Organization of a Clostridium thermocellum gene cluster encoding the cellulosomal scaffolding protein CipA and a protein possibly involved in attachment of the cellulosome to the cell surface

Cellulosomes: Highly Efficient Cellulolytic Complexes

Cellulosomes are elaborate multienzyme complexes capable of efficiently deconstructing lignocellulosic substrates, produced by cellulolytic anaerobic microorganisms, colonizing a large variety of ecological niches. These macromolecular structures have a modular architecture and are composed of two main elements: the cohesin-bearing scaffoldins, which are non-catalytic structural proteins, and the various dockerin-bearing enzymes that tenaciously bind to the scaffoldins. Cellulosome assembly is mediated by strong and highly specific interactions between the cohesin modules, present in the scaffoldins, and the dockerin modules, present in the catalytic units. Cellulosomal architecture and composition varies between species and can even change within the same organism. These differences seem to be largely influenced by external factors, including the nature of the available carbon-source. Even though cellulosome producing organisms are relatively few, the development of new genomic and proteomic technologies has allowed the identification of cellulosomal components in many archea, bacteria and even some primitive eukaryotes. This reflects the importance of this cellulolytic strategy and suggests that cohesin-dockerin interactions could be involved in other non-cellulolytic processes. Due to their building-block nature and highly cellulolytic capabilities, cellulosomes hold many potential biotechnological applications, such as the conversion of lignocellulosic biomass in the production of biofuels or the development of affinity based technologies.

Insights into Thermophilic Plant Biomass Hydrolysis from Caldicellulosiruptor Systems Biology

Plant polysaccharides continue to serve as a promising feedstock for bioproduct fermentation. However, the recalcitrant nature of plant biomass requires certain key enzymes, including cellobiohydrolases, for efficient solubilization of polysaccharides. Thermostable carbohydrate-active enzymes are sought for their stability and tolerance to other process parameters. Plant biomass degrading microbes found in biotopes like geothermally heated water sources, compost piles, and thermophilic digesters are a common source of thermostable enzymes. While traditional thermophilic enzyme discovery first focused on microbe isolation followed by functional characterization, metagenomic sequences are negating the initial need for species isolation. Here, we summarize the current state of knowledge about the extremely thermophilic genus Caldicellulosiruptor, including genomic and metagenomic analyses in addition to recent breakthroughs in enzymology and genetic manipulation of the genus. Ten years after completing the first Caldicellulosiruptor genome sequence, the tools required for systems biology of this non-model environmental microorganism are in place.

Surfaceome and Proteosurfaceome in Parietal Monoderm Bacteria: Focus on Protein Cell-Surface Display

The cell envelope of parietal monoderm bacteria (archetypal Gram-positive bacteria) is formed of a cytoplasmic membrane (CM) and a cell wall (CW). While the CM is composed of phospholipids, the CW is composed at least of peptidoglycan (PG) covalently linked to other biopolymers, such as teichoic acids, polysaccharides, and/or polyglutamate. Considering the CW is a porous structure with low selective permeability contrary to the CM, the bacterial cell surface hugs the molecular figure of the CW components as a well of the external side of the CM. While the surfaceome corresponds to the totality of the molecules found at the bacterial cell surface, the proteinaceous complement of the surfaceome is the proteosurfaceome. Once translocated across the CM, secreted proteins can either be released in the extracellular milieu or exposed at the cell surface by associating to the CM or the CW. Following the gene ontology (GO) for cellular components, cell-surface proteins at the CM can either be integral (GO: 0031226), i.e., the integral membrane proteins, or anchored to the membrane (GO: 0046658), i.e., the lipoproteins. At the CW (GO: 0009275), cell-surface proteins can be covalently bound, i.e., the LPXTG-proteins, or bound through weak interactions to the PG or wall polysaccharides, i.e., the cell wall binding proteins. Besides monopolypeptides, some proteins can associate to each other to form supramolecular protein structures of high molecular weight, namely the S-layer, pili, flagella, and cellulosomes. After reviewing the cell envelope components and the different molecular mechanisms involved in protein attachment to the cell envelope, perspectives in investigating the proteosurfaceome in parietal monoderm bacteria are further discussed.

Pan-Cellulosomics of Mesophilic Clostridia: Variations on a Theme

The bacterial cellulosome is an extracellular, multi-enzyme machinery, which efficiently depolymerizes plant biomass by degrading plant cell wall polysaccharides. Several cellulolytic bacteria have evolved various elaborate modular architectures of active cellulosomes. We present here a genome-wide analysis of a dozen mesophilic clostridia species, including both well-studied and yet-undescribed cellulosome-producing bacteria. We first report here, the presence of cellulosomal elements, thus expanding our knowledge regarding the prevalence of the cellulosomal paradigm in nature. We explored the genomic organization of key cellulosome components by comparing the cellulosomal gene clusters in each bacterial species, and the conserved sequence features of the specific cellulosomal modules (cohesins and dockerins), on the background of their phylogenetic relationship. Additionally, we performed comparative analyses of the species-specific repertoire of carbohydrate-degrading enzymes for each of the clostridial species, and classified each cellulosomal enzyme into a specific CAZy family, thus indicating their putative enzymatic activity (e.g., cellulases, hemicellulases, and pectinases). Our work provides, for this large group of bacteria, a broad overview of the blueprints of their multi-component cellulosomal complexes. The high similarity of their scaffoldin clusters and dockerin-based recognition residues suggests a common ancestor, and/or extensive horizontal gene transfer, and potential cross-species recognition. In addition, the sporadic spatial organization of the numerous dockerin-containing genes in several of the genomes, suggests the importance of the cellulosome paradigm in the given bacterial species. The information gained in this work may be utilized directly or developed further by genetically engineering and optimizing designer cellulosome systems for enhanced biotechnological biomass deconstruction and biofuel production.

Unique organization and unprecedented diversity of the Bacteroides (Pseudobacteroides) cellulosolvens cellulosome system

BACKGROUND

(Pseudo) Bacteroides cellulosolvens is an anaerobic, mesophilic, cellulolytic, cellulosome-producing clostridial bacterium capable of utilizing cellulose and cellobiose as carbon sources. Recently, we sequenced the B. cellulosolvens genome, and subsequent comprehensive bioinformatic analysis, herein reported, revealed an unprecedented number of cellulosome-related components, including 78 cohesin modules scattered among 31 scaffoldins and more than 200 dockerin-bearing ORFs. In terms of numbers, the B. cellulosolvens cellulosome system represents the most intricate, compositionally diverse cellulosome system yet known in nature.

RESULTS

The organization of the B. cellulosolvens cellulosome is unique compared to previously described cellulosome systems. In contrast to all other known cellulosomes, the cohesin types are reversed for all scaffoldins i.e., the type II cohesins are located on the enzyme-integrating primary scaffoldin, whereas the type I cohesins are located on the anchoring scaffoldins. Many of the type II dockerin-bearing ORFs include X60 modules, which are known to stabilize type II cohesin-dockerin interactions. In the present work, we focused on revealing the architectural arrangement of cellulosome structure in this bacterium by examining numerous interactions between the various cohesin and dockerin modules. In total, we cloned and expressed 43 representative cohesins and 27 dockerins. The results revealed various possible architectures of cell-anchored and cell-free cellulosomes, which serve to assemble distinctive cellulosome types via three distinct cohesin-dockerin specificities: type I, type II, and a novel-type designated R (distinct from type III interactions, predominant in ruminococcal cellulosomes).

