Fold recognition and accurate sequence-structure alignment of sequences directing beta-sheet proteins

The Pseudomonas aeruginosa Biofilm Matrix Protein CdrA Has Similarities to Other Fibrillar Adhesin Proteins

The ability of bacteria to adhere to each other and both biotic and abiotic surfaces is key to biofilm formation, and one way that bacteria adhere is using fibrillar adhesins. Fibrillar adhesins share several key characteristics, including (i) they are extracellular, surface-associated proteins, (ii) they contain an adhesive domain as well as a repetitive stalk domain, and (iii) they are either a monomer or homotrimer (i.e., identical, coiled-coil) of a high molecular weight protein. Pseudomonas aeruginosa uses the fibrillar adhesin called CdrA to promote bacterial aggregation and biofilm formation. Here, the current literature on CdrA is reviewed, including its transcriptional and posttranslational regulation by the second messenger c-di-GMP as well as what is known about its structure and ability to interact with other molecules. I highlight its similarities to other fibrillar adhesins and discuss open questions that remain to be answered toward a better understanding of CdrA.

A secondary structure-based position-specific scoring matrix applied to the improvement in protein secondary structure prediction

Protein secondary structure prediction (SSP) has a variety of applications; however, there has been relatively limited improvement in accuracy for years. With a vision of moving forward all related fields, we aimed to make a fundamental advance in SSP. There have been many admirable efforts made to improve the machine learning algorithm for SSP. This work thus took a step back by manipulating the input features. A secondary structure element-based position-specific scoring matrix (SSE-PSSM) is proposed, based on which a new set of machine learning features can be established. The feasibility of this new PSSM was evaluated by rigid independent tests with training and testing datasets sharing <25% sequence identities. In all experiments, the proposed PSSM outperformed the traditional amino acid PSSM. This new PSSM can be easily combined with the amino acid PSSM, and the improvement in accuracy was remarkable. Preliminary tests made by combining the SSE-PSSM and well-known SSP methods showed 2.0% and 5.2% average improvements in three- and eight-state SSP accuracies, respectively. If this PSSM can be integrated into state-of-the-art SSP methods, the overall accuracy of SSP may break the current restriction and eventually bring benefit to all research and applications where secondary structure prediction plays a vital role during development. To facilitate the application and integration of the SSE-PSSM with modern SSP methods, we have established a web server and standalone programs for generating SSE-PSSM available at http://10.life.nctu.edu.tw/SSE-PSSM.

The Right-Handed Parallel β-Helix Topology of Erwinia chrysanthemi Pectin Methylesterase Is Intimately Associated with Both Sequential Folding and Resistance to High Pressure

The complex topologies of large multi-domain globular proteins make the study of their folding and assembly particularly demanding. It is often characterized by complex kinetics and undesired side reactions, such as aggregation. The structural simplicity of tandem-repeat proteins, which are characterized by the repetition of a basic structural motif and are stabilized exclusively by sequentially localized contacts, has provided opportunities for dissecting their folding landscapes. In this study, we focus on the Erwinia chrysanthemi pectin methylesterase (342 residues), an all-β pectinolytic enzyme with a right-handed parallel β-helix structure. Chemicals and pressure were chosen as denaturants and a variety of optical techniques were used in conjunction with stopped-flow equipment to investigate the folding mechanism of the enzyme at 25 °C. Under equilibrium conditions, both chemical- and pressure-induced unfolding show two-state transitions, with average conformational stability (ΔG° = 35 ± 5 kJ·mol⁻¹) but exceptionally high resistance to pressure (P_m = 800 ± 7 MPa). Stopped-flow kinetic experiments revealed a very rapid (τ < 1 ms) hydrophobic collapse accompanied by the formation of an extended secondary structure but did not reveal stable tertiary contacts. This is followed by three distinct cooperative phases and the significant population of two intermediate species. The kinetics followed by intrinsic fluorescence shows a lag phase, strongly indicating that these intermediates are productive species on a sequential folding pathway, for which we propose a plausible model. These combined data demonstrate that even a large repeat protein can fold in a highly cooperative manner.

