Viral afterlife: SARS-CoV-2 as a reservoir of immunomimetic peptides that reassemble into proinflammatory supramolecular complexes.

It is unclear how severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection leads to the strong but ineffective inflammatory response that characterizes severe Coronavirus disease 2019 (COVID-19), with amplified immune activation in diverse cell types, including cells without angiotensin-converting enzyme 2 receptors necessary for infection. Proteolytic degradation of SARS-CoV-2 virions is a milestone in host viral clearance, but the impact of remnant viral peptide fragments from high viral loads is not known. Here, we examine the inflammatory capacity of fragmented viral components from the perspective of supramolecular self-organization in the infected host environment. Interestingly, a machine learning analysis to SARS-CoV-2 proteome reveals sequence motifs that mimic host antimicrobial peptides (xenoAMPs), especially highly cationic human cathelicidin LL-37 capable of augmenting inflammation. Such xenoAMPs are strongly enriched in SARS-CoV-2 relative to low-pathogenicity coronaviruses. Moreover, xenoAMPs from SARS-CoV-2 but not low-pathogenicity homologs assemble double-stranded RNA (dsRNA) into nanocrystalline complexes with lattice constants commensurate with the steric size of Toll-like receptor (TLR)-3 and therefore capable of multivalent binding. Such complexes amplify cytokine secretion in diverse uninfected cell types in culture (epithelial cells, endothelial cells, keratinocytes, monocytes, and macrophages), similar to cathelicidin's role in rheumatoid arthritis and lupus. The induced transcriptome matches well with the global gene expression pattern in COVID-19, despite using <0.3% of the viral proteome. Delivery of these complexes to uninfected mice boosts plasma interleukin-6 and CXCL1 levels as observed in COVID-19 patients.

Structural dynamics reveal subtype-specific activation and inhibition of influenza virus hemagglutinin.

Influenza hemagglutinin (HA) is a prototypical class 1 viral entry glycoprotein, responsible for mediating receptor binding and membrane fusion. Structures of its prefusion and postfusion forms, embodying the beginning and endpoints of the fusion pathway, have been extensively characterized. Studies probing HA dynamics during fusion have begun to identify intermediate states along the pathway, enhancing our understanding of how HA becomes activated and traverses its conformational pathway to complete fusion. HA is also the most variable, rapidly evolving part of influenza virus, and it is not known whether mechanisms of its activation and fusion are conserved across divergent viral subtypes. Here, we apply hydrogen-deuterium exchange mass spectrometry to compare fusion activation in two subtypes of HA, H1 and H3. Our data reveal subtype-specific behavior in the regions of HA that undergo structural rearrangement during fusion, including the fusion peptide and HA1/HA2 interface. In the presence of an antibody that inhibits the conformational change (FI6v3), we observe that acid-induced dynamic changes near the epitope are dampened, but the degree of protection at the fusion peptide is different for the two subtypes investigated. These results thus provide new insights into variation in the mechanisms of influenza HA's dynamic activation and its inhibition.

Long-range structural defects by pathogenic mutations in most severe glucose-6-phosphate dehydrogenase deficiency.

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common blood disorder, presenting multiple symptoms, including hemolytic anemia. It affects 400 million people worldwide, with more than 160 single mutations reported in G6PD. The most severe mutations (about 70) are classified as class I, leading to more than 90% loss of activity of the wild-type G6PD. The crystal structure of G6PD reveals these mutations are located away from the active site, concentrating around the noncatalytic NADP+-binding site and the dimer interface. However, the molecular mechanisms of class I mutant dysfunction have remained elusive, hindering the development of efficient therapies. To resolve this, we performed integral structural characterization of five G6PD mutants, including four class I mutants, associated with the noncatalytic NADP+ and dimerization, using crystallography, small-angle X-ray scattering (SAXS), cryogenic electron microscopy (cryo-EM), and biophysical analyses. Comparisons with the structure and properties of the wild-type enzyme, together with molecular dynamics simulations, bring forward a universal mechanism for this severe G6PD deficiency due to the class I mutations. We highlight the role of the noncatalytic NADP+-binding site that is crucial for stabilization and ordering two ß-strands in the dimer interface, which together communicate these distant structural aberrations to the active site through a network of additional interactions. This understanding elucidates potential paths for drug development targeting G6PD deficiency.

