Cohesin-dockerin code in cellulosomal dual binding modes and its allosteric regulation by proline isomerization

Cellulose is the most abundant organic molecule on Earth and represents a renewable and practically everlasting feedstock for the production of biofuels and chemicals. Self-assembled owing to the high-affinity cohesin-dockerin interaction, cellulosomes are huge multi-enzyme complexes with unmatched efficiency in the degradation of recalcitrant lignocellulosic substrates. The recruitment of diverse dockerin-borne enzymes into a multicohesin protein scaffold dictates the three-dimensional layout of the complex, and interestingly two alternative binding modes have been proposed. Using single-molecule fluorescence resonance energy transfer and molecular simulations on a range of cohesin-dockerin pairs, we directly detect varying distributions between these binding modes that follow a built-in cohesin-dockerin code. Surprisingly, we uncover a prolyl isomerase-modulated allosteric control mechanism, mediated by the isomerization state of a single proline residue, which regulates the distribution and kinetics of binding modes. Overall, our data provide a novel mechanistic understanding of the structural plasticity and dynamics of cellulosomes.

Discovery and mechanism of a pH-dependent dual-binding-site switch in the interaction of a pair of protein modules

Many important proteins undergo pH-dependent conformational changes resulting in "on-off" switches for protein function, which are essential for regulation of life processes and have wide application potential. Here, we report a pair of cellulosomal assembly modules, comprising a cohesin and a dockerin from Clostridium acetobutylicum, which interact together following a unique pH-dependent switch between two functional sites rather than on-off states. The two cohesin-binding sites on the dockerin are switched from one to the other at pH 4.8 and 7.5 with a 180° rotation of the bound dockerin. Combined analysis by nuclear magnetic resonance spectroscopy, crystal structure determination, mutagenesis, and isothermal titration calorimetry elucidates the chemical and structural mechanism of the pH-dependent switching of the binding sites. The pH-dependent dual-binding-site switch not only represents an elegant example of biological regulation but also provides a new approach for developing pH-dependent protein devices and biomaterials beyond an on-off switch for biotechnological applications.

Cell-surface exposure of a hybrid 3-cohesin scaffoldin allowing the functionalization of Escherichia coli envelope

Cellulosomes are large plant cell wall degrading complexes secreted by some anaerobic bacteria. They are typically composed of a major scaffolding protein containing multiple receptors called cohesins, which tightly anchor a small complementary module termed dockerin harbored by the cellulosomal enzymes. In the present study, we have successfully cell surface exposed in Escherichia coli a hybrid scaffoldin, Scaf6, fused to the curli protein CsgA, the latter is known to polymerize at the surface of E. coli to form extracellular fibers under stressful environmental conditions. The C-terminal part of the chimera encompasses the hybrid scaffoldin composed of three cohesins from different bacterial origins and a carbohydrate-binding module targeting insoluble cellulose. Using three cellulases hosting the complementary dockerin modules and labeled with different fluorophores, we have shown that the hybrid scaffoldin merged to CsgA is massively exposed at the cell surface of E. coli and that each cohesin module is fully operational. Altogether these data open a new route for a series of biotechnological applications exploiting the cell-surface exposure of CsgA-Scaf6 in various industrial sectors such as vaccines, biocatalysts or bioremediation, simply by grafting the small dockerin module to the desired proteins before incubation with the engineered E. coli.

Exploiting dCas9 fusion proteins for dynamic assembly of synthetic metabolons

Here we reported a new strategy to construct synthetic metabolons using dCas9-guided assembly. Three orthogonal dCas9 proteins were exploited to guide the independent and site-specific assembly of their fusion partners onto a single DNA scaffold. This new platform was applied towards the construction of a two-component cellulosome. Because of the superior binding affinity, the resulting structures exhibited both improved assembly and reducing sugar production. Conditional enzyme assembly was made possible by utilizing toehold-gated sgRNA (thgRNA), which blocks cellulosome formation until the spacer region is unblocked by a RNA trigger. This platform is highly modular owing to the ease of target synthesis, combinations of possible Cas9-fusion arrangements, and expansion to other metabolic pathways.

Substrate-Related Factors Affecting Cellulosome-Induced Hydrolysis for Lignocellulose Valorization

Cellulosomes are an extracellular supramolecular multienzyme complex that can efficiently degrade cellulose and hemicelluloses in plant cell walls. The structural and unique subunit arrangement of cellulosomes can promote its adhesion to the insoluble substrates, thus providing individual microbial cells with a direct competence in the utilization of cellulosic biomass. Significant progress has been achieved in revealing the structures and functions of cellulosomes, but a knowledge gap still exists in understanding the interaction between cellulosome and lignocellulosic substrate for those derived from biorefinery pretreatment of agricultural crops. The cellulosomic saccharification of lignocellulose is affected by various substrate-related physical and chemical factors, including native (untreated) wood lignin content, the extent of lignin and xylan removal by pretreatment, lignin structure, substrate size, and of course substrate pore surface area or substrate accessibility to cellulose. Herein, we summarize the cellulosome structure, substrate-related factors, and regulatory mechanisms in the host cells. We discuss the latest advances in specific strategies of cellulosome-induced hydrolysis, which can function in the reaction kinetics and the overall progress of biorefineries based on lignocellulosic feedstocks.

Artificial Cellulosome Complex from the Self-Assembly of Ni-NTA-Functionalized Polymeric Micelles and Cellulases

Polymer-protein core-shell nanoparticles have been explored for enzyme immobilization. This work reports on the development of functional polymeric micelles for immobilizing His6 -tagged cellulases with controlled spatial orientation of enzymes, resulting in "artificial cellulosomes" for effective cellulose hydrolysis. Poly(styrene)-b-poly(styrene-alt-maleic anhydride) was prepared through one-pot reversible addition-fragmentation chain-transfer polymerization and modified with nitrilotriacetic acid (NTA) to afford an amphiphilic block copolymer. The self-assembled polymer was mixed with a solution of NiSO4 to form Ni-NTA-functionalized micelles, which could successfully capture His6 -tagged cellulases and form hierarchically structured core-shell nanoparticles with cellulases as the corona. Because the anchored enzymes are site-specifically oriented and in close proximity, synergistic catalysis that results in over twofold activity enhancement has been achieved.

Resonance assignments of a cellulosomal double-dockerin from Clostridium thermocellum

Cellulosomes are highly efficient multienzyme complexes for lignocellulose degradation secreted by some lignocellulolytic bacteria. Cellulosomes are assembled through protein modules named cohesin and dockerin, and multiple cohesin modules in the scaffold protein generally determine the complexity of the cellulosomes. Some cellulosomal proteins contain multiple dockerin modules, which may generate more complex cellulosomal architectures. Genome mining revealed that cellulosomal proteins containing double dockerin modules and a protease module exist in many cellulosome-producing bacteria, and these proteins together with cellulosomal protease inhibitors were proposed to have regulatory roles. However, the structures and functions of these multiple-dockerin proteins in cellulosome have not been reported before. In this paper, we present the NMR chemical shift assignments of the double-dockerin of a cellulosomal protease from Clostridium thermocellum DSM1313. The secondary structures predicted from the chemical shifts agree with the structural arrangement of the tandem dockerin modules. The chemical shift assignments here provide the basis for the structural and functional studies of multiple-dockerin proteins in future.