Elucidating Self-Assembling Peptide Aggregation via Morphoscanner: A New Tool for Protein-Peptide Structural Characterization

Self-assembling and molecular folding are ubiquitous in Nature: they drive the organization of systems ranging from living creatures to DNA molecules. Elucidating the complex dynamics underlying these phenomena is of crucial importance. However, a tool for the analysis of the various phenomena involved in protein/peptide aggregation is still missing. Here, an innovative software is developed and validated for the identification and visualization of b-structuring and b-sheet formation in both simulated systems and crystal structures of proteins and peptides. The novel software suite, dubbed Morphoscanner, is designed to identify and intuitively represent b-structuring and b-sheet formation during molecular dynamics trajectories, paying attention to temporary strand-strand alignment, suboligomer formation and evolution of local order. Self-assembling peptides (SAPs) constitute a promising class of biomaterials and an interesting model to study the spontaneous assembly of molecular systems in vitro. With the help of coarse-grained molecular dynamics the self-assembling of diverse SAPs is simulated into molten aggregates. When applied to these systems, Morphoscanner highlights different b-structuring schemes and kinetics related to SAP sequences. It is demonstrated that Morphoscanner is a novel versatile tool designed to probe the aggregation dynamics of self-assembling systems, adaptable to the analysis of differently coarsened simulations of a variety of biomolecules.

Evolution of the Translocation and Assembly Module (TAM)

Bacterial outer membrane proteins require the beta-barrel assembly machinery (BAM) for their correct folding and function. The central component of this machinery is BamA, an Omp85 protein that is essential and found in all Gram-negative bacteria. An additional feature of the BAM is the translocation and assembly module (TAM), comprised TamA (an Omp85 family protein) and TamB. We report that TamA and a closely related protein TamL are confined almost exclusively to Proteobacteria and Bacteroidetes/Chlorobi respectively, whereas TamB is widely distributed across the majority of Gram-negative bacterial lineages. A comprehensive phylogenetic and secondary structure analysis of the TamB protein family revealed that TamB was present very early in the evolution of bacteria. Several sequence characteristics were discovered to define the TamB protein family: A signal-anchor linkage to the inner membrane, beta-helical structure, conserved domain architecture and a C-terminal region that mimics outer membrane protein beta-strands. Taken together, the structural and phylogenetic analyses suggest that the TAM likely evolved from an original combination of BamA and TamB, with a later gene duplication event of BamA, giving rise to an additional Omp85 sequence that evolved to be TamA in Proteobacteria and TamL in Bacteroidetes/Chlorobi.

An alternative outer membrane secretion mechanism for an autotransporter protein lacking a C-terminal stable core

Autotransporter (AT) proteins are a broad class of virulence factors from Gram-negative pathogens. AT outer membrane (OM) secretion appears simple in many regards, yet the mechanism that enables transport of the central AT 'passenger' across the OM remains unclear. OM secretion efficiency for two AT passengers is enhanced by approximately 20 kDa stable core at the C-terminus of the passenger, but studies on a broader range of AT proteins are needed in order to determine whether a stability difference between the passenger N- and C-terminus represents a truly common mechanistic feature. Yersinia pestis YapV is homologous to Shigella flexneri IcsA, and like IcsA, YapV recruits mammalian neural Wiskott-Aldrich syndrome protein (N-WASP). In vitro, the purified YapV passenger is functional and rich in β-sheet structure, but lacks a approximately 20 kDa C-terminal stable core. However, the N-terminal 49 residues of the YapV passenger globally destabilize the entire YapV passenger, enhancing its OM secretion efficiency. These results indicate that the contributions of AT passenger sequences to OM secretion efficiency extend beyond a C-terminal stable core, and highlight a role of the passenger N-terminus in reducing passenger stability in order to facilitate OM secretion of some AT proteins.

Compressive genomics for protein databases

MOTIVATION

The exponential growth of protein sequence databases has increasingly made the fundamental question of searching for homologs a computational bottleneck. The amount of unique data, however, is not growing nearly as fast; we can exploit this fact to greatly accelerate homology search. Acceleration of programs in the popular PSI/DELTA-BLAST family of tools will not only speed-up homology search directly but also the huge collection of other current programs that primarily interact with large protein databases via precisely these tools.

RESULTS

We introduce a suite of homology search tools, powered by compressively accelerated protein BLAST (CaBLASTP), which are significantly faster than and comparably accurate with all known state-of-the-art tools, including HHblits, DELTA-BLAST and PSI-BLAST. Further, our tools are implemented in a manner that allows direct substitution into existing analysis pipelines. The key idea is that we introduce a local similarity-based compression scheme that allows us to operate directly on the compressed data. Importantly, CaBLASTP's runtime scales almost linearly in the amount of unique data, as opposed to current BLASTP variants, which scale linearly in the size of the full protein database being searched. Our compressive algorithms will speed-up many tasks, such as protein structure prediction and orthology mapping, which rely heavily on homology search.