CONCLUSIONS

The results of this study provide novel insight into the architecture and function of the most intricate and extensive cellulosomal system known today, thereby extending significantly our overall knowledge base of cellulosome systems and their components. The robust cellulosome system of B. cellulosolvens, with its unique binding specificities and reversal of cohesin-dockerin types, has served to amend our view of the cellulosome paradigm. Revealing new cellulosomal interactions and arrangements is critical for designing high-efficiency artificial cellulosomes for conversion of plant-derived cellulosic biomass towards improved production of biofuels.

Cellulosomes: bacterial nanomachines for dismantling plant polysaccharides

Cellulosomes are multienzyme complexes that are produced by anaerobic cellulolytic bacteria for the degradation of lignocellulosic biomass. They comprise a complex of scaffoldin, which is the structural subunit, and various enzymatic subunits. The intersubunit interactions in these multienzyme complexes are mediated by cohesin and dockerin modules. Cellulosome-producing bacteria have been isolated from a large variety of environments, which reflects their prevalence and the importance of this microbial enzymatic strategy. In a given species, cellulosomes exhibit intrinsic heterogeneity, and between species there is a broad diversity in the composition and configuration of cellulosomes. With the development of modern technologies, such as genomics and proteomics, the full protein content of cellulosomes and their expression levels can now be assessed and the regulatory mechanisms identified. Owing to their highly efficient organization and hydrolytic activity, cellulosomes hold immense potential for application in the degradation of biomass and are the focus of much effort to engineer an ideal microorganism for the conversion of lignocellulose to valuable products, such as biofuels.

Emerging facets of prokaryotic glycosylation

Glycosylation of proteins is one of the most prevalent post-translational modifications occurring in nature, with a wide repertoire of biological implications. Pathways for the main types of this modification, the N- and O-glycosylation, can be found in all three domains of life-the Eukarya, Bacteria and Archaea-thereby following common principles, which are valid also for lipopolysaccharides, lipooligosaccharides and glycopolymers. Thus, studies on any glycoconjugate can unravel novel facets of the still incompletely understood fundamentals of protein N- and O-glycosylation. While it is estimated that more than two-thirds of all eukaryotic proteins would be glycosylated, no such estimate is available for prokaryotic glycoproteins, whose understanding is lagging behind, mainly due to the enormous variability of their glycan structures and variations in the underlying glycosylation processes. Combining glycan structural information with bioinformatic, genetic, biochemical and enzymatic data has opened up an avenue for in-depth analyses of glycosylation processes as a basis for glycoengineering endeavours. Here, the common themes of glycosylation are conceptualised for the major classes of prokaryotic (i.e. bacterial and archaeal) glycoconjugates, with a special focus on glycosylated cell-surface proteins. We describe the current knowledge of biosynthesis and importance of these glycoconjugates in selected pathogenic and beneficial microbes.

Diversity in S-layers

Surface layers, referred simply as S-layers, are the two-dimensional crystalline arrays of protein or glycoprotein subunits on cell surface. They are one of the most common outermost envelope components observed in prokaryotic organisms (Archaea and Bacteria). Over the past decades, S-layers have become an issue of increasing interest due to their ubiquitousness, special features and functions. Substantial work in this field provides evidences of an enormous diversity in S-layers. This paper reviews and illustrates the diversity from several different aspects, involving the S-layer-carrying strains, the structure of S-layers, the S-layer proteins and genes, as well as the functions of S-layers.

Nanoscale Engineering of Designer Cellulosomes

Biocatalysts showcase the upper limit obtainable for high-speed molecular processing and transformation. Efforts to engineer functionality in synthetic nanostructured materials are guided by the increasing knowledge of evolving architectures, which enable controlled molecular motion and precise molecular recognition. The cellulosome is a biological nanomachine, which, as a fundamental component of the plant-digestion machinery from bacterial cells, has a key potential role in the successful development of environmentally-friendly processes to produce biofuels and fine chemicals from the breakdown of biomass waste. Here, the progress toward so-called "designer cellulosomes", which provide an elegant alternative to enzyme cocktails for lignocellulose breakdown, is reviewed. Particular attention is paid to rational design via computational modeling coupled with nanoscale characterization and engineering tools. Remaining challenges and potential routes to industrial application are put forward.

Synergistic enhancement of cellulase pairs linked by consensus ankyrin repeats: Determination of the roles of spacing, orientation, and enzyme identity

Biomass deconstruction to small simple sugars is a potential approach to biofuels production; however, the highly recalcitrant nature of biomass limits the economic viability of this approach. Thus, research on efficient biomass degradation is necessary to achieve large-scale production of biofuels. Enhancement of cellulolytic activity by increasing synergism between cellulase enzymes holds promise in achieving high-yield biofuels production. Here we have inserted cellulase pairs from extremophiles into hyperstable α-helical consensus ankyrin repeat domain scaffolds. Such chimeric constructs allowed us to optimize arrays of enzyme pairs against a variety of cellulolytic substrates. We found that endocellulolytic domains CelA (CA) and Cel12A (C12A) act synergistically in the context of ankyrin repeats, with both three and four repeat spacing. The extent of synergy differs for different substrates. Also, having C12A N-terminal to CA provides greater synergy than the reverse construct, especially against filter paper. In contrast, we do not see synergy for these enzymes in tandem with CelK (CK) catalytic domain, a larger exocellulase, demonstrating the importance of enzyme identity in synergistic enhancement. Furthermore, we found endocellulases CelD and CA with three repeat spacing to act synergistically against filter paper. Importantly, connecting CA and C12A with a disordered linker of similar contour length shows no synergistic enhancement, indicating that synergism results from connecting these domains with folded ankyrin repeats. These results show that ankyrin arrays can be used to vary spacing and orientation between enzymes, helping to design and optimize artificial cellulosomes, providing a novel architecture for synergistic enhancement of enzymatic cellulose degradation. Proteins 2016; 84:1043-1054. © 2016 Wiley Periodicals, Inc.