Dynamic interactions of type I cohesin modules fine-tune the structure of the cellulosome of Clostridium thermocellum

Efficient degradation of plant cell walls by selected anaerobic bacteria is performed by large extracellular multienzyme complexes termed cellulosomes. The spatial arrangement within the cellulosome is organized by a protein called scaffoldin, which recruits the cellulolytic subunits through interactions between cohesin modules on the scaffoldin and dockerin modules on the enzymes. Although many structural studies of the individual components of cellulosomal scaffoldins have been performed, the role of interactions between individual cohesin modules and the flexible linker regions between them are still not entirely understood. Here, we report single-molecule measurements using FRET to study the conformational dynamics of a bimodular cohesin segment of the scaffoldin protein CipA of Clostridium thermocellum We observe compacted structures in solution that persist on the timescale of milliseconds. The compacted conformation is found to be in dynamic equilibrium with an extended state that shows distance fluctuations on the microsecond timescale. Shortening of the intercohesin linker does not destabilize the interactions but reduces the rate of contact formation. Upon addition of dockerin-containing enzymes, an extension of the flexible state is observed, but the cohesin-cohesin interactions persist. Using all-atom molecular-dynamics simulations of the system, we further identify possible intercohesin binding modes. Beyond the view of scaffoldin as "beads on a string," we propose that cohesin-cohesin interactions are an important factor for the precise spatial arrangement of the enzymatic subunits in the cellulosome that leads to the high catalytic synergy in these assemblies and should be considered when designing cellulosomes for industrial applications.

Correlation Between Size and Activity Enhancement of Recombinantly Assembled Cellulosomes

As multienzyme complexes, cellulosomes hydrolyze cellulosic biomass with high efficiency, which is believed to be attributed to either one or both factors: (1) synergy among the catalytic and substrate-binding entities and (2) the large size of cellulosome complexes. Although the former factor has been extensively documented, the correlation between size and specific activity of cellulosomes is still elusive to date. In this study, primary and secondary scaffoldins with 1, 3, or 5 copies of type I/II cohesin domains were recombinantly synthesized and various cellulosomes carrying 1, 3, 5, 9, 15, or 25 molecules of cellulase mixtures of family 5, 9, and 48 glycoside hydrolases were assembled. In addition, the assembled complex was annexed to cellulose with the aid of a family 3a carbohydrate-binding module (CBM3a). Measuring cellulolytic hydrolysis activities of assembled cellulosomes on crystalline Avicel revealed that higher degree of cellulosome complexity resulted in more efficient cellulose hydrolysis with plateaued synergic effects after the cellulosome size reaches certain degree.

Dual binding in cohesin-dockerin complexes: the energy landscape and the role of short, terminal segments of the dockerin module

The assembly of the polysaccharide degradating cellulosome machinery is mediated by tight binding between cohesin and dockerin domains. We have used an empirical model known as FoldX as well as molecular mechanics methods to determine the free energy of binding between a cohesin and a dockerin from Clostridium thermocellum in two possible modes that differ by an approximately 180° rotation. Our studies suggest that the full-length wild-type complex exhibits dual binding at room temperature, i.e., the two modes of binding have comparable probabilities at equilibrium. The ability to bind in the two modes persists at elevated temperatures. However, single-point mutations or truncations of terminal segments in the dockerin result in shifting the equilibrium towards one of the binding modes. Our molecular dynamics simulations of mechanical stretching of the full-length wild-type cohesin-dockerin complex indicate that each mode of binding leads to two kinds of stretching pathways, which may be mistakenly taken as evidence of dual binding.

PyContact: Rapid, Customizable, and Visual Analysis of Noncovalent Interactions in MD Simulations

Molecular dynamics (MD) simulations have become ubiquitous in all areas of life sciences. The size and model complexity of MD simulations are rapidly growing along with increasing computing power and improved algorithms. This growth has led to the production of a large amount of simulation data that need to be filtered for relevant information to address specific biomedical and biochemical questions. One of the most relevant molecular properties that can be investigated by all-atom MD simulations is the time-dependent evolution of the complex noncovalent interaction networks governing such fundamental aspects as molecular recognition, binding strength, and mechanical and structural stability. Extracting, evaluating, and visualizing noncovalent interactions is a key task in the daily work of structural biologists. We have developed PyContact, an easy-to-use, highly flexible, and intuitive graphical user interface-based application, designed to provide a toolkit to investigate biomolecular interactions in MD trajectories. PyContact is designed to facilitate this task by enabling identification of relevant noncovalent interactions in a comprehensible manner. The implementation of PyContact as a standalone application enables rapid analysis and data visualization without any additional programming requirements, and also preserves full in-program customization and extension capabilities for advanced users. The statistical analysis representation is interactively combined with full mapping of the results on the molecular system through the synergistic connection between PyContact and VMD. We showcase the capabilities and scientific significance of PyContact by analyzing and visualizing in great detail the noncovalent interactions underlying the ion permeation pathway of the human P2X3 receptor. As a second application, we examine the protein-protein interaction network of the mechanically ultrastable cohesin-dockering complex.

Combining in Vitro and in Silico Single-Molecule Force Spectroscopy to Characterize and Tune Cellulosomal Scaffoldin Mechanics

Cellulosomes are polyprotein machineries that efficiently degrade cellulosic material. Crucial to their function are scaffolds consisting of highly homologous cohesin domains, which serve a dual role by coordinating a multiplicity of enzymes as well as anchoring the microbe to its substrate. Here we combined two approaches to elucidate the mechanical properties of the main scaffold ScaA of Acetivibrio cellulolyticus. A newly developed parallelized one-pot in vitro transcription-translation and protein pull-down protocol enabled high-throughput atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) measurements of all cohesins from ScaA with a single cantilever, thus promising improved relative force comparability. Albeit very similar in sequence, the hanging cohesins showed considerably lower unfolding forces than the bridging cohesins, which are subjected to force when the microbe is anchored to its substrate. Additionally, all-atom steered molecular dynamics (SMD) simulations on homology models offered insight into the process of cohesin unfolding under force. Based on the differences among the individual force propagation pathways and their associated correlation communities, we designed mutants to tune the mechanical stability of the weakest hanging cohesin. The proposed mutants were tested in a second high-throughput AFM SMFS experiment revealing that in one case a single alanine to glycine point mutation suffices to more than double the mechanical stability. In summary, we have successfully characterized the force induced unfolding behavior of all cohesins from the scaffoldin ScaA, as well as revealed how small changes in sequence can have large effects on force resilience in cohesin domains. Our strategy provides an efficient way to test and improve the mechanical integrity of protein domains in general.

