Quantitation and characterization of serotype 6A activation for pneumococcal conjugate vaccine by cryo-EM and SEC methods.

Pneumococcal conjugate vaccines (PCV) typically consist of capsular polysaccharides from different S. pneumoniae serotypes which are covalently attached to carrier protein. A well-established process to manufacture PCV is through activating polysaccharide by oxidation of vicinal diols to aldehydes, followed by protein conjugation via reductive amination. Polysaccharide activation is a crucial step that affects vaccine product critical attributes including conjugate size and structure. Therefore, it is highly desired to have robust analytical methods to well characterize this activation process. In this study, using pneumococcal serotype 6A as the model, we present two complimentary analytical methods for characterization of activated polysaccharide. First, a size exclusion chromatography (SEC) method was developed for quantitative measurement of polysaccharide activation levels. This SEC method demonstrated good assay characteristics on accuracy, precision and linearity. Second, a gold nanoparticle labeled cryo-electron microscopy (Cryo-EM) technique was developed to visualize activation site distribution along polysaccharide chain and provide information on activation heterogeneity. These two complimentary methods can be utilized to control polysaccharide activation process and ensure consistent delivery of conjugate vaccine products.

Identification, structural, and biophysical characterization of a positive modulator of human Kv3.1 channels.

Voltage-gated potassium channels (Kv) are tetrameric membrane proteins that provide a highly selective pathway for potassium ions (K+) to diffuse across a hydrophobic cell membrane. These unique voltage-gated cation channels detect changes in membrane potential and, upon activation, help to return the depolarized cell to a resting state during the repolarization stage of each action potential. The Kv3 family of potassium channels is characterized by a high activation potential and rapid kinetics, which play a crucial role for the fast-spiking neuronal phenotype. Mutations in the Kv3.1 channel have been shown to have implications in various neurological diseases like epilepsy and Alzheimer's disease. Moreover, disruptions in neuronal circuitry involving Kv3.1 have been correlated with negative symptoms of schizophrenia. Here, we report the discovery of a novel positive modulator of Kv3.1, investigate its biophysical properties, and determine the cryo-EM structure of the compound in complex with Kv3.1. Structural analysis reveals the molecular determinants of positive modulation in Kv3.1 channels by this class of compounds and provides additional opportunities for rational drug design for the treatment of associated neurological disorders.

Discovery and multimerization of cross-reactive single-domain antibodies against SARS-like viruses to enhance potency and address emerging SARS-CoV-2 variants.

Coronaviruses have been the causative agent of three epidemics and pandemics in the past two decades, including the ongoing COVID-19 pandemic. A broadly-neutralizing coronavirus therapeutic is desirable not only to prevent and treat COVID-19, but also to provide protection for high-risk populations against future emergent coronaviruses. As all coronaviruses use spike proteins on the viral surface to enter the host cells, and these spike proteins share sequence and structural homology, we set out to discover cross-reactive biologic agents targeting the spike protein to block viral entry. Through llama immunization campaigns, we have identified single domain antibodies (VHHs) that are cross-reactive against multiple emergent coronaviruses (SARS-CoV, SARS-CoV-2, and MERS). Importantly, a number of these antibodies show sub-nanomolar potency towards all SARS-like viruses including emergent CoV-2 variants. We identified nine distinct epitopes on the spike protein targeted by these VHHs. Further, by engineering VHHs targeting distinct, conserved epitopes into multi-valent formats, we significantly enhanced their neutralization potencies compared to the corresponding VHH cocktails. We believe this approach is ideally suited to address both emerging SARS-CoV-2 variants during the current pandemic as well as potential future pandemics caused by SARS-like coronaviruses.

Cryo-EM structures of inhibitory antibodies complexed with arginase 1 provide insight into mechanism of action.