An ensemble of cadherin-catenin-vinculin complex employs vinculin as the major F-actin binding mode.

The cell-cell adhesion cadherin-catenin complexes recruit vinculin to the adherens junction (AJ) to modulate the mechanical couplings between neighboring cells. However, it is unclear how vinculin influences the AJ structure and function. Here, we identified two patches of salt bridges that lock vinculin in the head-tail autoinhibited conformation and reconstituted the full-length vinculin activation mimetics bound to the cadherin-catenin complex. The cadherin-catenin-vinculin complex contains multiple disordered linkers and is highly dynamic, which poses a challenge for structural studies. We determined the ensemble conformation of this complex using small-angle x-ray and selective deuteration/contrast variation small-angle neutron scattering. In the complex, both α-catenin and vinculin adopt an ensemble of flexible conformations, but vinculin has fully open conformations with the vinculin head and actin-binding tail domains well separated from each other. F-actin binding experiments show that the cadherin-catenin-vinculin complex binds and bundles F-actin. However, when the vinculin actin-binding domain is removed from the complex, only a minor fraction of the complex binds to F-actin. The results show that the dynamic cadherin-catenin-vinculin complex employs vinculin as the primary F-actin binding mode to strengthen AJ-cytoskeleton interactions.

Stabilization of glucose-6-phosphate dehydrogenase oligomers enhances catalytic activity and stability of clinical variants.

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a genetic trait that can cause hemolytic anemia. To date, over 150 nonsynonymous mutations have been identified in G6PD, with pathogenic mutations clustering near the dimer and/or tetramer interface and the allosteric NADP+-binding site. Recently, our lab identified a small molecule that activates G6PD variants by stabilizing the allosteric NADP+ and dimer complex, suggesting therapeutics that target these regions may improve structural defects. Here, we elucidated the connection between allosteric NADP+ binding, oligomerization, and pathogenicity to determine whether oligomer stabilization can be used as a therapeutic strategy for G6PD deficiency (G6PDdef). We first solved the crystal structure for G6PDK403Q, a mutant that mimics the physiological acetylation of wild-type G6PD in erythrocytes and demonstrated that loss of allosteric NADP+ binding induces conformational changes in the dimer. These structural changes prevent tetramerization, are unique to Class I variants (the most severe form of G6PDdef), and cause the deactivation and destabilization of G6PD. We also introduced nonnative cysteines at the oligomer interfaces and found that the tetramer complex is more catalytically active and stable than the dimer. Furthermore, stabilizing the dimer and tetramer improved protein stability in clinical variants, regardless of clinical classification, with tetramerization also improving the activity of G6PDK403Q and Class I variants. These findings were validated using enzyme activity and thermostability assays, analytical size-exclusion chromatography (SEC), and SEC coupled with small-angle X-ray scattering (SEC-SAXS). Taken together, our findings suggest a potential therapeutic strategy for G6PDdef and provide a foundation for future drug discovery efforts.

Avian paramyxovirus serotype-1 isolation from migratory birds and environmental water in southern Japan: An epidemiological survey during the 2018/19-2021/2022 winter seasons.

Newcastle disease caused by highly pathogenic viruses of avian paramyxovirus serotype-1 (APMV-1) is a highly contagious poultry disease. Although a large-scale epidemic of Newcastle disease had occurred in Japan between the 1950s and the 2000s, there have been no outbreaks anywhere since 2010. In addition, there are no reports of epidemiological surveys of APMV-1 in wild birds in Japan in the last 10 years. We conducted the first epidemiological survey of APMV-1 in the Izumi plain, Kagoshima prefecture of southern Japan from the winter of 2018 to 2022. A total of 15 APMV-1 strains were isolated, and isolation rates from roosting water and duck fecal samples were 2.51% and 0.10%, respectively. These results indicate that the isolation method from environmental water may be useful for efficient surveillance of APMV-1 in wild birds. Furthermore, this is the first report on the success of APMV-1 isolation from environmental water samples. Genetic analysis of the Fusion (F) gene showed that all APMV-1 isolates were closely related to virus strains circulating among waterfowl in Far East Asian countries. All isolates have avirulent motifs in their cleavage site of F genes, all of which were presumed to be low pathogenic viruses in poultry. However, pathogenicity test using embryonated chicken eggs demonstrated that some isolates killed all chicken embryos regardless of viral doses inoculated (102 -106 50% egg infectious dose). These results indicated that APMV-1 strains, which are potentially pathogenic to chickens, are continuously brought into the Izumi plain by migrating wild birds.