Assembly of Ruminococcus flavefaciens cellulosome revealed by structures of two cohesin-dockerin complexes

ABTRACT

Cellulosomes are sophisticated multi-enzymatic nanomachines produced by anaerobes to effectively deconstruct plant structural carbohydrates. Cellulosome assembly involves the binding of enzyme-borne dockerins (Doc) to repeated cohesin (Coh) modules located in a non-catalytic scaffoldin. Docs appended to cellulosomal enzymes generally present two similar Coh-binding interfaces supporting a dual-binding mode, which may confer increased positional adjustment of the different complex components. Ruminococcus flavefaciens' cellulosome is assembled from a repertoire of 223 Doc-containing proteins classified into 6 groups. Recent studies revealed that Docs of groups 3 and 6 are recruited to the cellulosome via a single-binding mode mechanism with an adaptor scaffoldin. To investigate the extent to which the single-binding mode contributes to the assembly of R. flavefaciens cellulosome, the structures of two group 1 Docs bound to Cohs of primary (ScaA) and adaptor (ScaB) scaffoldins were solved. The data revealed that group 1 Docs display a conserved mechanism of Coh recognition involving a single-binding mode. Therefore, in contrast to all cellulosomes described to date, the assembly of R. flavefaciens cellulosome involves single but not dual-binding mode Docs. Thus, this work reveals a novel mechanism of cellulosome assembly and challenges the ubiquitous implication of the dual-binding mode in the acquisition of cellulosome flexibility.

Production of Glucaric Acid from Hemicellulose Substrate by Rosettasome Enzyme Assemblies

Hemicellulose biomass is a complex polymer with many different chemical constituents that can be utilized as industrial feedstocks. These molecules can be released from the polymer and transformed into value-added chemicals through multistep enzymatic pathways. Some bacteria produce cellulosomes which are assemblies composed of lignocellulolytic enzymes tethered to a large protein scaffold. Rosettasomes are artificial engineered ring scaffolds designed to mimic the bacterial cellulosome. Both cellulosomes and rosettasomes have been shown to facilitate much higher rates of biomass hydrolysis compared to the same enzymes free in solution. We investigated whether tethering enzymes involved in both biomass hydrolysis and oxidative transformation to glucaric acid onto a rosettasome scaffold would result in an analogous production enhancement in a combined hydrolysis and bioconversion metabolic pathway. Three different enzymes were used to hydrolyze birchwood hemicellulose and convert the substituents to glucaric acid, a top-12 DOE value added chemical feedstock derived from biomass. It was demonstrated that colocalizing the three different enzymes to the synthetic scaffold resulted in up to 40 % higher levels of product compared to uncomplexed enzymes.

CBMs as Probes to Explore Plant Cell Wall Heterogeneity Using Immunocytochemistry

Immunocytochemistry is a widely used technique to localize antigen within intact tissues. Plant cell walls are complex matrixes of highly decorated polysaccharides and the large number of CBM families displaying specific substrate recognition reflects this complexity. The accessibility of large proteins, such as antibodies, to their cell wall epitopes may be sometimes difficult due to steric hindrance problems. Due to their smaller size, CBMs are interesting alternative probes. The aim of this chapter is to describe the use of CBM as probes to explore complex polysaccharide topochemistry in muro and to quantify enzymatic deconstruction.

Diverse specificity of cellulosome attachment to the bacterial cell surface

During the course of evolution, the cellulosome, one of Nature's most intricate multi-enzyme complexes, has been continuously fine-tuned to efficiently deconstruct recalcitrant carbohydrates. To facilitate the uptake of released sugars, anaerobic bacteria use highly ordered protein-protein interactions to recruit these nanomachines to the cell surface. Dockerin modules located within a non-catalytic macromolecular scaffold, whose primary role is to assemble cellulosomal enzymatic subunits, bind cohesin modules of cell envelope proteins, thereby anchoring the cellulosome onto the bacterial cell. Here we have elucidated the unique molecular mechanisms used by anaerobic bacteria for cellulosome cellular attachment. The structure and biochemical analysis of five cohesin-dockerin complexes revealed that cell surface dockerins contain two cohesin-binding interfaces, which can present different or identical specificities. In contrast to the current static model, we propose that dockerins utilize multivalent modes of cohesin recognition to recruit cellulosomes to the cell surface, a mechanism that maximises substrate access while facilitating complex assembly.

Insights into the functionality and stability of designer cellulosomes at elevated temperatures

Enzymatic breakdown of lignocellulose is a major limiting step in second generation biorefineries. Assembly of the necessary activities into designer cellulosomes increases the productivity of this step by enhancing enzyme synergy through the proximity effect. However, most cellulosomal components are obtained from mesophilic microorganisms, limiting the applications to temperatures up to 50 °C. We hypothesized that a scaffoldin, comprising modular components of mainly mesophilic origin, can function at higher temperatures when combined with thermophilic enzymes, and the resulting designer cellulosomes could be employed in higher temperature reactions. For this purpose, we used a tetravalent scaffoldin constituted of three cohesins of mesophilic origin as well as a cohesin and cellulose-binding module derived from the thermophilic bacterium Clostridium thermocellum. The scaffoldin was combined with four thermophilic enzymes from Geobacillus and Caldicellulosiruptor species, each fused with a dockerin whose specificity matched one of the cohesins. We initially verified that the biochemical properties and thermal stability of the resulting chimeric enzymes were not affected by the presence of the mesophilic dockerins. Then we examined the stability of the individual single-enzyme-scaffoldin complexes and the full tetravalent cellulosome showing that all complexes are stable and functional for at least 6 h at 60 °C. Finally, within this time frame and conditions, the full complex appeared over 50 % more efficient in the hydrolysis of corn stover compared to the free enzymes. Overall, the results support the utilization of scaffoldin components of mesophilic origin at relatively high temperatures and provide a framework for the production of designer cellulosomes suitable for high temperature biorefinery applications.

Complexity of the Ruminococcus flavefaciens cellulosome reflects an expansion in glycan recognition

The breakdown of plant cell wall (PCW) glycans is an important biological and industrial process. Noncatalytic carbohydrate binding modules (CBMs) fulfill a critical targeting function in PCW depolymerization. Defining the portfolio of CBMs, the CBMome, of a PCW degrading system is central to understanding the mechanisms by which microbes depolymerize their target substrates. Ruminococcus flavefaciens, a major PCW degrading bacterium, assembles its catalytic apparatus into a large multienzyme complex, the cellulosome. Significantly, bioinformatic analyses of the R. flavefaciens cellulosome failed to identify a CBM predicted to bind to crystalline cellulose, a key feature of the CBMome of other PCW degrading systems. Here, high throughput screening of 177 protein modules of unknown function was used to determine the complete CBMome of R. flavefaciens The data identified six previously unidentified CBM families that targeted β-glucans, β-mannans, and the pectic polysaccharide homogalacturonan. The crystal structures of four CBMs, in conjunction with site-directed mutagenesis, provide insight into the mechanism of ligand recognition. In the CBMs that recognize β-glucans and β-mannans, differences in the conformation of conserved aromatic residues had a significant impact on the topology of the ligand binding cleft and thus ligand specificity. A cluster of basic residues in CBM77 confers calcium-independent recognition of homogalacturonan, indicating that the carboxylates of galacturonic acid are key specificity determinants. This report shows that the extended repertoire of proteins in the cellulosome of R. flavefaciens contributes to an extended CBMome that supports efficient PCW degradation in the absence of CBMs that specifically target crystalline cellulose.

