mankind. The emergence of Plasmodium strains resistant to current therapeutics and prophylactics makes the development of new treatment strategies urgent. Among the potential targets for new antimalarial drugs is the BolA-like protein PFE0790c from Plasmodium falciparum (Pf-BolA). While the function of BolA is unknown, it has been linked to cell morphology by regulating transcription in response to stress. Using an NMR-based method, an ensemble of 20 structures of Pf-BolA was determined and deposited in the PDB (PDB entry 2kdn). The overall topology of the Pf-BolA structure, α1-ß1-ß2-η1-α2/η2-ß3-α3, with the ß-strands forming a mixed ß-sheet, is similar to the fold observed in other BolA structures. A helix-turn-helix motif similar to the class II KH fold associated with nucleic acid-binding proteins is present, but contains an FXGXXXL signature sequence that differs from the GXXG signature sequence present in class II KH folds, suggesting that the BolA family of proteins may use a novel protein-nucleic acid interface. A well conserved arginine residue, Arg50, hypothesized to play a role in governing the formation of the C-terminal α-helix in the BolA family of proteins, is too distant to form polar contacts with any side chains in this α-helix in Pf-BolA, suggesting that this conserved arginine may only serve a role in guiding the orientation of this C-terminal helix in some BolA proteins. A survey of BolA structures suggests that the C-terminal helix may not have a functional role and that the third helix (α2/η2) has a `kink' that appears to be conserved among the BolA protein structures. Circular dichroism spectroscopy shows that Pf-BolA is fairly robust, partially unfolding when heated to 353 K and refolding upon cooling to 298 K.

The crystal structure of hypothetical protein MTH1491 from Methanobacterium thermoautotrophicum.

As part of our structural proteomics initiative, we have determined the crystal structure of MTH1491, a previously uncharacterized hypothetical protein from Methanobacterium thermoautotrophicum. MTH1491 is one of numerous structural genomics targets selected in a genome-wide survey of uncharacterized proteins. It belongs to a family of proteins whose biological function is not known. The crystal structure of MTH1491, the first structure for this family of proteins, consists of an overall five-stranded parallel beta-sheet with strand order 51234 and flanking helices. The oligomeric form of this molecule is a trimer as seen from both crystal contacts and gel filtration studies. Analysis revealed that the structure of MTH1491 is similar to that of dehydrogenases, amidohydrolases, and oxidoreductases. Using a combination of sequence and structural analyses, we showed that MTH1491 does not belong to either the dehydrogenase or the amidohydrolase superfamilies of proteins.

Solution structure of ribosomal protein S28E from Methanobacterium thermoautotrophicum.

The ribosomal protein S28E from the archaeon Methanobacterium thermoautotrophicum is a component of the 30S ribosomal subunit. Sequence homologs of S28E are found only in archaea and eukaryotes. Here we report the three-dimensional solution structure of S28E by NMR spectroscopy. S28E contains a globular region and a long C-terminal tail protruding from the core. The globular region consists of four antiparallel beta-strands that are arranged in a Greek-key topology. Unique features of S28E include an extended loop L2-3 that folds back onto the protein and a 12-residue charged C-terminal tail with no regular secondary structure and greater flexibility relative to the rest of the protein. The structural and surface resemblance to OB-fold family of proteins and the presence of highly conserved basic residues suggest that S28E may bind to RNA. A broad positively charged surface extending over one side of the beta-barrel and into the flexible C terminus may present a putative binding site for RNA.

Strategies for structural proteomics of prokaryotes: Quantifying the advantages of studying orthologous proteins and of using both NMR and X-ray crystallography approaches.

Only about half of non-membrane-bound proteins encoded by either bacterial or archaeal genomes are soluble when expressed in Escherichia coli (Yee et al., Proc Natl Acad Sci USA 2002;99:1825-1830; Christendat et al., Prog Biophys Mol Biol 200;73:339-345). This property limits genome-scale functional and structural proteomics studies, which depend on having a recombinant, soluble version of each protein. An emerging strategy to increase the probability of deriving a soluble derivative of a protein is to study different sequence homologues of the same protein, including representatives from thermophilic organisms, based on the assumption that the stability of these proteins will facilitate structural analysis. To estimate the relative merits of this strategy, we compared the recombinant expression, solubility, and suitability for structural analysis by NMR and/or X-ray crystallography for 68 pairs of homologous proteins from E. coli and Thermotoga maritima. A sample suitable for structural studies was obtained for 62 of the 68 pairs of homologs under standardized growth and purification procedures. Fourteen (eight E. coli and six T. maritima proteins) samples generated NMR spectra of a quality suitable for structure determination and 30 (14 E. coli and 16 T. maritima proteins) samples formed crystals. Only three (one E. coli and two T. maritima proteins) samples both crystallized and had excellent NMR properties. The conclusions from this work are: (1) The inclusion of even a single ortholog of a target protein increases the number of samples for structural studies almost twofold; (2) there was no clear advantage to the use of thermophilic proteins to generate samples for structural studies; and (3) for the small proteins analyzed here, the use of both NMR and crystallography approaches almost doubled the number of samples for structural studies.

Screening proteins for NMR suitability.

NMR spectroscopy is a valuable tool in structural genomics. Identification of protein samples that are amenable to structure determination by NMR spectroscopy requires efficient screening. The preparation of multiple samples in parallel and screening by NMR is described. The method described is applicable to large structural genomics projects but can easily be scaled down for application to small structural biology projects. All the equipment used is commonly found in any NMR structural biology laboratory.

Structural characterization of a flexible two-domain protein in solution using small angle X-ray scattering and NMR data.

Multidomain proteins in which individual domains are connected by linkers often possess inherent interdomain flexibility that significantly complicates their structural characterization in solution using either nuclear magnetic resonance (NMR) spectroscopy or small-angle X-ray scattering (SAXS) alone. Here, we report a protocol for joint refinement of flexible multidomain protein structures against NMR distance and angular restraints, residual dipolar couplings, and SAXS data. The protocol is based on the ensemble optimization method principle (Bernadó et al., 2007) and is compared with different refinement strategies for the structural characterization of the flexible two-domain protein sf3636 from Shigella flexneri 2a. The results of our refinement suggest the existence of a dominant population of configurational states in solution possessing an overall elongated shape and restricted relative twisting of the two domains.