AVAILABILITY

CaBLASTP is available under the GNU Public License at http://cablastp.csail.mit.edu/

CONTACT

bab@mit.edu.

Sequence and structural characterization of great salt lake bacteriophage CW02, a member of the T7-like supergroup

Halophage CW02 infects a Salinivibrio costicola-like bacterium, SA50, isolated from the Great Salt Lake. Following isolation, cultivation, and purification, CW02 was characterized by DNA sequencing, mass spectrometry, and electron microscopy. A conserved module of structural genes places CW02 in the T7 supergroup, members of which are found in diverse aquatic environments, including marine and freshwater ecosystems. CW02 has morphological similarities to viruses of the Podoviridae family. The structure of CW02, solved by cryogenic electron microscopy and three-dimensional reconstruction, enabled the fitting of a portion of the bacteriophage HK97 capsid protein into CW02 capsid density, thereby providing additional evidence that capsid proteins of tailed double-stranded DNA phages have a conserved fold. The CW02 capsid consists of bacteriophage lambda gpD-like densities that likely contribute to particle stability. Turret-like densities were found on icosahedral vertices and may represent a unique adaptation similar to what has been seen in other extremophilic viruses that infect archaea, such as Sulfolobus turreted icosahedral virus and halophage SH1.

STITCHER: Dynamic assembly of likely amyloid and prion β-structures from secondary structure predictions

The supersecondary structure of amyloids and prions, proteins of intense clinical and biological interest, are difficult to determine by standard experimental or computational means. In addition, significant conformational heterogeneity is known or suspected to exist in many amyloid fibrils. Previous work has demonstrated that probability-based prediction of discrete β-strand pairs can offer insight into these structures. Here, we devise a system of energetic rules that can be used to dynamically assemble these discrete β-strand pairs into complete amyloid β-structures. The STITCHER algorithm progressively 'stitches' strand-pairs into full β-sheets based on a novel free-energy model, incorporating experimentally observed amino-acid side-chain stacking contributions, entropic estimates, and steric restrictions for amyloidal parallel β-sheet construction. A dynamic program computes the top 50 structures and returns both the highest scoring structure and a consensus structure taken by polling this list for common discrete elements. Putative structural heterogeneity can be inferred from sequence regions that compose poorly. Predictions show agreement with experimental models of Alzheimer's amyloid beta peptide and the Podospora anserina Het-s prion. Predictions of the HET-s homolog HET-S also reflect experimental observations of poor amyloid formation. We put forward predicted structures for the yeast prion Sup35, suggesting N-terminal structural stability enabled by tyrosine ladders, and C-terminal heterogeneity. Predictions for the Rnq1 prion and alpha-synuclein are also given, identifying a similar mix of homogenous and heterogeneous secondary structure elements. STITCHER provides novel insight into the energetic basis of amyloid structure, provides accurate structure predictions, and can help guide future experimental studies.

Tandem repeats in proteins: from sequence to structure

The bioinformatics analysis of proteins containing tandem repeats requires special computer programs and databases, since the conventional approaches predominantly developed for globular domains have limited success. Here, I survey bioinformatics tools which have been developed recently for identification and proteome-wide analysis of protein repeats. The last few years have also been marked by an emergence of new 3D structures of these proteins. Appraisal of the known structures and their classification uncovers a straightforward relationship between their architecture and the length of the repetitive units. This relationship and the repetitive character of structural folds suggest rules for better prediction of the 3D structures of such proteins. Furthermore, bioinformatics approaches combined with low resolution structural data, from biophysical techniques, especially, the recently emerged cryo-electron microscopy, lead to reliable prediction of the protein repeat structures and their mode of binding with partners within molecular complexes. This hybrid approach can actively be used for structural and functional annotations of proteomes.

Remote protein homology detection and fold recognition using two-layer support vector machine classifiers