Human Arginase 1 (hArg1) is a metalloenzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea, and modulates T-cell-mediated immune response. Arginase-targeted therapies have been pursued across several disease areas including immunology, oncology, nervous system dysfunction, and cardiovascular dysfunction and diseases. Currently, all published hArg1 inhibitors are small molecules usually less than 350 Da in size. Here we report the cryo-electron microscopy structures of potent and inhibitory anti-hArg antibodies bound to hArg1 which form distinct macromolecular complexes that are greater than 650 kDa. With local resolutions of 3.5 Å or better we unambiguously mapped epitopes and paratopes for all five antibodies and determined that the antibodies act through orthosteric and allosteric mechanisms. These hArg1:antibody complexes present an alternative mechanism to inhibit hArg1 activity and highlight the ability to utilize antibodies as probes in the discovery and development of peptide and small molecule inhibitors for enzymes in general.

Structure and mechanism of B-family DNA polymerase ζ specialized for translesion DNA synthesis.

DNA polymerase ζ (Polζ) belongs to the same B-family as high-fidelity replicative polymerases, yet is specialized for the extension reaction in translesion DNA synthesis (TLS). Despite its importance in TLS, the structure of Polζ is unknown. We present cryo-EM structures of the Saccharomyces cerevisiae Polζ holoenzyme in the act of DNA synthesis (3.1 Å) and without DNA (4.1 Å). Polζ displays a pentameric ring-like architecture, with catalytic Rev3, accessory Pol31' Pol32 and two Rev7 subunits forming an uninterrupted daisy chain of protein-protein interactions. We also uncover the features that impose high fidelity during the nucleotide-incorporation step and those that accommodate mismatches and lesions during the extension reaction. Collectively, we decrypt the molecular underpinnings of Polζ's role in TLS and provide a framework for new cancer therapeutics.

Structural basis for active single and double ring complexes in human mitochondrial Hsp60-Hsp10 chaperonin.

mHsp60-mHsp10 assists the folding of mitochondrial matrix proteins without the negative ATP binding inter-ring cooperativity of GroEL-GroES. Here we report the crystal structure of an ATP (ADP:BeF3-bound) ground-state mimic double-ring mHsp6014-(mHsp107)2 football complex, and the cryo-EM structures of the ADP-bound successor mHsp6014-(mHsp107)2 complex, and a single-ring mHsp607-mHsp107 half-football. The structures explain the nucleotide dependence of mHsp60 ring formation, and reveal an inter-ring nucleotide symmetry consistent with the absence of negative cooperativity. In the ground-state a two-fold symmetric H-bond and a salt bridge stitch the double-rings together, whereas only the H-bond remains as the equatorial gap increases in an ADP football poised to split into half-footballs. Refolding assays demonstrate obligate single- and double-ring mHsp60 variants are active, and complementation analysis in bacteria shows the single-ring variant is as efficient as wild-type mHsp60. Our work provides a structural basis for active single- and double-ring complexes coexisting in the mHsp60-mHsp10 chaperonin reaction cycle.

Vitamin B12 modulates Parkinson's disease LRRK2 kinase activity through allosteric regulation and confers neuroprotection.

Missense mutations in Leucine-Rich Repeat Kinase 2 (LRRK2) cause the majority of familial and some sporadic forms of Parkinson's disease (PD). The hyperactivity of LRRK2 kinase induced by the pathogenic mutations underlies neurotoxicity, promoting the development of LRRK2 kinase inhibitors as therapeutics. Many potent and specific small-molecule LRRK2 inhibitors have been reported with promise. However, nearly all inhibitors are ATP competitive-some with unwanted side effects and unclear clinical outcome-alternative types of LRRK2 inhibitors are lacking. Herein we identify 5'-deoxyadenosylcobalamin (AdoCbl), a physiological form of the essential micronutrient vitamin B12 as a mixed-type allosteric inhibitor of LRRK2 kinase activity. Multiple assays show that AdoCbl directly binds LRRK2, leading to the alterations of protein conformation and ATP binding in LRRK2. STD-NMR analysis of a LRRK2 homologous kinase reveals the contact sites in AdoCbl that interface with the kinase domain. Furthermore, we provide evidence that AdoCbl modulates LRRK2 activity through disrupting LRRK2 dimerization. Treatment with AdoCbl inhibits LRRK2 kinase activity in cultured cells and brain tissue, and prevents neurotoxicity in cultured primary rodent neurons as well as in transgenic C. elegans and D. melanogaster expressing LRRK2 disease variants. Finally, AdoCbl alleviates deficits in dopamine release sustainability caused by LRRK2 disease variants in mouse models. Our study uncovers vitamin B12 as a novel class of LRRK2 kinase modulator with a distinct mechanism, which can be harnessed to develop new LRRK2-based PD therapeutics in the future.