Reversible and multi-cyclic protein-protein interaction in bacterial cellulosome-mimic system using rod-shaped viral nanostructure

The type II cohesin domain and type II dockerin of bacterial cellulosome were cloned from Clostridium thermocellum and expressed with the fusion of tobacco mosaic virus coat protein (TMVcp) and enhanced green fluorescent protein (EGFP), respectively, in Escherichia coli. The TMVcp-cohesin fusion protein was assembled to the stable and rod-shaped nanostructure (TMVcp-Coh rod) under a particular buffer condition, where many active cohesin proteins are biologically and densely displayed around the 3-dimensional surface of TMVcp-Coh rod. Using EGFP-dockerin as a fluorescent reporter, we confirmed that the Ca(2+)-dependent binding and dissociation between native cohesin and dockerin were reproduced with the two recombinant fusion proteins, TMVcp-cohesin and EGFP-dockerin. The multi-cyclic binding-dissociation operation of TMVcp-Coh rod and EGFP-dockerin was successfully performed with maintaining the reversible cohesin-dockerin interaction in every cycle. EGFP that was fused to dockerin as a proof-of-concept here can be switched to other functional proteins/peptides that need to be used in multi-cyclic operation.

Stoichiometrically Controlled Immobilization of Multiple Enzymes on Magnetic Nanoparticles by the Magnetosome Display System for Efficient Cellulose Hydrolysis

The immobilization of multiple cellulase complexes receiving attention for use in the efficient hydrolysis of celluloses. In this study, the magnetosome display system was employed for the preparation of systems mimicking natural multiple cellulase complexes (cellulosomes) on magnetic nanoparticles (MNPs). Initially, two fluorescent proteins, namely, green fluorescent protein and mCherry, were immobilized on MNPs. Fluorescence analysis revealed the close proximity of two different proteins on the MNPs. Enzyme-linked immunosorbent assay analysis showed that stoichiometrically equivalent amounts of the proteins were immobilized on the MNPs. Next, endoglucanase (EG) and β-glucosidase (BG) were immobilized on MNPs to give EG/BG-MNPs. The resulting MNPs were applied for the hydrolysis of celluloses, with rapid hydrolysis of carboxymethyl cellulose being observed. Furthermore, the fusion of the cellulose-binding domain to EG/BG-MNPs promoted improved hydrolysis activity against the insoluble cellulose. We could therefore conclude that the magnetosome display system can expand the possibilities of mimicking natural cellulosome organization on MNPs.

Expression, purification, crystallization and preliminary X-ray analysis of CttA, a putative cellulose-binding protein from Ruminococcus flavefaciens

A number of anaerobic microorganisms produce multi-modular, multi-enzyme complexes termed cellulosomes. These extracellular macromolecular nanomachines are designed for the efficient degradation of plant cell-wall carbohydrates to smaller sugars that are subsequently used as a source of carbon and energy. Cellulolytic strains from the rumens of mammals, such as Ruminococcus flavefaciens, have been shown to have one of the most complex cellulosomal systems known. Cellulosome assembly requires the binding of dockerin modules located in cellulosomal enzymes to cohesin modules located in a macromolecular scaffolding protein. Over 220 genes encoding dockerin-containing proteins have been identified in the R. flavefaciens genome. The dockerin-containing enzymes can be incorporated into the primary scaffoldin (ScaA), which in turn can bind to adaptor scaffoldins (ScaB or ScaC) and subsequently to anchoring scaffoldin (ScaE), thereby attaching the whole complex to the cell surface. However, unlike other cellulosomes such as that from Clostridium thermocellum, the Ruminococcus species lack a specific carbohydrate-binding module (CBM) on ScaA which recruits the entire complex onto the surface of the substrate. Instead, a cellulose-binding protein, CttA, comprising two putative tandem novel carbohydrate-binding modules and a C-terminal X-dockerin module, which can bind to the cohesin of ScaE, may mediate the attachment of bacterial cells to cellulose. Here, the expression, purification and crystallization of the carbohydrate-binding modular part of the CttA from R. flavefaciens are described. X-ray data have been collected to resolutions of 3.23 and to 1.61 Å in space groups P3(1)21 or P3(2)21 and P2(1), respectively. The structure was phased using bound iodide from the crystallization buffer by SAD experiments.

Statistical radii associated with amino acids to determine the contact map: fixing the structure of a type I cohesin domain in the Clostridium thermocellum cellulosome

We propose to improve and simplify protein refinement procedures through consideration of which pairs of amino acid residues should form native contacts. We first consider 11 330 proteins from the CATH database to determine statistical distributions of contacts associated with a given type of amino acid. The distributions are set across the distances between the α-C atoms that are in contact. Based on this data, we determine typical radii of effective spheres that can be placed on the α-C atoms in order to reconstruct the distribution of the contact lengths. This is done by checking for overlaps with enlarged van der Waals spheres associated with heavy atoms on other amino acids.The resulting contacts can be used to identify non-native contacts that may arise during the time evolution of structure-based models. Here, the radii are used to guide reconstruction of nine missing side chains in a type I cohesin domain with the Protein Data Bank code 1AOH. We first identify the likely missing contacts and then sculpt the corresponding side chains by standard refinement tools to achieve consistency with the expected contact map. One ambiguity in refinement is resolved by determining all-atom conformational energies.

Cell-surface Attachment of Bacterial Multienzyme Complexes Involves Highly Dynamic Protein-Protein Anchors

Protein-protein interactions play a pivotal role in the assembly of the cellulosome, one of nature's most intricate nanomachines dedicated to the depolymerization of complex carbohydrates. The integration of cellulosomal components usually occurs through the binding of type I dockerin modules located at the C terminus of the enzymes to cohesin modules located in the primary scaffoldin subunit. Cellulosomes are typically recruited to the cell surface via type II cohesin-dockerin interactions established between primary and cell-surface anchoring scaffoldin subunits. In contrast with type II interactions, type I dockerins usually display a dual binding mode that may allow increased conformational flexibility during cellulosome assembly. Acetivibrio cellulolyticus produces a highly complex cellulosome comprising an unusual adaptor scaffoldin, ScaB, which mediates the interaction between the primary scaffoldin, ScaA, through type II cohesin-dockerin interactions and the anchoring scaffoldin, ScaC, via type I cohesin-dockerin interactions. Here, we report the crystal structure of the type I ScaB dockerin in complex with a type I ScaC cohesin in two distinct orientations. The data show that the ScaB dockerin displays structural symmetry, reflected by the presence of two essentially identical binding surfaces. The complex interface is more extensive than those observed in other type I complexes, which results in an ultra-high affinity interaction (Ka ∼10(12) M). A subset of ScaB dockerin residues was also identified as modulating the specificity of type I cohesin-dockerin interactions in A. cellulolyticus. This report reveals that recruitment of cellulosomes onto the cell surface may involve dockerins presenting a dual binding mode to incorporate additional flexibility into the quaternary structure of highly populated multienzyme complexes.