Multidomain, Surface Layer-associated Glycoside Hydrolases Contribute to Plant Polysaccharide Degradation by Caldicellulosiruptor Species

The genome of the extremely thermophilic bacterium Caldicellulosiruptor kronotskyensisencodes 19 surface layer (S-layer) homology (SLH) domain-containing proteins, the most in any Caldicellulosiruptorspecies genome sequenced to date. These SLH proteins include five glycoside hydrolases (GHs) and one polysaccharide lyase, the genes for which were transcribed at high levels during growth on plant biomass. The largest GH identified so far in this genus, Calkro_0111 (2,435 amino acids), is completely unique toC. kronotskyensisand contains SLH domains. Calkro_0111 was produced recombinantly inEscherichia colias two pieces, containing the GH16 and GH55 domains, respectively, as well as putative binding and spacer domains. These displayed endo- and exoglucanase activity on the β-1,3-1,6-glucan laminarin. A series of additional truncation mutants of Calkro_0111 revealed the essential architectural features required for catalytic function. Calkro_0402, another of the SLH domain GHs inC. kronotskyensis, when produced inE. coli, was active on a variety of xylans and β-glucans. Unlike Calkro_0111, Calkro_0402 is highly conserved in the genus Caldicellulosiruptorand among other biomass-degrading Firmicutes but missing from Caldicellulosiruptor bescii As such, the gene encoding Calkro_0402 was inserted into the C. besciigenome, creating a mutant strain with its S-layer extensively decorated with Calkro_0402. This strain consequently degraded xylans more extensively than wild-typeC. bescii The results here provide new insights into the architecture and role of SLH domain GHs and demonstrate that hemicellulose degradation can be enhanced through non-native SLH domain GHs engineered into the genomes of Caldicellulosiruptorspecies.

Enzyme Systems of Anaerobes for Biomass Conversion

Biofuels from abundantly available cellulosic biomass are an attractive alternative to current petroleum-based fuels (fossil fuels). Although several strategies exist for commercial production of biofuels, conversion of biomass to biofuels via consolidated bioprocessing offers the potential to reduce production costs and increase processing efficiencies. In consolidated bioprocessing (CBP), enzyme production, cellulose hydrolysis, and fermentation are all carried out in a single-step by microorganisms that efficiently employ a multitude of intricate enzymes which act synergistically to breakdown cellulose and its associated cell wall components. Various strategies employed by anaerobic cellulolytic bacteria for biomass hydrolysis are described in this chapter. In addition, the regulation of CAZymes, the role of "omics" technologies in assessing lignocellulolytic ability, and current strategies for improving biomass hydrolysis for optimum biofuel production are highlighted.

S-layers: principles and applications

Monomolecular arrays of protein or glycoprotein subunits forming surface layers (S-layers) are one of the most commonly observed prokaryotic cell envelope components. S-layers are generally the most abundantly expressed proteins, have been observed in species of nearly every taxonomical group of walled bacteria, and represent an almost universal feature of archaeal envelopes. The isoporous lattices completely covering the cell surface provide organisms with various selection advantages including functioning as protective coats, molecular sieves and ion traps, as structures involved in surface recognition and cell adhesion, and as antifouling layers. S-layers are also identified to contribute to virulence when present as a structural component of pathogens. In Archaea, most of which possess S-layers as exclusive wall component, they are involved in determining cell shape and cell division. Studies on structure, chemistry, genetics, assembly, function, and evolutionary relationship of S-layers revealed considerable application potential in (nano)biotechnology, biomimetics, biomedicine, and synthetic biology.

Comparative analysis of carbohydrate active enzymes in Clostridium termitidis CT1112 reveals complex carbohydrate degradation ability

Clostridium termitidis strain CT1112 is an anaerobic, gram positive, mesophilic, cellulolytic bacillus isolated from the gut of the wood-feeding termite, Nasutitermes lujae. It produces biofuels such as hydrogen and ethanol from cellulose, cellobiose, xylan, xylose, glucose, and other sugars, and therefore could be used for biofuel production from biomass through consolidated bioprocessing. The first step in the production of biofuel from biomass by microorganisms is the hydrolysis of complex carbohydrates present in biomass. This is achieved through the presence of a repertoire of secreted or complexed carbohydrate active enzymes (CAZymes), sometimes organized in an extracellular organelle called cellulosome. To assess the ability and understand the mechanism of polysaccharide hydrolysis in C. termitidis, the recently sequenced strain CT1112 of C. termitidis was analyzed for both CAZymes and cellulosomal components, and compared to other cellulolytic bacteria. A total of 355 CAZyme sequences were identified in C. termitidis, significantly higher than other Clostridial species. Of these, high numbers of glycoside hydrolases (199) and carbohydrate binding modules (95) were identified. The presence of a variety of CAZymes involved with polysaccharide utilization/degradation ability suggests hydrolysis potential for a wide range of polysaccharides. In addition, dockerin-bearing enzymes, cohesion domains and a cellulosomal gene cluster were identified, indicating the presence of potential cellulosome assembly.

Bacterial cell-envelope glycoconjugates

Prokaryotic glycosylation fulfills an important role in maintaining and protecting the structural integrity and function of the bacterial cell wall, as well as serving as a flexible adaption mechanism to evade environmental and host-induced pressure. The scope of bacterial and archaeal protein glycosylation has considerably expanded over the past decade(s), with numerous examples covering the glycosylation of flagella, pili, glycosylated enzymes, as well as surface-layer proteins. This article addresses structure, analysis, function, genetic basis, biosynthesis, and biomedical and biotechnological applications of cell-envelope glycoconjugates, S-layer glycoprotein glycans, and "nonclassical" secondary-cell wall polysaccharides. The latter group of polymers mediates the important attachment and regular orientation of the S-layer to the cell wall. The structures of these glycopolymers reveal an enormous diversity, resembling the structural variability of bacterial lipopolysaccharides and capsular polysaccharides. While most examples are presented for Gram-positive bacteria, the S-layer glycan of the Gram-negative pathogen Tannerella forsythia is also discussed. In addition, archaeal S-layer glycoproteins are briefly summarized.

Different Binding Specificities of S-Layer Homology Modules fromClostridium thermocellumAncA, Slp1, and Slp2

S-layer homology (SLH) module polypeptides were derived from Clostridium thermocellum S-layer proteins Slp1 and Slp2 and cellulosome anchoring protein AncA as rSlp1-SLH, rSlp2-SLH, and rAncA-SLH respectively. Their binding specificities were investigated using C. thermocellum cell-wall preparations. rAncA-SLH associated with native peptidoglycan-containing sacculi from C. thermocellum, including both peptidoglycan and secondary cell wall polymers (SCWP), but not to hydrofluoric acid-extracted peptidoglycan-containing sacculi (HF-EPCS) lacking SCWPs, suggesting that SCWPs are responsible for binding with SLH modules of AncA. On the other hand, rSlp1-SLH and rSlp2-SLH associated with HF-EPCS, suggesting that these polypeptides had an affinity for peptidoglycan. A binding assay using a peptidoglycan fraction prepared from Escherichia coli cells definitely confirmed that rSlp1-SLH and rSlp2-SLH specifically interacted with peptidoglycan but not with SCWP.

Direct glucose production from lignocellulose using Clostridium thermocellum cultures supplemented with a thermostable β-glucosidase

BACKGROUND

Cellulases continue to be one of the major costs associated with the lignocellulose hydrolysis process. Clostridium thermocellum is an anaerobic, thermophilic, cellulolytic bacterium that produces cellulosomes capable of efficiently degrading plant cell walls. The end-product cellobiose, however, inhibits degradation. To maximize the cellulolytic ability of C. thermocellum, it is important to eliminate this end-product inhibition.