A bacterial surface layer protein exploits multistep crystallization for rapid self-assembly.

Surface layers (S-layers) are crystalline protein coats surrounding microbial cells. S-layer proteins (SLPs) regulate their extracellular self-assembly by crystallizing when exposed to an environmental trigger. However, molecular mechanisms governing rapid protein crystallization in vivo or in vitro are largely unknown. Here, we demonstrate that the Caulobacter crescentus SLP readily crystallizes into sheets in vitro via a calcium-triggered multistep assembly pathway. This pathway involves 2 domains serving distinct functions in assembly. The C-terminal crystallization domain forms the physiological 2-dimensional (2D) crystal lattice, but full-length protein crystallizes multiple orders of magnitude faster due to the N-terminal nucleation domain. Observing crystallization using a time course of electron cryo-microscopy (Cryo-EM) imaging reveals a crystalline intermediate wherein N-terminal nucleation domains exhibit motional dynamics with respect to rigid lattice-forming crystallization domains. Dynamic flexibility between the 2 domains rationalizes efficient S-layer crystal nucleation on the curved cellular surface. Rate enhancement of protein crystallization by a discrete nucleation domain may enable engineering of kinetically controllable self-assembling 2D macromolecular nanomaterials.

Genetically Diverse Highly Pathogenic Avian Influenza A(H5N1/H5N8) Viruses among Wild Waterfowl and Domestic Poultry, Japan, 2021.

Genetic analyses of highly pathogenic avian influenza H5 subtype viruses isolated from the Izumi Plain, Japan, revealed cocirculation of 2 genetic groups of clade 2.3.4.4b viruses among migratory waterfowl. Our findings demonstrate that both continuous surveillance and timely information sharing of avian influenza viruses are valuable for rapid risk assessment.

An ensemble of flexible conformations underlies mechanotransduction by the cadherin-catenin adhesion complex.

The cadherin-catenin adhesion complex is the central component of the cell-cell adhesion adherens junctions that transmit mechanical stress from cell to cell. We have determined the nanoscale structure of the adherens junction complex formed by the α-catenin•ß-catenin•epithelial cadherin cytoplasmic domain (ABE) using negative stain electron microscopy, small-angle X-ray scattering, and selective deuteration/small-angle neutron scattering. The ABE complex is highly pliable and displays a wide spectrum of flexible structures that are facilitated by protein-domain motions in α- and ß-catenin. Moreover, the 107-residue intrinsically disordered N-terminal segment of ß-catenin forms a flexible "tongue" that is inserted into α-catenin and participates in the assembly of the ABE complex. The unanticipated ensemble of flexible conformations of the ABE complex suggests a dynamic mechanism for sensitivity and reversibility when transducing mechanical signals, in addition to the catch/slip bond behavior displayed by the ABE complex under mechanical tension. Our results provide mechanistic insight into the structural dynamics for the cadherin-catenin adhesion complex in mechanotransduction.

Dynamics and stability in the maturation of a eukaryotic virus: a paradigm for chemically programmed large-scale macromolecular reorganization.

Virus maturation is found in all animal viruses and dsDNA bacteriophages that have been studied. It is a programmed process, cued by cellular environmental factors, that transitions a noninfectious, initial assembly product (provirus) to an infectious particle (virion). Nudaurelia capensis omega virus (NωV) is an ssRNA insect virus with T=4 quasi-symmetry. Over the last 20 years, NωV virus-like particles (VLPs) have been an attractive model for the detailed study of maturation. The novel feature of the system is the progressive transition from procapsid to capsid controlled by pH. Homogeneous populations of maturation intermediates can be readily produced at arbitrary intervals by adjusting the pH between 7.6 and 5.0. These intermediates were investigated using biochemical and biophysical methods to create a stop-frame transition series of this complex process. The studies reviewed here characterized the large-scale subunit reorganization during maturation (the particle changes size from 48 nm to 41 nm) as well as the mechanism of a maturation cleavage, a time-resolved study of cleavage site formation, and specific roles of quasi-equivalent subunits in the release of membrane lytic peptides required for cellular entry.