Insights into a type III cohesin-dockerin recognition interface from the cellulose-degrading bacterium Ruminococcus flavefaciens

Cellulosomes are large multicomponent cellulose-degrading assemblies found on the surfaces of cellulolytic microorganisms. Often containing hundreds of components, the self-assembly of cellulosomes is mediated by the ultra-high-affinity cohesin-dockerin interaction, which allows them to adopt the complex architectures necessary for degrading recalcitrant cellulose. Better understanding of how the cellulosome assembles and functions and what kinds of structures it adopts will further effort to develop industrial applications of cellulosome components, including their use in bioenergy production. Ruminococcus flavefaciens is a well-studied anaerobic cellulolytic bacteria found in the intestinal tracts of ruminants and other herbivores. Key to cellulosomal self-assembly in this bacterium is the dockerin ScaADoc, found on the non-catalytic structural subunit scaffoldin ScaA, which is responsible for assembling arrays of cellulose-degrading enzymes. This work expands on previous efforts by conducting a series of binding studies on ScaADoc constructs that contain mutations in their cohesin recognition interface, in order to identify which residues play important roles in binding. Molecular dynamics simulations were employed to gain insight into the structural basis for our findings. A specific residue pair in the first helix of ScaADoc, as well as a glutamate near the C-terminus, was identified to be essential for cohesin binding. By advancing our understanding of the cohesin binding of ScaADoc, this study serves as a foundation for future work to more fully understand the structural basis of cellulosome assembly in R. flavefaciens.

Crystallization and preliminary crystallographic studies of a novel noncatalytic carbohydrate-binding module from the Ruminococcus flavefaciens cellulosome

Microbial degradation of the plant cell wall is a fundamental biological process with considerable industrial importance. Hydrolysis of recalcitrant polysaccharides is orchestrated by a large repertoire of carbohydrate-active enzymes that display a modular architecture in which a catalytic domain is connected via linker sequences to one or more noncatalytic carbohydrate-binding modules (CBMs). CBMs direct the appended catalytic modules to their target substrates, thus potentiating catalysis. The genome of the most abundant ruminal cellulolytic bacterium, Ruminococcus flavefaciens strain FD-1, provides an opportunity to discover novel cellulosomal proteins involved in plant cell-wall deconstruction. It encodes a modular protein comprising a glycoside hydrolase family 9 catalytic module (GH9) linked to two unclassified tandemly repeated CBMs (termed CBM-Rf6A and CBM-Rf6B) and a C-terminal dockerin. The novel CBM-Rf6A from this protein has been crystallized and data were processed for the native and a selenomethionine derivative to 1.75 and 1.5 Å resolution, respectively. The crystals belonged to orthorhombic and cubic space groups, respectively. The structure was solved by a single-wavelength anomalous dispersion experiment using the CCP4 program suite and SHELXC/D/E.

Mechanostability of cohesin-dockerin complexes in a structure-based model: anisotropy and lack of universality in the force profiles

We use a structure-based coarse grained model to elucidate stretching of three cohesin-dockerin complexes that are found in the cellulosome. The average strength of mechanostability is comparable to that of the I27 domain of titin, but the force profiles depend on the pulling direction and anisotropy effects can be substantial. Even though the force profiles for individual cohesins and dockerins are similar, those for their complexes are visibly distinct for any pulling direction.

Signal amplification by a self-assembled biosensor system designed on the principle of dockerin-cohesin interactions in a cellulosome complex

To construct a self-assembled biosensor with signal amplification, a cellulosome system, comprising type I and type II dockerin-cohesin interactions with different specificities, from the anaerobic Clostridia bacterium was applied. The self-assembled biosensor was highly sensitive and achieved 128.1-fold increase in detection levels compared to the control.

Construction of GH16 β-glucanase mini-cellulosomes to improve the nutritive value of barley-based diets for broilers

Anaerobic cellulolytic bacteria organize a comprehensive range of cellulases and hemicellulases in high molecular weight multienzyme complexes termed cellulosomes. Integration of cellulosomal components occurs via highly ordered protein-protein interactions between cohesins and dockerins. This paper reports the production of mini-cellulosomes containing one (GH16-1C) or three (GH16-3C) copies of Clostridium thermocellum glucanase 16A (CtGlc16A). Barley β-1,3-1,4-glucans are known to be antinutritive for monogastric animals, particularly for poultry. GH16-1C and GH16-3C were used to supplement barley-based diets for broilers. The data revealed that the two mini-cellulosomes effectively improved the nutritive value of barley-based diets for broilers. Analysis of mini-cellulosome molecular integrity revealed that linker sequences separating protein domains in scaffoldins and cellulosomal catalytic units are highly susceptible to proteolytic attack in vivo. The data suggest that linker protection could result in further improvements in enzyme efficacy to improve the nutritive value of barley-based diets for monogastric animals.

Structural characterization of a novel autonomous cohesin from Ruminococcus flavefaciens

Ruminococcus flavefaciens is a cellulolytic bacterium found in the rumen of herbivores and produces one of the most elaborate and variable cellulosome systems. The structure of an R. flavefaciens protein (RfCohG, ZP_06142108), representing a freestanding (non-cellulosomal) type III cohesin module, has been determined. A selenomethionine derivative with a C-terminal histidine tag was crystallized and diffraction data were measured to 2.44 Å resolution. Its structure was determined by single-wavelength anomalous dispersion, revealing eight molecules in the asymmetric unit. RfCohG exhibits the most complex among all known cohesin structures, possessing four α-helical elements and a topographical protuberance on the putative dockerin-binding surface.

Investigating receptor-ligand systems of the cellulosome with AFM-based single-molecule force spectroscopy

Cellulosomes are discrete multienzyme complexes used by a subset of anaerobic bacteria and fungi to digest lignocellulosic substrates. Assembly of the enzymes onto the noncatalytic scaffold protein is directed by interactions among a family of related receptor-ligand pairs comprising interacting cohesin and dockerin modules. The extremely strong binding between cohesin and dockerin modules results in dissociation constants in the low picomolar to nanomolar range, which may hamper accurate off-rate measurements with conventional bulk methods. Single-molecule force spectroscopy (SMFS) with the atomic force microscope measures the response of individual biomolecules to force, and in contrast to other single-molecule manipulation methods (i.e. optical tweezers), is optimal for studying high-affinity receptor-ligand interactions because of its ability to probe the high-force regime (>120 pN). Here we present our complete protocol for studying cellulosomal protein assemblies at the single-molecule level. Using a protein topology derived from the native cellulosome, we worked with enzyme-dockerin and carbohydrate binding module-cohesin (CBM-cohesin) fusion proteins, each with an accessible free thiol group at an engineered cysteine residue. We present our site-specific surface immobilization protocol, along with our measurement and data analysis procedure for obtaining detailed binding parameters for the high-affinity complex. We demonstrate how to quantify single subdomain unfolding forces, complex rupture forces, kinetic off-rates, and potential widths of the binding well. The successful application of these methods in characterizing the cohesin-dockerin interaction responsible for assembly of multidomain cellulolytic complexes is further described.

Unraveling enzyme discrimination during cellulosome assembly independent of cohesin-dockerin affinity

Bacterial cellulosomes are generally believed to assemble at random, like those produced by Clostridium cellulolyticum. They are composed of one scaffolding protein bearing eight homologous type I cohesins that bind to any of the type I dockerins borne by the 62 cellulosomal subunits, thus generating highly heterogeneous complexes. In the present study, the heterogeneity and random assembly of the cellulosomes were evaluated with a simpler model: a miniscaffoldin containing three C. cellulolyticum cohesins and three cellulases of the same bacterium bearing the cognate dockerin (Cel5A, Cel48F, and Cel9G). Surprisingly, rather than the expected randomized integration of enzymes, the assembly of the minicellulosome generated only three distinct types of complex out of the 10 possible combinations, thus indicating preferential integration of enzymes upon binding to the scaffoldin. A hybrid scaffoldin that displays one cohesin from C. cellulolyticum and one from C. thermocellum, thus allowing sequential integration of enzymes, was exploited to further characterize this phenomenon. The initial binding of a given enzyme to the C. thermocellum cohesin was found to influence the type of enzyme that subsequently bound to the C. cellulolyticum cohesin. The preferential integration appears to be related to the length of the inter-cohesin linker. The data indicate that the binding of a cellulosomal enzyme to a cohesin has a direct influence on the dockerin-bearing proteins that will subsequently interact with adjacent cohesins. Thus, despite the general lack of specificity of the cohesin-dockerin interaction within a given species and type, bacterial cellulosomes are not necessarily assembled at random.