Structural model of the dimeric Parkinson's protein LRRK2 reveals a compact architecture involving distant interdomain contacts.

Leucine-rich repeat kinase 2 (LRRK2) is a large, multidomain protein containing two catalytic domains: a Ras of complex proteins (Roc) G-domain and a kinase domain. Mutations associated with familial and sporadic Parkinson's disease (PD) have been identified in both catalytic domains, as well as in several of its multiple putative regulatory domains. Several of these mutations have been linked to increased kinase activity. Despite the role of LRRK2 in the pathogenesis of PD, little is known about its overall architecture and how PD-linked mutations alter its function and enzymatic activities. Here, we have modeled the 3D structure of dimeric, full-length LRRK2 by combining domain-based homology models with multiple experimental constraints provided by chemical cross-linking combined with mass spectrometry, negative-stain EM, and small-angle X-ray scattering. Our model reveals dimeric LRRK2 has a compact overall architecture with a tight, multidomain organization. Close contacts between the N-terminal ankyrin and C-terminal WD40 domains, and their proximity-together with the LRR domain-to the kinase domain suggest an intramolecular mechanism for LRRK2 kinase activity regulation. Overall, our studies provide, to our knowledge, the first structural framework for understanding the role of the different domains of full-length LRRK2 in the pathogenesis of PD.

The architecture of yeast DNA polymerase ζ.

DNA polymerase ζ (Polζ) is specialized for the extension step of translesion DNA synthesis (TLS). Despite its central role in maintaining genome integrity, little is known about its overall architecture. Initially identified as a heterodimer of the catalytic subunit Rev3 and the accessory subunit Rev7, yeast Polζ has recently been shown to form a stable four-subunit enzyme (Polζ-d) upon the incorporation of Pol31 and Pol32, the accessory subunits of yeast Polδ. To understand the 3D architecture and assembly of Polζ and Polζ-d, we employed electron microscopy. We show here how the catalytic and accessory subunits of Polζ and Polζ-d are organized relative to each other. In particular, we show that Polζ-d has a bilobal architecture resembling the replicative polymerases and that Pol32 lies in proximity to Rev7. Collectively, our study provides views of Polζ and Polζ-d and a structural framework for understanding their roles in DNA damage bypass.

Structure of gamma-secretase and its trimeric pre-activation intermediate by single-particle electron microscopy.

The γ-secretase membrane protein complex is responsible for proteolytic maturation of signaling precursors and catalyzes the final step in the production of the amyloid ß-peptides implicated in the pathogenesis of Alzheimer disease. The incorporation of PEN-2 (presenilin enhancer 2) into a pre-activation intermediate, composed of the catalytic subunit presenilin and the accessory proteins APH-1 (anterior pharynx-defective 1) and nicastrin, triggers the endoproteolysis of presenilin and results in an active tetrameric γ-secretase. We have determined the three-dimensional reconstruction of a mature and catalytically active γ-secretase using single-particle cryo-electron microscopy. γ-Secretase has a cup-like shape with a lateral belt of ∼40-50 Å in height that encloses a water-accessible internal chamber. Active site labeling with a gold-coupled transition state analog inhibitor suggested that the γ-secretase active site faces this chamber. Comparison with the structure of a trimeric pre-activation intermediate suggested that the incorporation of PEN-2 might contribute to the maturation of the active site architecture.

Gating of a G protein-sensitive mammalian Kir3.1 prokaryotic Kir channel chimera in planar lipid bilayers.