Quaternary structure of α-amino-ß-carboxymuconate-ϵ-semialdehyde decarboxylase (ACMSD) controls its activity.

α-Amino-ß-carboxymuconate-ϵ-semialdehyde decarboxylase (ACMSD) plays an important role in l-tryptophan degradation via the kynurenine pathway. ACMSD forms a homodimer and is functionally inactive as a monomer because its catalytic assembly requires an arginine residue from a neighboring subunit. However, how the oligomeric state and self-association of ACMSD are controlled in solution remains unexplored. Here, we demonstrate that ACMSD from Pseudomonas fluorescens can self-assemble into homodimer, tetramer, and higher-order structures. Using size-exclusion chromatography coupled with small-angle X-ray scattering (SEC-SAXS) analysis, we investigated the ACMSD tetramer structure, and fitting the SAXS data with X-ray crystal structures of the monomeric component, we could generate a pseudo-atomic structure of the tetramer. This analysis revealed a tetramer model of ACMSD as a head-on dimer of dimers. We observed that the tetramer is catalytically more active than the dimer and is in equilibrium with the monomer and dimer. Substituting a critical residue of the dimer-dimer interface, His-110, altered the tetramer dissociation profile by increasing the higher-order oligomer portion in solution without changing the X-ray crystal structure. ACMSD self-association was affected by pH, ionic strength, and other electrostatic interactions. Alignment of ACMSD sequences revealed that His-110 is highly conserved in a few bacteria that utilize nitrobenzoic acid as a sole source of carbon and energy, suggesting a dedicated functional role of ACMSD's self-assembly into the tetrameric and higher-order structures. These results indicate that the dynamic oligomerization status potentially regulates ACMSD activity and that SEC-SAXS coupled with X-ray crystallography is a powerful tool for studying protein self-association.

Antibody Probes of Module 1 of the 6-Deoxyerythronolide B Synthase Reveal an Extended Conformation During Ketoreduction.

The 6-deoxyerythronolide B synthase (DEBS) is a prototypical assembly line polyketide synthase (PKS) that synthesizes the macrocyclic core of the antibiotic erythromycin. Each of its six multidomain modules presumably sample distinct conformations, as biosynthetic intermediates tethered to their acyl carrier proteins interact with multiple active sites during the courses of their catalytic cycles. The spatiotemporal details underlying these protein dynamics remain elusive. Here, we investigate one aspect of this conformational flexibility using two domain-specific monoclonal antibody fragments (Fabs) isolated from a very large naïve human antibody library. Both Fabs, designated 1D10 and 2G10, were bound specifically and with high affinity to the ketoreductase domain of DEBS module 1 (KR1). Comparative kinetic analysis of stand-alone KR1 as well as a truncated bimodular derivative of DEBS revealed that 1D10 inhibited KR1 activity whereas 2G10 did not. Co-crystal structures of each KR1-Fab complex provided a mechanistic rationale for this difference. A hybrid PKS module harboring KR1 was engineered, whose individual catalytic domains have been crystallographically characterized at high resolution. Size exclusion chromatography coupled to small-angle X-ray scattering (SEC-SAXS) of this hybrid module bound to 1D10 provided further support for the catalytic relevance of the "extended" model of a PKS module. Our findings reinforce the power of monoclonal antibodies as tools to interrogate structure-function relationships of assembly line PKSs.

Transition in genetic constellations of H3N8 and H4N6 low-pathogenic avian influenza viruses isolated from an overwintering site in Japan throughout different winter seasons.

The Izumi plain in Kagoshima Prefecture, Japan, is an overwintering site for migratory ducks and endangered cranes. We have surveyed avian influenza viruses (AIVs) in this area since 2012 and isolated low-pathogenic AIVs (LPAIVs) of various subtypes every winter season. H3N8 LPAIVs were isolated during the 2012/13 and 2016/17 seasons, and H4N6 LPAIVs were isolated during the 2012/13 and 2013/14 seasons. In the 2017/18 season, one H3N8 and two H4N6 LPAIV strains were isolated from environmental water samples. Genetic and phylogenetic analysis for each gene segment from these H3N8 and H4N6 LPAIVs suggested that our isolates were genetic reassortants generated by intermixing between AIVs circulating not only in Eurasia but also in Africa and/or North America. Comparison of the genetic constellations of our three isolates with their counterparts isolated during previous seasons from the Izumi plain revealed a drastic transition in the genetic constellations of both subtypes. These findings emphasize the importance of continuous surveillance of AIVs on the Izumi plain.