Insights into cellulosome assembly and dynamics: from dissection to reconstruction of the supramolecular enzyme complex

Cellulosomes are multi-enzyme complexes produced by anaerobic bacteria for the efficient deconstruction of plant cell wall polysaccharides. The assembly of enzymatic subunits onto a central non-catalytic scaffoldin subunit is mediated by a highly specific interaction between the enzyme-bearing dockerin modules and the resident cohesin modules of the scaffoldin, which affords their catalytic activities to work synergistically. The scaffoldin also imparts substrate-binding and bacterial-anchoring properties, the latter of which involves a second cohesin-dockerin interaction. Recent structure-function studies reveal an ever-growing array of unique and increasingly complex cohesin-dockerin complexes and cellulosomal enzymes with novel activities. A 'build' approach involving multimodular cellulosomal segments has provided a structural model of an organized yet conformationally dynamic supramolecular assembly with the potential to form higher order structures.

Self-assembly of synthetic metabolons through synthetic protein scaffolds: one-step purification, co-immobilization, and substrate channeling

One-step purification of a multi-enzyme complex was developed based on a mixture of cell extracts containing three dockerin-containing enzymes and one family 3 cellulose-binding module (CBM3)-containing scaffoldin through high-affinity adsorption on low-cost solid regenerated amorphous cellulose (RAC). The three-enzyme complex, called synthetic metabolon, was self-assembled through the high-affinity interaction between the dockerin in each enzyme and three cohesins in the synthetic scaffoldin. The metabolons were either immobilized on the external surface of RAC or free when the scaffoldin contained an intein between the CBM3 and three cohesins. The immobilized and free metabolons containing triosephosphate isomerase, aldolase, and fructose 1,6-biphosphatase exhibited initial reaction rates 48 and 38 times, respectively, that of the non-complexed three-enzyme mixture at the same enzyme loading. Such reaction rate enhancements indicated strong substrate channeling among synthetic metabolons due to the close spatial organization among cascade enzymes. These results suggested that the construction of synthetic metabolons by using cohesins, dockerins, and cellulose-binding modules from cellulosomes not only decreased protein purification labor and cost for in vitro synthetic biology projects but also accelerated reaction rates by 1 order of magnitude compared to non-complexed enzymes. Synthetic metabolons would be an important biocatalytic module for in vitro and in vivo synthetic biology projects.

Structural and functional characterization of a novel type-III dockerin from Ruminococcus flavefaciens

Phylogenetic analysis of known dockerins in Ruminococcus flavefaciens revealed a novel subtype, type-III, in the scaffoldin proteins, ScaA, ScaB, ScaC and ScaE. In this study, we explored the Ca²⁺-binding properties of the type-III dockerin from the ScaA scaffoldin (ScaADoc) using a battery of structural and biophysical approaches including circular dichroism spectroscopy, isothermal titration calorimetry, differential scanning calorimetry, and nuclear magnetic resonance spectroscopy. Despite the lack of a second canonical Ca²⁺-binding loop, the behaviour of ScaADoc is similar with respect to other dockerin protein modules in terms of its responsiveness to Ca²⁺ and affinity for the cohesin from the ScaB scaffoldin. Our results highlight the robustness of dockerin modules and how their Ca²⁺-binding properties can be exploited in the construction of designer cellulosomes.

Novel Clostridium thermocellum type I cohesin-dockerin complexes reveal a single binding mode

Protein-protein interactions play a pivotal role in a large number of biological processes exemplified by the assembly of the cellulosome. Integration of cellulosomal components occurs through the binding of type I cohesin modules located in a non-catalytic molecular scaffold to type I dockerin modules located at the C terminus of cellulosomal enzymes. The majority of type I dockerins display internal symmetry reflected by the presence of two essentially identical cohesin-binding surfaces. Here we report the crystal structures of two novel Clostridium thermocellum type I cohesin-dockerin complexes (CohOlpC-Doc124A and CohOlpA-Doc918). The data revealed that the two dockerins, Doc918 and Doc124A, are unusual because they lack the structural symmetry required to support a dual binding mode. Thus, in both cases, cohesin recognition is dominated by residues located at positions 11, 12, and 19 of one of the dockerin binding surfaces. The alternative binding mode is not possible (Doc918) or highly limited (Doc124A) because residues that assume the critical interacting positions, when dockerins are reoriented by 180°, make steric clashes with the cohesin. In common with a third dockerin (Doc258) that also presents a single binding mode, Doc124A directs the appended cellulase, Cel124A, to the surface of C. thermocellum and not to cellulosomes because it binds preferentially to type I cohesins located at the cell envelope. Although there are a few exceptions, such as Doc918 described here, these data suggest that there is considerable selective pressure for the evolution of a dual binding mode in type I dockerins that direct enzymes into cellulosomes.

Indirect ELISA-based approach for comparative measurement of high-affinity cohesin-dockerin interactions

The interaction between the cohesin and dockerin modules serves to attach cellulolytic enzymes (carrying dockerins) to non-catalytic scaffoldin units (carrying multiple cohesins) in cellulosome, a multienzyme plant cell-wall degrading complex. This interaction is species-specific, for example, the enzyme-borne dockerin from Clostridium thermocellum bacteria binds to scaffoldin cohesins from the same bacteria but not to cohesins from Clostridium cellulolyticum and vice versa. We studied the role of interface residues, contributing either to affinity or specificity, by mutating these residues on the cohesin counterpart from C. thermocellum. The high affinity of the cognate interactions makes it difficult to evaluate the effect of these mutations by common methods used for measuring protein-protein interactions, especially when subtle discrimination between the mutants is needed. We described in this article an approach based on indirect enzyme-linked immunosorbent assay (ELISA) that is able to detect differences in binding between the various cohesin mutants, whereas surface plasmon resonance and standard ELISA failed to distinguish between high-affinity interactions. To be able to calculate changes in energy of binding (ΔΔG) and dissociation constants (K(d)) of mutants relative to wild type, a pre-equilibrium step was added to the standard indirect ELISA procedure. Thus, the cohesin-dockerin interaction under investigation occurs in solution rather than between soluble and immobilized proteins. Unbound dockerins are then detected through their interaction with immobilized cohesins. Because our method allows us to assess the effect of mutations on particularly tenacious protein-protein interactions much more accurately than do other prevalent methods used to measure binding affinity, we therefore suggest this approach as a method of choice for comparing relative binding in high-affinity interactions.