RESULTS

This work describes a system for biological saccharification that leads to glucose production following hydrolysis of lignocellulosic biomass. C. thermocellum cultures supplemented with thermostable beta-glucosidases make up this system. This approach does not require any supplementation with cellulases and hemicellulases. When C. thermocellum strain S14 was cultured with a Thermoanaerobacter brockii beta-glucosidase (CglT with activity 30 U/g cellulose) in medium containing 100 g/L cellulose (617 mM initial glucose equivalents), we observed not only high degradation of cellulose, but also accumulation of 426 mM glucose in the culture broth. In contrast, cultures without CglT, or with less thermostable beta-glucosidases, did not efficiently hydrolyze cellulose and accumulated high levels of glucose. Glucose production required a cellulose load of over 10 g/L. When alkali-pretreated rice straw containing 100 g/L glucan was used as the lignocellulosic biomass, approximately 72% of the glucan was saccharified, and glucose accumulated to 446 mM in the culture broth. The hydrolysate slurry containing glucose was directly fermented to 694 mM ethanol by addition of Saccharomyces cerevisiae, giving an 85% theoretical yield without any inhibition.

CONCLUSIONS

Our process is the first instance of biological saccharification with exclusive production and accumulation of glucose from lignocellulosic biomass. The key to its success was the use of C. thermocellum supplemented with a thermostable beta-glucosidase and cultured under a high cellulose load. We named this approach biological simultaneous enzyme production and saccharification (BSES). BSES may resolve a significant barrier to economical production by providing a platform for production of fermentable sugars with reduced enzyme amounts.

A synthetic biology approach for evaluating the functional contribution of designer cellulosome components to deconstruction of cellulosic substrates

BACKGROUND

Select cellulolytic bacteria produce multi-enzymatic cellulosome complexes that bind to the plant cell wall and catalyze its efficient degradation. The multi-modular interconnecting cellulosomal subunits comprise dockerin-containing enzymes that bind cohesively to cohesin-containing scaffoldins. The organization of the modules into functional polypeptides is achieved by intermodular linkers of different lengths and composition, which provide flexibility to the complex and determine its overall architecture.

RESULTS

Using a synthetic biology approach, we systematically investigated the spatial organization of the scaffoldin subunit and its effect on cellulose hydrolysis by designing a combinatorial library of recombinant trivalent designer scaffoldins, which contain a carbohydrate-binding module (CBM) and 3 divergent cohesin modules. The positions of the individual modules were shuffled into 24 different arrangements of chimaeric scaffoldins. This basic set was further extended into three sub-sets for each arrangement with intermodular linkers ranging from zero (no linkers), 5 (short linkers) and native linkers of 27-35 amino acids (long linkers). Of the 72 possible scaffoldins, 56 were successfully cloned and 45 of them expressed, representing 14 full sets of chimaeric scaffoldins. The resultant 42-component scaffoldin library was used to assemble designer cellulosomes, comprising three model C. thermocellum cellulases. Activities were examined using Avicel as a pure microcrystalline cellulose substrate and pretreated cellulose-enriched wheat straw as a model substrate derived from a native source. All scaffoldin combinations yielded active trivalent designer cellulosome assemblies on both substrates that exceeded the levels of the free enzyme systems. A preferred modular arrangement for the trivalent designer scaffoldin was not observed for the three enzymes used in this study, indicating that they could be integrated at any position in the designer cellulosome without significant effect on cellulose-degrading activity. Designer cellulosomes assembled with the long-linker scaffoldins achieved higher levels of activity, compared to those assembled with short-and no-linker scaffoldins.

CONCLUSIONS

The results demonstrate the robustness of the cellulosome system. Long intermodular scaffoldin linkers are preferable, thus leading to enhanced degradation of cellulosic substrates, presumably due to the increased flexibility and spatial positioning of the attached enzymes in the complex. These findings provide a general basis for improved designer cellulosome systems as a platform for bioethanol production.

Are cellulosome scaffolding protein CipC and CBM3-containing protein HycP, involved in adherence of Clostridium cellulolyticum to cellulose?

Clostridium cellulolyticum, a mesophilic anaerobic bacterium, produces highly active enzymatic complexes called cellulosomes. This strain was already shown to bind to cellulose, however the molecular mechanism(s) involved is not known. In this context we focused on the gene named hycP, encoding a 250-kDa protein of unknown function, containing a Family-3 Carbohydrate Binding Module (CBM3) along with 23 hyaline repeat modules (HYR modules). In the microbial kingdom the gene hycP is only found in C. cellulolyticum and the very close strain recently sequenced Clostridium sp BNL1100. Its presence in C. cellulolyticum guided us to analyze its function and its putative role in adhesion of the cells to cellulose. The CBM3 of HycP was shown to bind to crystalline cellulose and was assigned to the CBM3b subfamily. No hydrolytic activity on cellulose was found with a mini-protein displaying representative domains of HycP. A C. cellulolyticum inactivated hycP mutant strain was constructed, and we found that HycP is neither involved in binding of the cells to cellulose nor that the protein has an obvious role in cell growth on cellulose. We also characterized the role of the cellulosome scaffolding protein CipC in adhesion of C. cellulolyticum to cellulose, since cellulosome scaffolding protein has been proposed to mediate binding of other cellulolytic bacteria to cellulose. A second mutant was constructed, where cipC was inactivated. We unexpectedly found that CipC is only partly involved in binding of C. cellulolyticum to cellulose. Other mechanisms for cellulose adhesion may therefore exist in C. cellulolyticum. In addition, no cellulosomal protuberances were observed at the cellular surface of C. cellulolyticum, what is in contrast to reports from several other cellulosomes producing strains. These findings may suggest that C. cellulolyticum has no dedicated molecular mechanism to aggregate the cellulosomes at the cellular surface.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

Structure of surface layer homology (SLH) domains from Bacillus anthracis surface array protein

Surface (S)-layers, para-crystalline arrays of protein, are deposited in the envelope of most bacterial species. These surface organelles are retained in the bacterial envelope through the non-covalent association of proteins with cell wall carbohydrates. Bacillus anthracis, a Gram-positive pathogen, produces S-layers of the protein Sap, which uses three consecutive repeats of the surface-layer homology (SLH) domain to engage secondary cell wall polysaccharides (SCWP). Using x-ray crystallography, we reveal here the structure of these SLH domains, which assume the shape of a three-prong spindle. Each SLH domain contributes to a three-helical bundle at the spindle base, whereas another α-helix and its connecting loops generate the three prongs. The inter-prong grooves contain conserved cationic and anionic residues, which are necessary for SLH domains to bind the B. anthracis SCWP. Modeling experiments suggest that the SLH domains of other S-layer proteins also fold into three-prong spindles and capture bacterial envelope carbohydrates by a similar mechanism.