Remote protein homology detection and fold recognition refer to detection of structural homology in proteins where there are small or no similarities in the sequence. To detect protein structural classes from protein primary sequence information, homology-based methods have been developed, which can be divided to three types: discriminative classifiers, generative models for protein families and pairwise sequence comparisons. Support Vector Machines (SVM) and Neural Networks (NN) are two popular discriminative methods. Recent studies have shown that SVM has fast speed during training, more accurate and efficient compared to NN. We present a comprehensive method based on two-layer classifiers. The 1st layer is used to detect up to superfamily and family in SCOP hierarchy using optimized binary SVM classification rules. It used the kernel function known as the Bio-kernel, which incorporates the biological information in the classification process. The 2nd layer uses discriminative SVM algorithm with string kernel that will detect up to protein fold level in SCOP hierarchy. The results obtained were evaluated using mean ROC and mean MRFP and the significance of the result produced with pairwise t-test was tested. Experimental results show that our approaches significantly improve the performance of remote protein homology detection and fold recognition for all three different version SCOP datasets (1.53, 1.67 and 1.73). We achieved 4.19% improvements in term of mean ROC in SCOP 1.53, 4.75% in SCOP 1.67 and 4.03% in SCOP 1.73 datasets when compared to the result produced by well-known methods. The combination of first layer and second layer of BioSVM-2L performs well in remote homology detection and fold recognition even in three different versions of datasets.

The Waddlia genome: a window into chlamydial biology

Growing evidence suggests that a novel member of the Chlamydiales order, Waddlia chondrophila, is a potential agent of miscarriage in humans and abortion in ruminants. Due to the lack of genetic tools to manipulate chlamydia, genomic analysis is proving to be the most incisive tool in stimulating investigations into the biology of these obligate intracellular bacteria. 454/Roche and Solexa/Illumina technologies were thus used to sequence and assemble de novo the full genome of the first representative of the Waddliaceae family, W. chondrophila. The bacteria possesses a 2′116′312bp chromosome and a 15′593 bp low-copy number plasmid that might integrate into the bacterial chromosome. The Waddlia genome displays numerous repeated sequences indicating different genome dynamics from classical chlamydia which almost completely lack repetitive elements. Moreover, W. chondrophila exhibits many virulence factors also present in classical chlamydia, including a functional type III secretion system, but also a large complement of specific factors for resistance to host or environmental stresses. Large families of outer membrane proteins were identified indicating that these highly immunogenic proteins are not Chlamydiaceae specific and might have been present in their last common ancestor. Enhanced metabolic capability for the synthesis of nucleotides, amino acids, lipids and other co-factors suggests that the common ancestor of the modern Chlamydiales may have been less dependent on their eukaryotic host. The fine-detailed analysis of biosynthetic pathways brings us closer to possibly developing a synthetic medium to grow W. chondrophila, a critical step in the development of genetic tools. As a whole, the availability of the W. chondrophila genome opens new possibilities in Chlamydiales research, providing new insights into the evolution of members of the order Chlamydiales and the biology of the Waddliaceae.

UpaH is a newly identified autotransporter protein that contributes to biofilm formation and bladder colonization by uropathogenic Escherichia coli CFT073

Escherichia coli is the primary cause of urinary tract infection (UTI) in the developed world. The major factors associated with virulence of uropathogenic E. coli (UPEC) are fimbrial adhesins, which mediate specific attachment to host receptors and trigger innate host responses. Another group of adhesins is represented by the autotransporter (AT) subgroup of proteins. In this study, we identified a new AT-encoding gene, termed upaH, present in a 6.5-kb unannotated intergenic region in the genome of the prototypic UPEC strain CFT073. Cloning and sequencing of the upaH gene from CFT073 revealed an intact 8.535-kb coding region, contrary to the published genome sequence. The upaH gene was widely distributed among a large collection of UPEC isolates as well as the E. coli Reference (ECOR) strain collection. Bioinformatic analyses suggest beta-helix as the predominant structure in the large N-terminal passenger (alpha) domain and a 12-strand beta-barrel for the C-terminal beta-domain of UpaH. We demonstrated that UpaH is expressed at the cell surface of CFT073 and promotes biofilm formation. In the mouse UTI model, deletion of the upaH gene in CFT073 and in two other UPEC strains did not significantly affect colonization of the bladder in single-challenge experiments. However, in competitive colonization experiments, CFT073 significantly outcompeted its upaH isogenic mutant strain in urine and the bladder.

Markov random fields reveal an N-terminal double beta-propeller motif as part of a bacterial hybrid two-component sensor system

The recent explosion in newly sequenced bacterial genomes is outpacing the capacity of researchers to try to assign functional annotation to all the new proteins. Hence, computational methods that can help predict structural motifs provide increasingly important clues in helping to determine how these proteins might function. We introduce a Markov Random Field approach tailored for recognizing proteins that fold into mainly beta-structural motifs, and apply it to build recognizers for the beta-propeller shapes. As an application, we identify a potential class of hybrid two-component sensor proteins, that we predict contain a double-propeller domain.