Identification of a distinct lineage of aviadenovirus from crane feces.

Viruses are believed to be ubiquitous; however, the diversity of viruses is largely unknown because of the bias of previous research toward pathogenic viruses. Deep sequencing is a promising and unbiased approach to detect viruses from animal-derived materials. Although cranes are known to be infected by several viruses such as influenza A viruses, previous studies targeted limited species of viruses, and thus viruses that infect cranes have not been extensively studied. In this study, we collected crane fecal samples in the Izumi plain in Japan, which is an overwintering site for cranes, and performed metagenomic shotgun sequencing analyses. We detected aviadenovirus-like sequences in the fecal samples and tentatively named the discovered virus crane-associated adenovirus 1 (CrAdV-1). We determined that our sequence accounted for approximately three-fourths of the estimated CrAdV-1 genome size (33,245 bp). The GC content of CrAdV-1 genome is 34.1%, which is considerably lower than that of other aviadenoviruses. Phylogenetic analyses revealed that CrAdV-1 clusters with members of the genus Aviadenovirus, but is distantly related to the previously identified aviadenoviruses. The protein sequence divergence between the DNA polymerase of CrAdV-1 and those of other aviadenoviruses is 45.2-46.8%. Based on these results and the species demarcation for the family Adenoviridae, we propose that CrAdV-1 be classified as a new species in the genus Aviadenovirus. Results of this study contribute to a deeper understanding of the diversity and evolution of viruses and provide additional information on viruses that infect cranes, which might lead to protection of the endangered species of cranes.

α-Catenin Structure and Nanoscale Dynamics in Solution and in Complex with F-Actin.

As a core component of the adherens junction, α-catenin stabilizes the cadherin/catenin complexes to the actin cytoskeleton for the mechanical coupling of cell-cell adhesion. α-catenin also modulates actin dynamics, cell polarity, and cell-migration functions that are independent of the adherens junction. We have determined the solution structures of the α-catenin monomer and dimer using in-line size-exclusion chromatography small-angle X-ray scattering, as well as the structure of α-catenin dimer in complex to F-actin filament using selective deuteration and contrast-matching small angle neutron scattering. We further present the first observation, to our knowledge, of the nanoscale dynamics of α-catenin by neutron spin-echo spectroscopy, which explicitly reveals the mobile regions of α-catenin that are crucial for binding to F-actin. In solution, the α-catenin monomer is more expanded than either protomer shown in the crystal structure dimer, with the vinculin-binding M fragment and the actin-binding domain being able to adopt different configurations. The α-catenin dimer in solution is also significantly more expanded than the dimer crystal structure, with fewer interdomain and intersubunit contacts than the crystal structure. When in complex to F-actin, the α-catenin dimer has an even more open and extended conformation than in solution, with the actin-binding domain further separated from the main body of the dimer. The α-catenin-assembled F-actin bundle develops into an ordered filament packing arrangement at increasing α-catenin/F-actin molar ratios. Together, the structural and dynamic studies reveal that α-catenin possesses dynamic molecular conformations that prime this protein to function as a mechanosensor protein.

Discovery and Characterization of a Thioesterase-Specific Monoclonal Antibody That Recognizes the 6-Deoxyerythronolide B Synthase.

Assembly line polyketide synthases (PKSs) are large multimodular enzymes responsible for the biosynthesis of diverse antibiotics in bacteria. Structural and mechanistic analysis of these megasynthases can benefit from the discovery of reagents that recognize individual domains or linkers in a site-specific manner. Monoclonal antibodies not only have proven themselves as premier tools in analogous applications but also have the added benefit of constraining the conformational flexibility of their targets in unpredictable but often useful ways. Here we have exploited a library based on the naïve human antibody repertoire to discover a Fab (3A6) that recognizes the terminal thioesterase (TE) domain of the 6-deoxyerythronolide B synthase with high specificity. Biochemical assays were used to verify that 3A6 binding does not inhibit enzyme turnover. The co-crystal structure of the TE-3A6 complex was determined at 2.45 Å resolution, resulting in atomic characterization of this protein-protein recognition mechanism. Fab binding had minimal effects on the structural integrity of the TE. In turn, these insights were used to interrogate via small-angle X-ray scattering the solution-phase conformation of 3A6 complexed to a catalytically competent PKS module and bimodule. Altogether, we have developed a high-affinity monoclonal antibody tool that recognizes the TE domain of the 6-deoxyerythronolide B synthase while maintaining its native function.