A Bacillus anthracis S-layer homology protein that binds heme and mediates heme delivery to IsdC

The sequestration of iron by mammalian hosts represents a significant obstacle to the establishment of a bacterial infection. In response, pathogenic bacteria have evolved mechanisms to acquire iron from host heme. Bacillus anthracis, the causative agent of anthrax, utilizes secreted hemophores to scavenge heme from host hemoglobin, thereby facilitating iron acquisition from extracellular heme pools and delivery to iron-regulated surface determinant (Isd) proteins covalently attached to the cell wall. However, several Gram-positive pathogens, including B. anthracis, contain genes that encode near iron transporter (NEAT) proteins that are genomically distant from the genetically linked Isd locus. NEAT domains are protein modules that partake in several functions related to heme transport, including binding heme and hemoglobin. This finding raises interesting questions concerning the relative role of these NEAT proteins, relative to hemophores and the Isd system, in iron uptake. Here, we present evidence that a B. anthracis S-layer homology (SLH) protein harboring a NEAT domain binds and directionally transfers heme to the Isd system via the cell wall protein IsdC. This finding suggests that the Isd system can receive heme from multiple inputs and may reflect an adaptation of B. anthracis to changing iron reservoirs during an infection. Understanding the mechanism of heme uptake in pathogenic bacteria is important for the development of novel therapeutics to prevent and treat bacterial infections.

Cell surface xylanases of the glycoside hydrolase family 10 are essential for xylan utilization by Paenibacillus sp. W-61 as generators of xylo-oligosaccharide inducers for the xylanase genes

Paenibacillus sp. W-61 is capable of utilizing water-insoluble xylan for carbon and energy sources and has three xylanase genes, xyn1, xyn3, and xyn5. Xyn1, Xyn3, and Xyn5 are extracellular enzymes of the glycoside hydrolase (GH) families 11, 30, and 10, respectively. Xyn5 contains several domains including those of carbohydrate-binding modules (CBMs) similar to a surface-layer homologous (SLH) protein. This study focused on the role of Xyn5, localized on the cell surface, in water-insoluble xylan utilization. Electron microscopy using immunogold staining revealed Xyn5 clusters over the entire cell surface. Xyn5 was bound to cell wall fractions through its SLH domain. A Deltaxyn5 mutant grew poorly and produced minimal amounts of Xyn1 and Xyn3 on water-insoluble xylan. A Xyn5 mutant lacking the SLH domain (Xyn5DeltaSLH) grew poorly, secreting Xyn5DeltaSLH into the medium and producing minimal Xyn1 and Xyn3 on water-insoluble xylan. A mutant with an intact xyn5 produced Xyn5 on the cell surface, grew normally, and actively synthesized Xyn1 and Xyn3 on water-insoluble xylan. Quantitative reverse transcription-PCR showed that xylobiose, generated from water-insoluble xylan decomposition by Xyn5, is the most active inducer for xyn1 and xyn3. Luciferase assays using a Xyn5-luciferase fusion protein suggested that xylotriose is the best inducer for xyn5. The cell surface Xyn5 appears to play two essential roles in water-insoluble xylan utilization: (i) generation of the xylo-oligosaccharide inducers of all the xyn genes from water-insoluble xylan and (ii) attachment of the cells to the substrate so that the generated inducers can be immediately taken up by cells to activate expression of the xyn system.

Insights into higher-order organization of the cellulosome revealed by a dissect-and-build approach: crystal structure of interacting Clostridium thermocellum multimodular components

Cellulosomes are large, multienzyme, plant cell wall-degrading protein complexes found affixed to the surface of a variety of anaerobic microbes. The core of the cellulosome is a noncatalytic scaffoldin protein, which contains several type-I cohesin modules that bind type-I dockerin-containing enzymatic subunits, a cellulose-binding module, an X module, and a type-II dockerin that interacts with type-II cohesin-containing cell surface proteins. The unique arrangement of the enzymatic subunits in the cellulosome complex, made possible by the scaffoldin subunit, promotes enhanced substrate degradation relative to the enzymes free in solution. Despite representative high-resolution structures of all of the individual modules of the cellulosome, this mechanism of enzymatic synergy remains poorly understood. Consequently, a model of the entire cellulosome and a detailed picture of intermodular contacts will provide more detailed insight into cellulosome activity. Toward this goal, we have solved the structure of a multimodular heterodimeric complex from Clostridium thermocellum composed of the type-II cohesin module of the cell surface protein SdbA bound to a trimodular C-terminal fragment of the scaffoldin subunit CipA to a resolution of 1.95 A. The linker that connects the ninth type-I cohesin module and the X module has elevated temperature factors, reflecting an inherent flexibility within this region. Interestingly, a novel dimer interface was observed between CipA and a second, symmetry-related CipA molecule within the crystal structure, mediated by contacts between a type-I cohesin and an X module of a symmetry mate, resulting in two intertwined scaffoldins. Sedimentation velocity experiments confirmed that dimerization also occurs in solution. These observations support the intriguing possibility that individual cellulosomes can associate with one another via inter-scaffoldin interactions, which may play a role in the mechanism of action of the complex.

Cellulosomes: highly efficient nanomachines designed to deconstruct plant cell wall complex carbohydrates

Cellulosomes can be described as one of nature's most elaborate and highly efficient nanomachines. These cell bound multienzyme complexes orchestrate the deconstruction of cellulose and hemicellulose, two of the most abundant polymers on Earth, and thus play a major role in carbon turnover. Integration of cellulosomal components occurs via highly ordered protein:protein interactions between cohesins and dockerins, whose specificity allows the incorporation of cellulases and hemicellulases onto a molecular scaffold. Cellulosome assembly promotes the exploitation of enzyme synergism because of spatial proximity and enzyme-substrate targeting. Recent structural and functional studies have revealed how cohesin-dockerin interactions mediate both cellulosome assembly and cell-surface attachment, while retaining the spatial flexibility required to optimize the catalytic synergy within the enzyme complex. These emerging advances in our knowledge of cellulosome function are reviewed here.

Cohesin diversity revealed by the crystal structure of the anchoring cohesin from Ruminococcus flavefaciens

The cellulosome is an intriguing multienzyme complex found in cellulolytic bacteria that plays a key role in the degradation of plant cell-wall polysaccharides. In Ruminococcus flavefaciens, a predominant fiber-degrading bacterium found in ruminants, the cellulosome is anchored to the bacterial cell wall through a relatively short ScaE scaffoldin. Determination of the crystal structure of the lone type-III ScaE cohesin from R. flavefaciens (Rf-CohE) was initiated as a part of a structural effort to define cellulosome assembly. The structure was determined at 1.95 A resolution by single-wavelength anomalous diffraction. This is the first detailed description of a crystal structure for a type-III cohesin, and its features were compared with those of the known type-I and type-II cohesin structures. The Rf-CohE module folds into a nine-stranded beta-sandwich with jellyroll topology, typically observed for cohesins, and includes two beta-flaps in the midst of beta-strands 4 and 8, similar to the type-II cohesin structures. However, the presence in Rf-CohE of an additional 13-residue alpha-helix located between beta-strands 8 and 9 represents a dramatic divergence from other known cohesin structures. The prominent alpha-helix is enveloped by an extensive N-terminal loop, not observed in any other known cohesin, which embraces the helix presumably enhancing its stability. A planar surface at the upper portion of the front face of the molecule, bordered by beta-flap 8, exhibits plausible dimensions and exposed amino acid residues to accommodate the dockerin-binding site.