Kir3 channels control heart rate and neuronal excitability through GTP-binding (G) protein and phosphoinositide signaling pathways. These channels were the first characterized effectors of the ßγ subunits of G proteins. Because we currently lack structures of complexes between G proteins and Kir3 channels, their interactions leading to modulation of channel function are not well understood. The recent crystal structure of a chimera between the cytosolic domain of a mammalian Kir3.1 and the transmembrane region of a prokaryotic KirBac1.3 (Kir3.1 chimera) has provided invaluable structural insight. However, it was not known whether this chimera could form functional K(+) channels. Here, we achieved the functional reconstitution of purified Kir3.1 chimera in planar lipid bilayers. The chimera behaved like a bona fide Kir channel displaying an absolute requirement for PIP(2) and Mg(2+)-dependent inward rectification. The channel could also be blocked by external tertiapin Q. The three-dimensional reconstruction of the chimera by single particle electron microscopy revealed a structure consistent with the crystal structure. Channel activity could be stimulated by ethanol and activated G proteins. Remarkably, the presence of both activated Gα and Gßγ subunits was required for gating of the channel. These results confirm the Kir3.1 chimera as a valid structural and functional model of Kir3 channels.

Modeling experimental image formation for likelihood-based classification of electron microscopy data.

The coexistence of multiple distinct structural states often obstructs the application of three-dimensional cryo-electron microscopy to large macromolecular complexes. Maximum likelihood approaches are emerging as robust tools for solving the image classification problems that are posed by such samples. Here, we propose a statistical data model that allows for a description of the experimental image formation within the formulation of 2D and 3D maximum-likelihood refinement. The proposed approach comprises a formulation of the probability calculations in Fourier space, including a spatial frequency-dependent noise model and a description of defocus-dependent imaging effects. The Expectation-Maximization-like algorithms presented are generally applicable to the alignment and classification of structurally heterogeneous projection data. Their effectiveness is demonstrated with various examples, including 2D classification of top views of the archaeal helicase MCM and 3D classification of 70S E. coli ribosome and Simian Virus 40 large T-antigen projections.

Double hexamer disruption and biochemical activities of Methanobacterium thermoautotrophicum MCM.

Methanobacterium thermoautotrophicum MCM (mtMCM) is a helicase required for DNA replication. Previous electron microscopy studies have shown mtMCM in several oligomeric forms. However, biochemical studies suggest that mtMCM is a dodecamer, likely a double hexamer (dHex). The crystal structure of the N-terminal fragment of mtMCM reveals a stable dHex architecture. To further confirm that the dHex is not an artifact of crystal packing of two hexamers, we investigated the relevance of the dHex by disrupting the hexamer-hexamer interactions seen in the crystal structure via site-directed mutagenesis and examining various biochemical activities of the mutants in vitro. Using a combination of biochemical and structural assays, we demonstrated that changing arginine to alanine at amino acid position 161 or the insertion of a six-aminoacid peptide at the hexamer-hexamer interface disrupted dHex formation and produced stable single hexamers (sHex). Furthermore, we showed that the sHex mutants retained wild-type level of ATPase and DNA binding activities but had decreased helicase activity when compared with the wild type dHex protein. These biochemical properties of mtMCM are reminiscent of those of SV40 large T antigen, suggesting that the dHex form of mtMCM may be the active helicase for DNA unwinding during the bidirectional DNA replication.

Polymorphism and double hexamer structure in the archaeal minichromosome maintenance (MCM) helicase from Methanobacterium thermoautotrophicum.

Methanobacterium thermoautotrophicum minichromosome maintenance complex (mtMCM), a cellular replicative helicase, is a useful model for the more complex eukaryotic MCMs. Biochemical and crystallographic evidence indicates that mtMCM assembles as a double hexamer (dHex), but previous electron microscopy studies reported only the presence of single heptamers or single hexamers (Pape, T., Meka, H., Chen, S., Vicentini, G., Van Heel, M., and Onesti, S. (2003) EMBO Rep. 4, 1079-1083; Yu, X., VanLoock, M. S., Poplawski, A., Kelman, Z., Xiang, T., Tye, B. K., and Egelman, E. H. (2002) EMBO Rep. 3, 792-797). Here we present the first three-dimensional electron microscopy reconstruction of the full-length mtMCM dHex in which two hexamers contact each other via the structurally well defined N-terminal domains. The dHex has obvious side openings that resemble the side channels of LTag (large T antigen). 6-fold and 7-fold rings were observed in the same mtMCM preparation, but we determined that assembly as a double ring favors 6-fold structures. Additionally, open rings were also detected, which suggests a direct mtMCM loading mechanism onto DNA.