Structure-Function Analysis of the Extended Conformation of a Polyketide Synthase Module.

Catalytic modules of assembly-line polyketide synthases (PKSs) have previously been observed in two very different conformations-an "extended" architecture and an "arch-shaped" architecture-although the catalytic relevance of neither has been directly established. By the use of a fully human naïve antigen-binding fragment (Fab) library, a high-affinity antibody was identified that bound to the extended conformation of a PKS module, as verified by X-ray crystallography and tandem size-exclusion chromatography-small-angle X-ray scattering (SEC-SAXS). Kinetic analysis proved that this antibody-stabilized module conformation was fully competent for catalysis of intermodular polyketide chain translocation as well as intramodular polyketide chain elongation and functional group modification of a growing polyketide chain. Thus, the extended conformation of a PKS module is fully competent for all of its essential catalytic functions.

Phospho-Priming Confers Functionally Relevant Specificities for Rad53 Kinase Autophosphorylation.

The vast majority of in vitro structural and functional studies of the activation mechanism of protein kinases use the kinase domain alone. Well-demonstrated effects of regulatory domains or allosteric factors are scarce for serine/threonine kinases. Here we use a site-specifically phosphorylated SCD1-FHA1-kinase three-domain construct of the serine/threonine kinase Rad53 to show the effect of phospho-priming, an in vivo regulatory mechanism, on the autophosphorylation intermediate and specificity. Unphosphorylated Rad53 is a flexible monomer in solution but is captured in an asymmetric enzyme:substrate complex in crystal with the two FHA domains separated from each other. Phospho-priming induces formation of a stable dimer via intermolecular pT-FHA binding in solution. Importantly, autophosphorylation of unprimed and phospho-primed Rad53 produced predominantly inactive pS350-Rad53 and active pT354-Rad53, respectively. The latter mechanism was also demonstrated in vivo. Our results show that, while Rad53 can display active conformations under various conditions, simulation of in vivo regulatory conditions confers functionally relevant autophosphorylation.

iTRAQ-Based Quantitative Proteomics Analysis on Rice Anther Responding to High Temperature.

As one of the most important crops, rice provides the major food for more than half of the world population. However, its production is limited by many environmental factors, among which high temperature stress (HS) frequently occurs during anthesis and reduces its spikelet fertility. To explore the mechanism of HS tolerance in rice, we conducted a comparative proteomics analysis on the anthers between HS resistant and sensitive cultivars under different levels of high temperature. Under the same HS treatment, the resistant cultivar showed much higher spikelet fertility than the sensitive cultivar. Proteomic data showed that HS lead to the degradation of ribosomal proteins in the sensitive cultivar but not in the resistant one, which might result in the injury of protein biosynthetic machinery. In contrast, HS induced the increase of sHSP, ß-expansins and lipid transfer proteins in the resistant cultivar, which might contribute to its ability to tolerate HS. The results provide some new insights into the mechanism of rice HS response.

Solution structure of an "open" E. coli Pol III clamp loader sliding clamp complex.

Sliding clamps are opened and loaded onto primer template junctions by clamp loaders, and once loaded on DNA, confer processivity to replicative polymerases. Previously determined crystal structures of eukaryotic and T4 clamp loader-clamp complexes have captured the sliding clamps in either closed or only partially open interface conformations. In these solution structure studies, we have captured for the first time the clamp loader-sliding clamp complex from Escherichia coli using size exclusion chromatography coupled to small angle X-ray scattering (SEC-SAXS). The data suggests the sliding clamp is in an open conformation which is wide enough to permit duplex DNA binding. The data also provides information about spatial arrangement of the sliding clamp with respect to the clamp loader subunits and is compared to complex crystal structures determined from other organisms.