From cellulosomes to cellulosomics

Cellulosomes are intricate multienzyme systems produced by several cellulolytic bacteria, the first example of which was discovered in the anaerobic thermophilic bacterium, Clostridium thermocellum. Cellulosomes are designed for efficient degradation of plant cell wall polysaccharides, notably cellulose--the most abundant renewable polymer on earth. The component parts of the multicomponent complex are integrated by virtue of a unique family of integrating modules, the cohesins and the dockerins, whose distribution and specificity dictate the overall cellulosome architecture. A full generation of research has elapsed since the original publications that documented the cellulosome concept. In this review, we provide a personal account on the discovery process, while describing how divergent cellulosome systems were identified and investigated, culminating in the collaboration of several labs worldwide to tackle together the challenging field of cellulosome genomics and metagenomics.

Physical and genetic characterization of an outer-membrane protein (OmpM1) containing an N-terminal S-layer-like homology domain from the phylogenetically Gram-positive gut anaerobe Mitsuokella multacida

Thin sectioning and freeze-fracture-etch of the ovine ruminal isolate Mitsuokella multacida strain 46/5(2) revealed a Gram-negative envelope ultra-structure consisting of a peptidoglycan wall overlaid by an outer membrane. Sodium-dodecyl-sulfate-polyacrylamide gel electrophoretic (SDS-PAGE) analysis of whole cells, cell envelopes and Triton X-100 extracted envelopes in combination with thin-section and N-terminal sequence analyses demonstrated that the outer membrane contained two major proteins (45 and 43 kDa) sharing identical N-termini (A-A-N-P-F-S-D-V-P-A-D-H-W-A-Y-D). A gene encoding a protein with a predicted N-terminus identical to those of the 43 and 45 kDa outer-membrane proteins was cloned. The 1290 bp open reading frame encoded a 430 amino acid polypeptide with a predicted molecular mass of 47,492 Da. Cleavage of a predicted 23 amino acid leader sequence would yield a protein with a molecular mass of 45,232 Da. Mass spectroscopic analysis confirmed that the cloned gene (ompM1) encoded the 45 kDa outer-membrane protein. The N-terminus of the mature OmpM1 protein (residues 24-70) shared homology with surface-layer homology (SLH) domains found in a wide variety of regularly structured surface-layers (S-layers). However, the outer-membrane locale, resistance to denaturation by SDS and high temperatures and the finding that the C-terminal residue was a phenylalanine suggested that ompM1 encoded a porin. Threading analysis in combination with the identification of membrane spanning domains indicated that the C-terminal region of OmpM1 (residues 250-430) likely forms a 16-strand beta-barrel and appears to be related to the unusual N-terminal SLH-domain-containing beta-barrel-porins previously described in the cyanobacterium Synechococcus PCC6301.

Cohesin-dockerin microarray: Diverse specificities between two complementary families of interacting protein modules

The cellulosome is an intricate multienzyme complex, designed for efficient degradation of plant cell wall polysaccharides, notably cellulose. The supramolecular cellulosome architecture in different bacteria is the consequence of the types and specificities of the interacting cohesin and dockerin modules, borne by the different cellulosomal subunits. In this study, we describe a microarray system for determining cohesin-dockerin specificity, which allows global comparison among the interactions between various members of these two complementary families of interacting protein modules. Matching recombinant fusion proteins were prepared that contained one of the interacting modules: cohesins were joined to an appropriate cellulose-binding module (CBM) and the dockerins were fused to a thermostable xylanase that served to enhance expression and proper folding. The CBM-fused cohesins were immobilized on cellulose-coated glass slides, to which xylanase-fused dockerin samples were applied. Knowledge of the specificity characteristics of native and mutated members of the cohesin and dockerin families provides insight into the architecture of the parent cellulosome and allows selection of suitable cohesin-dockein pairs for biotechnological and nanotechnological application. Using this approach, extensive cross-species interaction among type-II cohesins and dockerins is shown for the first time. Selective intraspecies binding of an archaeal dockerin to two complementary cohesins is also demonstrated.

Xylanase attachment to the cell wall of the hyperthermophilic bacterium Thermotoga maritima

The cellular localization and processing of the endo-xylanases (1,4-beta-D-xylan-xylanohydrolase; EC 3.2.1.8) of the hyperthermophile Thermotoga maritima were investigated, in particular with respect to the unusual outer membrane ("toga") of this gram-negative bacterium. XynB (40 kDa) was detected in the periplasmic fraction of T. maritima cells and in the culture supernatant. XynA (120 kDa) was partially released to the surrounding medium, but most XynA remained cell associated. Immunogold labeling of thin sections revealed that cell-bound XynA was localized mainly in the outer membranes of T. maritima cells. Amino-terminal sequencing of purified membrane-bound XynA revealed processing of the signal peptide after the eighth residue, thereby leaving the hydrophobic core of the signal peptide attached to the enzyme. This mode of processing is reminiscent of type IV prepilin signal peptide cleavage. Removal of the entire XynA signal peptide was necessary for release from the cell because enzyme purified from the culture supernatant lacked 44 residues at the N terminus, including the hydrophobic part of the signal peptide. We conclude that toga association of XynA is mediated by residues 9 to 44 of the signal peptide. The biochemical and electron microscopic localization studies together with the amino-terminal processing data indicate that XynA is held at the cell surface of T. maritima via a hydrophobic peptide anchor, which is highly unusual for an outer membrane protein.

The C. elegans gene dig-1 encodes a giant member of the immunoglobulin superfamily that promotes fasciculation of neuronal processes

The adhesion of growing neurites into appropriate bundles or fascicles is important for the development of correct synaptic connectivity in the nervous system. We describe fasciculation defects of animals with mutations in the C. elegans gene dig-1 and show that dig-1 encodes a giant molecule (13,100 amino acids) of the immunoglobulin superfamily. Five new alleles of dig-1 were isolated in a screen for mutations affecting the morphology or function of several classes of head sensory neurons. Mutants showed process defasciculation of several classes of neurons. Analysis of a temperature-sensitive allele revealed that dig-1 is required during embryogenesis for normal process fasciculation of one class of head sensory neuron. Partial sequencing of two alleles, RNA interference (RNAi) and rescuing experiments showed that dig-1 encodes a giant molecule of the immunoglobulin superfamily. DIG-1 protein contains many domains associated with adhesion, is likely secreted, and has some features of proteoglycans. dig-1 mutants were originally isolated due to their displaced gonads [Thomas, J.H., Stern, M.J., Horvitz, H.R., 1990. Cell interactions coordinate the development of the C. elegans egg-laying system. Cell 62, 1041-52]; thus, dig-1 alleles were also characterized for their effects on gonad placement. Mutant phenotypes suggest that DIG-1 may mediate cell movement as well as process fasciculation and that different regions of the protein may mediate these functions.