BETASCAN: probable beta-amyloids identified by pairwise probabilistic analysis

Amyloids and prion proteins are clinically and biologically important β-structures, whose supersecondary structures are difficult to determine by standard experimental or computational means. In addition, significant conformational heterogeneity is known or suspected to exist in many amyloid fibrils. Recent work has indicated the utility of pairwise probabilistic statistics in β-structure prediction. We develop here a new strategy for β-structure prediction, emphasizing the determination of β-strands and pairs of β-strands as fundamental units of β-structure. Our program, BETASCAN, calculates likelihood scores for potential β-strands and strand-pairs based on correlations observed in parallel β-sheets. The program then determines the strands and pairs with the greatest local likelihood for all of the sequence's potential β-structures. BETASCAN suggests multiple alternate folding patterns and assigns relative a priori probabilities based solely on amino acid sequence, probability tables, and pre-chosen parameters. The algorithm compares favorably with the results of previous algorithms (BETAPRO, PASTA, SALSA, TANGO, and Zyggregator) in β-structure prediction and amyloid propensity prediction. Accurate prediction is demonstrated for experimentally determined amyloid β-structures, for a set of known β-aggregates, and for the parallel β-strands of β-helices, amyloid-like globular proteins. BETASCAN is able both to detect β-strands with higher sensitivity and to detect the edges of β-strands in a richly β-like sequence. For two proteins (Aβ and Het-s), there exist multiple sets of experimental data implying contradictory structures; BETASCAN is able to detect each competing structure as a potential structure variant. The ability to correlate multiple alternate β-structures to experiment opens the possibility of computational investigation of prion strains and structural heterogeneity of amyloid. BETASCAN is publicly accessible on the Web at http://betascan.csail.mit.edu.

Amyloid is a highly ordered form of protein aggregation that a wide variety of proteins can form. While the earliest discovered amyloids were associated with systemic and neurodegenerative diseases, recent findings indicate amyloids may have myriad roles and functions ranging from learning and memory, to yeast epigenetics, to biofilm and melanin production. In this study, we expand the range and flexibility of our ability to understand how amyloid properties arise from their polypeptide sequence. By taking advantage of the intrinsic properties of a characteristic amyloid structure—parallel β-strands—and data from available protein structures, we construct and test an algorithm to predict the probability that particular portions of a protein will form amyloid. Our method has the advantage of more accurate detection of the edges of such zones, as well as the ability to consider and evaluate the likelihood of multiple folding patterns.

Invivo folding efficiencies for mutants of the P22 tailspike beta-helix protein correlate with predicted stability changes

Parallel beta-helices are among the simplest repetitive structural elements in proteins. The folding behavior of beta-helix proteins has been studied intensively, also to gain insight on the formation of amyloid fibrils, which share the parallel beta-helix as a central structural motif. An important system for investigating beta-helix folding is the tailspike protein from the Salmonella bacteriophage P22. The central domain of this protein is a right-handed parallel beta-helix with 13 windings. Extensive mutational analyses of the P22 tailspike protein have revealed two main phenotypes: temperature-sensitive-folding (tsf) mutations that reduce the folding efficiency at elevated temperatures, and global suppressor (su) mutations that increase the tailspike folding efficiency. A central question is whether these phenotypes can be understood from changes in the protein stability induced by the mutations. Experimental determination of the protein stability is complicated by the nearly irreversible trimerization of the folded tailspike protein. Here, we present calculations of stability changes with the program FoldX, focusing on a recently published extensive data set of 145 singe-residue alanine mutants. We find that the calculated stability changes are correlated with the experimentally measured invivo folding efficiencies. In addition, we determine the free-energy landscape of the P22 tailspike protein in a nucleation-propagation model to explore the folding mechanism of this protein, and obtain a processive folding route on which the protein nucleates in the N-terminal region of the helix.

Insight into DNA and protein transport in double-stranded DNA viruses: the structure of bacteriophage N4

Bacteriophage N4 encapsidates a 3500-aa-long DNA-dependent RNA polymerase (vRNAP), which is injected into the host along with the N4 genome upon infection. The three-dimensional structures of wild-type and mutant N4 viruses lacking gp17, gp50, or gp65 were determined by cryoelectron microscopy. The virion has an icosahedral capsid with T=9 quasi-symmetry that encapsidates well-organized double-stranded DNA and vRNAP. The tail, attached at a unique pentameric vertex of the head, consists of a neck, 12 appendages, and six ribbons that constitute a non-contractile sheath around a central tail tube. Comparison of wild-type and mutant virus structures in conjunction with bioinformatics established the identity and virion locations of the major capsid protein (gp56), a decorating protein (gp17), the vRNAP (gp50), the tail sheath (gp65), the appendages (gp66), and the portal protein (gp59). The N4 virion organization provides insight into its assembly and suggests a mechanism for genome and vRNAP transport strategies utilized by this unique system.