Crystallization and preliminary X-ray characterization of a type III cohesin-dockerin complex from the cellulosome system of Ruminococcus flavefaciens

In Ruminococcus flavefaciens, a predominant fibre-degrading bacterium found in ruminants, cellulosomal proteins are anchored to the bacterial cell wall through a relatively small ScaE scaffoldin which includes a single type III cohesin. The cotton-binding protein CttA consists of two cellulose-binding modules and a C-terminal modular pair (XDoc) comprising an X-module and a contiguous dockerin, which exhibits high affinity towards the ScaE cohesin. Seleno-L-methionine-labelled derivatives of the ScaE cohesin module and the XDoc from CttA have been expressed, copurified and cocrystallized. The crystals belonged to the tetragonal space group P4(3)2(1)2, with unit-cell parameters a = b = 78.7, c = 203.4 Å, and the unit cell contains a single cohesin-XDoc complex in the asymmetric unit. The diffraction data were phased to 2.0 Å resolution using the anomalous signal of the Se atoms.

Genome-wide analysis of acetivibrio cellulolyticus provides a blueprint of an elaborate cellulosome system

BACKGROUND

Microbial degradation of plant cell walls and its conversion to sugars and other byproducts is a key step in the carbon cycle on Earth. In order to process heterogeneous plant-derived biomass, specialized anaerobic bacteria use an elaborate multi-enzyme cellulosome complex to synergistically deconstruct cellulosic substrates. The cellulosome was first discovered in the cellulolytic thermophile, Clostridium thermocellum, and much of our knowledge of this intriguing type of protein composite is based on the cellulosome of this environmentally and biotechnologically important bacterium. The recently sequenced genome of the cellulolytic mesophile, Acetivibrio cellulolyticus, allows detailed comparison of the cellulosomes of these two select cellulosome-producing bacteria.

RESULTS

Comprehensive analysis of the A. cellulolyticus draft genome sequence revealed a very sophisticated cellulosome system. Compared to C. thermocellum, the cellulosomal architecture of A. cellulolyticus is much more extensive, whereby the genome encodes for twice the number of cohesin- and dockerin-containing proteins. The A. cellulolyticus genome has thus evolved an inflated number of 143 dockerin-containing genes, coding for multimodular proteins with distinctive catalytic and carbohydrate-binding modules that play critical roles in biomass degradation. Additionally, 41 putative cohesin modules distributed in 16 different scaffoldin proteins were identified in the genome, representing a broader diversity and modularity than those of Clostridium thermocellum. Although many of the A. cellulolyticus scaffoldins appear in unconventional modular combinations, elements of the basic structural scaffoldins are maintained in both species. In addition, both species exhibit similarly elaborate cell-anchoring and cellulosome-related gene- regulatory elements.

CONCLUSIONS

This work portrays a particularly intricate, cell-surface cellulosome system in A. cellulolyticus and provides a blueprint for examining the specific roles of the various cellulosomal components in the degradation of complex carbohydrate substrates of the plant cell wall by the bacterium.

Analysis of selective, high protein-protein binding interaction of cohesin-dockerin complex using biosensing methods

Optical biosensors that use fluorescence are promising tools for the analysis of target materials such as protein, DNA and other biomaterial. To analyze the binding properties of a protein-protein interaction, we constructed fluorescent biomarkers based on the cohesin-dockerin interaction, which coordinates the assembly of cellulolytic enzymes and scaffolding proteins to produce a cell surface multiprotein complex known as the "cellulosome" in some anaerobic bacteria. Our 2D-PAGE results displayed diverse binding profiles to the dockerin containing cellulosomal proteins produced by Clostridium cellulovorans grown on different carbon sources, such as Avicel, xylan and AXP (Avicel:xylan:pectin (3:1:1)). Fluorescence intensity analysis indicated that EngE and EngH bound more efficiently to Coh6 than to Coh2 or Coh9 (2-fold to 6-fold and 1.5-fold to 5-fold, respectively), while others cellulosomal proteins displayed similar results. In addition, both an enzyme-linked interaction assay (ELIA) and surface plasmon resonance (SPR) analyses demonstrated that both EngE and EngH preferentially bound cohesin6 versus the other two cohesin molecules. This work demonstrated the analysis of the binding patterns between interacting proteins using fluorescent biomarkers. We also illustrated the potential of this sensitive approach to quantify specific target analytical materials via the example of the cohesin-dockerin interaction.

Escherichia coli expression, purification, crystallization, and structure determination of bacterial cohesin-dockerin complexes

Cellulosomes are highly efficient nanomachines that play a fundamental role during the anaerobic deconstruction of complex plant cell wall carbohydrates. The assembly of these complex nanomachines results from the very tight binding of repetitive cohesin modules, located in a noncatalytic molecular scaffold, and dockerin domains located at the C-terminus of the enzyme components of the cellulosome. The number of enzymes found in a cellulosome varies but may reach more than 100 catalytic subunits if cellulosomes are further organized in polycellulosomes, through a second type of cohesin-dockerin interaction. Structural studies have revealed how the cohesin-dockerin interaction mediates cellulosome assembly and cell-surface attachment, while retaining the flexibility required to potentiate catalytic synergy within the complex. Methods that might be applied for the production, purification, and structure determination of cohesin-dockerin complexes are described here.

Structure of CBM3b of the major cellulosomal scaffoldin subunit ScaA from Acetivibrio cellulolyticus

The carbohydrate-binding module (CBM) of the major scaffoldin subunit ScaA of the cellulosome of Acetivibrio cellulolyticus is classified as a family 3b CBM and binds strongly to cellulose. The CBM3b was overexpressed, purified and crystallized, and its three-dimensional structure was determined. The structure contained a nickel-binding site located at the N-terminal region in addition to a 'classical' CBM3b calcium-binding site. The structure was also determined independently by the SAD method using data collected at the Ni-absorption wavelength of 1.48395 Å and even at a wavelength of 0.97625 Å in a favourable case. The new scaffoldin-borne CBM3 structure reported here provides clear evidence for the proposition that a family 3b CBM may be accommodated in scaffoldin subunits and functions as the major substrate-binding entity of the cellulosome assembly.

Probing the mechanism of cellulosome attachment to the Clostridium thermocellum cell surface: computer simulation of the Type II cohesin-dockerin complex and its variants

The recalcitrance of lignocellulosic biomass to hydrolysis is the bottleneck in cellulosic ethanol production. Efficient degradation of biomass by the anaerobic bacterium Clostridium thermocellum is carried out by the multicomponent cellulosome complex. The bacterial cell-surface attachment of the cellulosome is mediated by high-affinity protein-protein interactions between the Type II cohesin domain borne by the cell envelope protein and the Type II dockerin domain, together with neighboring X-module present at the C-terminus of the scaffolding protein (Type II coh-Xdoc). Here, the Type II coh-Xdoc interaction is probed using molecular dynamics simulations, free-energy calculations and essential dynamics analyses on both the wild type and various mutants of the C. thermocellum Type II coh-Xdoc in aqueous solution. The simulations identify the hot spots, i.e. the amino acid residues that may lead to a dramatic decrease in binding affinity upon mutation and also probe the effects of mutations on the mode of binding. The results suggest that bulky and hydrophobic residues at the protein interface, which make specific contacts with their counterparts, may play essential roles in retaining a rigid cohesin-dockerin interface. Moreover, dynamical cross-correlation analysis indicates that the X-module has a dramatic effect on the cohesin-dockerin interaction and is required for the dynamical integrity of the interface.