Mutagenesis of conserved charged amino acids in SLH domains of Thermoanaerobacterium thermosulfurigenes EM1 affects attachment to cell wall sacculi

SLH domains (for surface layer homology) are involved in the attachment of proteins to bacterial cell walls. The data presented here assign the conserved TRAE motif within SLH domains a key role for the binding. The charged amino acids arginine (R) or/and glutamic acid (E) were replaced via site-directed mutagenesis by different amino acids. Effects were visualized in an in vitro binding assay using native cell wall sacculi of Thermoanaerobacterium thermosulfurigenes EM1 and different variants of an SLH protein which consisted of the triplicate SLH domain of xylanase XynA of this bacterium and which was purified after expression in Escherichia coli. The results indicated (1) that the TRAE motif is critical for the binding function of SLH domains, (2) that a functional TRAE motif is necessary in all three domains, (3) that a least one (preferentially positively) charged amino acid in the TRAE motif is required for the functionality of the SLH domain, and (4) that the position of the negatively and positively charged amino acids is important. The finding that the cell wall of T. thermosulfurigenes EM1 contains pyruvate (4 microg mg(-1)) is in agreement with the hypothesis that pyruvylated secondary cell wall polymers function as ligand for SLH domains.

The cytosolic, cell surface and extracellular proteomes of the biotechnologically important soil bacteriumCorynebacterium efficiens YS-314 in comparison to those ofCorynebacterium glutamicum ATCC 13032

Reference maps of the cytosolic, cell surface and extracellular proteome fractions of the amino acid-producing soil bacterium Corynebacterium efficiens YS-314 were established. The analysis window covers a pI range from 3 to 7 along with a molecular mass range from 10 to 130 kDa. After second-dimensional separation on SDS-PAGE and Coomassie staining, computational analysis detected 635 protein spots in the cytosolic proteome fraction, whereas 76 and 102 spots were detected in the cell surface and extracellular proteomes, respectively. By means of MALDI-TOF-MS and tryptic peptide mass fingerprinting, 164 cytosolic proteins, 49 proteins of the cell surface and 89 extracellular protein spots were identified, representing in total 177 different proteins. Additionally, reference maps of the three cellular proteome fractions of the close phylogenetic relative Corynebacterium glutamicum ATCC 13032 were generated and used for comparative proteomics. Classification according to the Clusters of Orthologous Groups of proteins scheme and abundance analysis of the identified proteins revealed species-specific differences. The high abundance of molecular chaperones and amino acid biosynthesis enzymes in C. efficiens points to environmental adaptations of this recently discovered amino acid-producing bacterium.

Matching fusion protein systems for affinity analysis of two interacting families of proteins: the cohesin-dockerin interaction

Cellulosomes are multi-enzyme complexes that orchestrate the efficient degradation of cellulose and related plant cell wall polysaccharides. The complex is maintained by the high-affinity protein-protein interaction between two complementary modules: the cohesin and the dockerin. In order to characterize the interaction between different cohesins and dockerins, we have developed matching fusion-protein systems, which harbor either the cohesin or the dockerin component. For this purpose, corresponding plasmid cassettes were designed, which encoded for the following carrier proteins: (i) a thermostable xylanase with an appended His-tag; and (ii) a highly stable cellulose-binding module (CBM). The resultant xylanase-dockerin and CBM-cohesin fusion products exhibited high expression levels of soluble protein. The expressed, affinity-purified proteins were extremely stable, and the functionality of the cohesin or dockerin component was retained. The fusion protein system was used to establish a sensitive and reliable, semi-quantitative enzyme-linked affinity assay for determining multiple samples of cohesin-dockerin interactions in microtiter plates. A variety of cohesin-dockerin systems, which had been examined previously using other methodologies, were revisited applying the affinity-based enzyme assay, the results of which served to verify the validity of the approach.

Unconventional mode of attachment of the Ruminococcus flavefaciens cellulosome to the cell surface

Sequence extension of the scaffoldin gene cluster from Ruminococcus flavefaciens revealed a new gene (scaE) that encodes a protein with an N-terminal cohesin domain and a C terminus with a typical gram-positive anchoring signal for sortase-mediated attachment to the bacterial cell wall. The recombinant cohesin of ScaE was recovered after expression in Escherichia coli and was shown to bind to the C-terminal domain of the cellulosomal structural protein ScaB, as well as to three unknown polypeptides derived from native cellulose-bound Ruminococcus flavefaciens protein extracts. The ScaB C terminus includes a cryptic dockerin domain that is unusual in its sequence, and considerably larger than conventional dockerins. The ScaB dockerin binds to ScaE, suggesting that this interaction occurs through a novel cohesin-dockerin pairing. The novel ScaB dockerin was expressed as a xylanase fusion protein, which was shown to bind tenaciously and selectively to a recombinant form of the ScaE cohesin. Thus, ScaE appears to play a role in anchoring the cellulosomal complex to the bacterial cell envelope via its interaction with ScaB. This sortase-mediated mechanism for covalent cell-wall anchoring of the cellulosome in R. flavefaciens differs from those reported thus far for any other cellulosome system.

The structure of secondary cell wall polymers: how Gram-positive bacteria stick their cell walls together

The cell wall of Gram-positive bacteria has been a subject of detailed chemical study over the past five decades. Outside the cytoplasmic membrane of these organisms the fundamental polymer is peptidoglycan (PG), which is responsible for the maintenance of cell shape and osmotic stability. In addition, typical essential cell wall polymers such as teichoic or teichuronic acids are linked to some of the peptidoglycan chains. In this review these compounds are considered as 'classical' cell wall polymers. In the course of recent investigations of bacterial cell surface layers (S-layers) a different class of 'non-classical' secondary cell wall polymers (SCWPs) has been identified, which is involved in anchoring of S-layers to the bacterial cell surface. Comparative analyses have shown considerable differences in chemical composition, overall structure and charge behaviour of these SCWPs. This review discusses the progress that has been made in understanding the structural principles of SCWPs, which may have useful applications in S-layer-based 'supramolecular construction kits' in nanobiotechnology.

Cellulase, clostridia, and ethanol

Biomass conversion to ethanol as a liquid fuel by the thermophilic and anaerobic clostridia offers a potential partial solution to the problem of the world's dependence on petroleum for energy. Coculture of a cellulolytic strain and a saccharolytic strain of Clostridium on agricultural resources, as well as on urban and industrial cellulosic wastes, is a promising approach to an alternate energy source from an economic viewpoint. This review discusses the need for such a process, the cellulases of clostridia, their presence in extracellular complexes or organelles (the cellulosomes), the binding of the cellulosomes to cellulose and to the cell surface, cellulase genetics, regulation of their synthesis, cocultures, ethanol tolerance, and metabolic pathway engineering for maximizing ethanol yield.