Crystal structure of the Helicobacter pylori vacuolating toxin p55 domain

Helicobacter pylori VacA, a pore-forming toxin secreted by an autotransporter pathway, causes multiple alterations in human cells, contributes to the pathogenesis of peptic ulcer disease and gastric cancer, and is a candidate antigen for inclusion in an H. pylori vaccine. Here, we present a 2.4-A crystal structure of the VacA p55 domain, which has an important role in mediating VacA binding to host cells. The structure is predominantly a right-handed parallel beta-helix, a feature that is characteristic of autotransporter passenger domains but unique among known bacterial protein toxins. Notable features of VacA p55 include disruptions in beta-sheet contacts that result in five beta-helix subdomains and a C-terminal domain that contains a disulfide bond. Analysis of VacA protein sequences from unrelated H. pylori strains, including m1 and m2 forms of VacA, allows us to identify structural features of the VacA surface that may be important for interactions with host receptors. Docking of the p55 structure into a 19-A cryo-EM map of a VacA dodecamer allows us to propose a model for how VacA monomers assemble into oligomeric structures capable of membrane channel formation.

Predicting transmembrane beta-barrels and interstrand residue interactions from sequence

Transmembrane beta-barrel (TMB) proteins are embedded in the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts. The cellular location and functional diversity of beta-barrel outer membrane proteins (omps) makes them an important protein class. At the present time, very few nonhomologous TMB structures have been determined by X-ray diffraction because of the experimental difficulty encountered in crystallizing transmembrane proteins. A novel method using pairwise interstrand residue statistical potentials derived from globular (nonouter membrane) proteins is introduced to predict the supersecondary structure of transmembrane beta-barrel proteins. The algorithm transFold employs a generalized hidden Markov model (i.e., multitape S-attribute grammar) to describe potential beta-barrel supersecondary structures and then computes by dynamic programming the minimum free energy beta-barrel structure. Hence, the approach can be viewed as a "wrapping" component that may capture folding processes with an initiation stage followed by progressive interaction of the sequence with the already-formed motifs. This approach differs significantly from others, which use traditional machine learning to solve this problem, because it does not require a training phase on known TMB structures and is the first to explicitly capture and predict long-range interactions. TransFold outperforms previous programs for predicting TMBs on smaller (<or=200 residues) proteins and matches their performance for straightforward recognition of longer proteins. An exception is for multimeric porins where the algorithm does perform well when an important functional motif in loops is initially identified. We verify our simulations of the folding process by comparing them with experimental data on the functional folding of TMBs. A Web server running transFold is available and outputs contact predictions and locations for sequences predicted to form TMBs.

β‐Rolls, β‐Helices, and Other β‐Solenoid Proteins

Beta-rolls and beta-helices belong to a larger group of topologically similar proteins with solenoid folds: because their regular secondary structure elements are exclusively beta-strands, they are referred to as beta-solenoids. The number of beta-solenoids whose structures are known is now large enough to support a systematic analysis. Here we survey the distinguishing structural features of beta-solenoids, also documenting their notable diversity. Appraisal of these structures suggests a classification based on handedness, twist, oligomerization state, and coil shape. In addition, beta-solenoids are distinguished by the number of chains that wind around a common axis: the majority are single-stranded but there is a recently discovered subset of triple-stranded beta-solenoids. This survey has revealed some relationships of the amino acid sequences of beta-solenoids with their structures and functions-in particular, the repetitive character of the coil sequences and conformations that recur in tracts of tandem repeats. We have proposed the term beta-arc for the distinctive turns found in beta-solenoids and beta-arch for the corresponding strand-turn-strand motifs. The evolutionary mechanisms underlying these proteins are also discussed. This analysis has direct implications for sequence-based detection, structural prediction, and de novo design of other beta-solenoid proteins. The abundance of virulence factors, toxins and allergens among beta-solenoids, as well as commonalities of beta-solenoids with amyloid fibrils, imply that this class of folds may have a broader role in human diseases than was previously recognized. Thus, identification of genes with putative beta-solenoid domains promises to be a fertile direction in the search for viable targets in the development of new antibiotics and vaccines.