Modular arrangement of a cellulosomal scaffoldin subunit revealed from the crystal structure of a cohesin dyad

The cellulosome complex is composed of a conglomerate of subunits, each of which comprises a set of interacting functional modules. Scaffoldin (Sca), a major cellulosomal subunit, is responsible for organizing the cellulolytic subunits into the complex. This is accomplished by the interaction of two complementary classes of modules-a cohesin (Coh) module on the Sca subunit and a dockerin module on each of the enzymatic subunits. Although individual Coh modules from different cellulosomal scaffoldins have been subjected to intensive structural investigation, the Sca subunit in its entirety has not, and there remains a paucity of information on the arrangement and interactions of Cohs within the Sca subunit. In the present work, we describe the crystal structure of a type II Coh dyad from the ScaB "adaptor" Sca of Acetivibrio cellulolyticus. The ScaB Cohs are oriented in an "antiparallel" manner relative to one another, with their dockerin-interacting surfaces (beta-strands 8-3-6-5) facing the same direction-aligned on the same plane. A set of extensive hydrophobic and hydrogen-bond contacts between the Cohs and the short interconnecting linker segment between them stabilizes the modular orientation. This Coh dyad structure provides novel information about Coh-Coh association and arrangement in the Sca and further insight into intermodular linker interactions. Putative structural arrangements of a hexamodular complex, composed of the Coh dyad bound to two X-dockerin modules, were suggested.

Purification and crystallization of a multimodular heterotrimeric complex containing both type I and type II cohesin-dockerin interactions from the cellulosome of Clostridium thermocellum

The multimodular scaffoldin subunit CipA is the central component of the cellulosome, a multienzyme plant cell-wall-degrading complex, from Clostridium thermocellum. It captures secreted cellulases and hemicellulases and anchors the entire complex to the cell surface via high-affinity calcium-dependent interactions between cohesin and dockerin modules termed type I and type II interactions. The crystallization of a heterotrimeric complex comprising the type II cohesin module from the cell-surface protein SdbA, a trimodular C-terminal fragment of the scaffoldin CipA and the type I dockerin module from the CelD cellulase is reported. The crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 119.37, b = 186.31, c = 191.17 A. The crystals diffracted to 2.7 A resolution with four or eight molecules of the ternary protein complex in the asymmetric unit.

Intermodular linker flexibility revealed from crystal structures of adjacent cellulosomal cohesins of Acetivibrio cellulolyticus

Cellulosome complexes comprise an intercalated set of multimodular dockerin-containing enzymatic subunits connected to cohesin-containing nonenzymatic subunits called scaffoldins. The adjoining modules in each cellulosomal subunit are interconnected by a variety of linker segments of different lengths and composition. The exact role of the cellulosomal linkers has yet to be described, although it is assumed that they contribute to the architecture and action of the cellulosome by providing the protein subunits with flexibility and by providing spacers between the enzymatic modules that could enhance interactions with the cellulose substrate. Here we present four crystal structures of Acetivibrio cellulolyticus cellulosomal type II cohesins with linker extensions. Two of the structures represent two different crystal forms (trigonal and orthorhombic) of the same N-terminal cohesin module (CohB1) together with its full (6-residue) native C-terminal linker, derived from scaffoldin B. The other two structures belong to the adjacent (second) cohesin module (CohB2), each of which was crystallized with the same 6-residue linker segment, but now positioned at the N-terminus and with either a truncated (5-residue) or a full-length (45-residue) C-terminal linker, respectively. Comparison between the two CohB1 structures revealed significant differences in the conformation of their equivalent C-terminal linker segment. In one crystal form a helical conformation was observed, as opposed to an extended conformation in the other. The CohB2 structures also displayed diverse conformations in their linker segments. In these structures, different linker conformations were observed in the individual molecules within the asymmetric unit of each structure. This conformational diversity implies that the linkers may adopt alternative conformations in their natural environment, consistent with varying environmental conditions. The findings suggest that linkers can play an important role in the assembly, dynamics and function of the cellulosomal components.

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.

A serpin in the cellulosome of the anaerobic fungus Piromyces sp. strain E2

A gene encoding a novel component of the cellulolytic complex (cellulosome) of the anaerobic fungus Piromyces sp. strain E2 was identified. The encoded 538 amino acid protein, named celpin, consists of a signal peptide, a positively charged domain of unknown function followed by two fungal dockerins, typical for components of the extracellular fungal cellulosome. The C-terminal end consists of a 380 amino acid serine proteinase inhibitor (or serpin) domain homologue, sharing 30% identity and 50% similarity to vertebrate and bacterial serpins. Detailed protein sequence analysis of the serpin domain revealed that it contained all features of a functional serpin. It possesses the conserved amino acids present in more than 70% of known serpins, and it contained the consensus of inhibiting serpins. Because of the confined space of the fungal cellulosome inside plant tissue and the auto-proteolysis of plant material in the rumen, the fungal serpin is presumably involved in protection of the cellulosome against plant proteinases. The celpin protein of Piromyces sp. strain E2 is the first non-structural, non-hydrolytic fungal cellulosome component. Furthermore, the celpin protein of Piromyces sp. strain E2 is the first representative of a serine proteinase inhibitor of the fungal kingdom.

Synthesis of Clostridium cellulovorans minicellulosomes by intercellular complementation

The ability of two strains of bacteria to cooperate in the synthesis of an enzyme complex (a minicellulosome) was examined. Three strains of Bacillus subtilis were constructed to express Clostridium cellulovorans genes engB, xynB, and minicbpA. MiniCbpA, EngB, and XynB were synthesized and secreted into the medium by B. subtilis. When the strains with the minicbpA and engB genes or with xynB were cocultured, minicellulosomes were synthesized, consisting in one case of miniCbpA and EngB and in the second case of miniCbpA and XynB. Both minicellulosomes showed their respective enzymatic activities. We call this phenomenon "intercellular complementation." Interesting implications concerning bacterial cooperation are suggested from these results.

Conservation and divergence in cellulosome architecture between two strains of Ruminococcus flavefaciens

A 17-kb scaffoldin gene cluster in Ruminococcus flavefaciens strain FD-1 was compared with the homologous segment published for strain 17. Although the general design of the cluster is identical in the two strains, significant differences in the modular architecture of the scaffoldin proteins were discovered, implying strain-specific divergence in cellulosome organization.

Degradation of cellulosome-produced cello-oligosaccharides by an extracellular non-cellulosomal beta-glucan glucohydrolase, BglA, from Clostridium cellulovorans

Clostridium cellulovorans degrades cellulose efficiently to small oligosaccharides, which are used as an energy source. To characterize enzymes related to degrading small oligosaccharides, a gene was cloned for an extracellular non-cellulosomal beta-glucan glucohydrolase (BglA) classified as a family-1 glycosyl hydrolase in C. cellulovorans. Recombinant BglA (rBglA) had higher activity on long glucooligomers than on cellobiose. When cellulosomes and rBglA were incubated with cellulose, the oligosaccharides produced were degraded more effectively to cellobiose and glucose, than with cellulosomes alone, indicating that BglA facilitated the degradation of accessible cello-oligosaccharides produced from cellulose by C. cellulovorans cellulosomes. Thus, this is an example of an extracellular non-cellulosomal enzyme working in a cooperative manner with cellulosomes to degrade cellulose to sugars.