Regulation of major cellulosomal endoglucanases of Clostridium thermocellum differs from that of a prominent cellulosomal xylanase

The expression of scaffoldin-anchoring genes and one of the major processive endoglucanases (CelS) from the cellulosome of Clostridium thermocellum has been shown to be dependent on the growth rate. For the present work, we studied the gene regulation of selected cellulosomal endoglucanases and a major xylanase in order to examine the previously observed substrate-linked alterations in cellulosome composition. For this purpose, the transcript levels of genes encoding endoglucanases CelB, CelG, and CelD and the family 10 xylanase XynC were determined in batch cultures, grown on either cellobiose or cellulose, and in carbon-limited continuous cultures at different dilution rates. Under all conditions tested, the transcript levels of celB and celG were at least 10-fold higher than that of celD. Like the major processive endoglucanase CelS, the transcript levels of these endoglucanase genes were also dependent on the growth rate. Thus, at a rate of 0.04 h(-1), the levels of celB, celG, and celD were threefold higher than those obtained in cultures grown at maximal rates (0.35 h(-1)) on cellobiose. In contrast, no clear correlation was observed between the transcript level of xynC and the growth rate-the levels remained relatively high, fluctuating between 30 and 50 transcripts per cell. The results suggest that the regulation of C. thermocellum endoglucanases is similar to that of the processive endoglucanase celS but differs from that of a major cellulosomal xylanase in that expression of the latter enzyme is independent of the growth rate.

Structural characterization of type II dockerin module from the cellulosome of Clostridium thermocellum: calcium-induced effects on conformation and target recognition

The assembly of a functional cellulose-degrading complex termed the cellulosome involves two specific calcium-dependent cohesin-dockerin interactions: type I and type II. Extensive structural and mutagenesis studies have been performed on the type I modules and their interaction in an attempt to identify the underlying molecular determinants responsible for this specificity. However, very little structural information exists for the type II interaction. We have performed a variety of biophysical studies on the type II dockerin-X-module modular pair (DocX), which comprises the C-terminal region of cellulosomal scaffoldin subunit from Clostridium thermocellum, to determine the effect of calcium on its structure and interaction with type II cohesin. Our results indicate that calcium binding to type II dockerin occurs with an apparent dissociation constant (K(d)) of 7 microM, induces stable secondary and tertiary structure, and leads to the exposure of a hydrophobic surface. Calcium binding also results in the homodimerization of DocX. Analytical ultracentrifugation experiments indicate that the DocX homodimer has an elongated shape and a K(d) of approximately 40 microM. However, addition of the SdbA type II cohesin binding partner led to the dissociation of the DocX homodimer and to the formation of a 1:1 heterodimer. We propose that the exposed hydrophobic surface forms, at least in part, the type II cohesin-binding site, which in the absence of cohesin results in the dimerization of DocX.

Hydrophilic domains of scaffolding protein CbpA promote glycosyl hydrolase activity and localization of cellulosomes to the cell surface of Clostridium cellulovorans

CbpA, the scaffolding protein of Clostridium cellulovorans cellulosomes, possesses one family 3 cellulose binding domain, nine cohesin domains, and four hydrophilic domains (HLDs). Among the three types of domains, the function of the HLDs is still unknown. We proposed previously that the HLDs of CbpA play a role in attaching the cellulosome to the cell surface, since they showed some homology to the surface layer homology domains of EngE. Several recombinant proteins with HLDs (rHLDs) and recombinant EngE (rEngE) were examined to determine their binding to the C. cellulovorans cell wall fraction. Tandemly linked rHLDs showed higher affinity for the cell wall than individual rHLDs showed. EngE was shown to have a higher affinity for cell walls than rHLDs have. C. cellulovorans native cellulosomes were found to have higher affinity for cell walls than rHLDs have. When immunoblot analysis was carried out with the native cellulosome fraction bound to cell wall fragments, the presence of EngE was also confirmed, suggesting that the mechanism anchoring CbpA to the C. cellulovorans cell surface was mediated through EngE and that the HLDs play a secondary role in the attachment of the cellulosome to the cell surface. During a study of the role of HLDs on cellulose degradation, the mini-cellulosome complexes with HLDs degraded cellulose more efficiently than complexes without HLDs degraded cellulose. The rHLDs also showed binding affinity for crystalline cellulose and carboxymethyl cellulose. These results suggest that the CbpA HLDs play a major role and a minor role in C. cellulovorans cellulosomes. The primary role increases cellulose degradation activity by binding the cellulosome complex to the cellulose substrate; secondarily, HLDs aid the binding of the CbpA/cellulosome to the C. cellulovorans cell surface.

A novel Acetivibrio cellulolyticus anchoring scaffoldin that bears divergent cohesins

Sequencing of a cellulosome-integrating gene cluster in Acetivibrio cellulolyticus was completed. The cluster contains four tandem scaffoldin genes (scaA, scaB, scaC, and scaD) bounded upstream and downstream, respectively, by a presumed cellobiose phosphorylase and a nucleotide methylase. The sequences and properties of scaA, scaB, and scaC were reported previously, and those of scaD are reported here. The scaD gene encodes an 852-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. The calculated molecular weight of the mature ScaD is 88,960; a 67-residue linker segment separates cohesins 1 and 2, and two approximately 30-residue linkers separate cohesin 2 from 3 and cohesin 3 from the SLH module. The presence of an SLH module in ScaD indicates its role as an anchoring protein. The first two ScaD cohesins can be classified as type II, similar to the four cohesins of ScaB. Surprisingly, the third ScaD cohesin belongs to the type I cohesins, like the seven ScaA cohesins. ScaD is the first scaffoldin to be described that contains divergent types of cohesins as integral parts of the polypeptide chain. The recognition properties among selected recombinant cohesins and dockerins from the different scaffoldins of the gene cluster were investigated by affinity blotting. The results indicated that the divergent types of ScaD cohesins also differ in their preference of dockerins. ScaD thus plays a dual role, both as a primary scaffoldin, capable of direct incorporation of a single dockerin-borne enzyme, and as a secondary scaffoldin that anchors the major primary scaffoldin, ScaA and its complement of enzymes to the cell surface.

The bacterial scaffoldin: structure, function and potential applications in the nanosciences

Natural protein complexes may provide the best templates for nanometer-scale technology and new biomaterials. The bacterial cellulosome is becoming a well-understood multi-protein complex found in cellulolytic microorganisms. The scaffoldin subunits of the bacterial cellulosome function to organize and position other protein subunits into the complex. The scaffoldins can also serve as an attachment device for harnessing the cellulosome onto the cell surface and/or for its targeting to substrate. Biochemical and molecular biological evidence have identified a receptor/adaptor type of protein domain pair, called "cohesin and dockerin," which is responsible for cellulosome self-assembly. The recognition between cohesin and dockerin is generally type and/or species specific. More than 80 cohesin and 100 dockerin sequences have been found, mostly from anaerobic bacteria. X-ray crystallography and NMR have been used to determine the three-dimensional structures of representative cohesin and dockerin domains, respectively. The cohesin peptide is about 140 amino acids in length and highly conserved in sequence and domain structure. The dockerin domain comprises about 70 amino acids and contains two 22 amino acid duplicated regions, each of which includes an "F-hand" modification of the EF-hand calcium-binding motif. Biochemical evidence and site-directed mutagenesis have confirmed that the two F-hand motifs are required for function and calcium dependence; at least two amino acids from each motif are critical for cohesin-dockerin recognition. In this report, we review the structure and function of the scaffoldin of the bacterial cellulosome and emphasize a detailed sequence analysis of the cohesin and dockerin domains. We also speculate about potential applications in nanoscience that may be based on cohesin-dockerin recognition.