Dimension, shape, and conformational flexibility of a two domain fungal cellulase in solution probed by small angle X-ray scattering

Glycosylation of cellulase: a novel strategy for improving cellulase

Protein glycosylation is the most complex posttranslational modification process. Most cellulases from filamentous fungi contain N-glycosylation and O-glycosylation. Here, we discuss the potential roles of glycosylation on the characteristics and function of cellulases. The use of certain cultivation, inducer, and alteration of engineering glycosylation pathway can enable the rational control of cellulase glycosylation. Glycosylation does not occur arbitrarily and may tend to modify the 3D structure of cellulases by using specially distributed glycans. Therefore, glycoengineering should be considered comprehensively along with the spatial structure of cellulases. Cellulase glycosylation may be an evolution phenomenon, which has been considered as an economical way for providing different functions from identical proteins. In addition to gene and transcription regulations, glycosylation may be another regulation on the protein expression level. Enhanced understanding of the potential regulatory role of cellulase glycosylation will enable synthetic biology approaches for the development of commercial cellulase.

The effect of linker conformation on performance and stability of a two-domain lytic polysaccharide monooxygenase

A considerable number of lytic polysaccharide monooxygenases (LPMOs) and other carbohydrate-active enzymes are modular, with catalytic domains being tethered to additional domains, such as carbohydrate-binding modules, by flexible linkers. While such linkers may affect the structure, function, and stability of the enzyme, their roles remain largely enigmatic, as do the reasons for natural variation in length and sequence. Here, we have explored linker functionality using the two-domain cellulose-active ScLPMO10C from Streptomyces coelicolor as a model system. In addition to investigating the WT enzyme, we engineered three linker variants to address the impact of both length and sequence and characterized these using small-angle X-ray scattering, NMR, molecular dynamics simulations, and functional assays. The resulting data revealed that, in the case of ScLPMO10C, linker length is the main determinant of linker conformation and enzyme performance. Both the WT and a serine-rich variant, which have the same linker length, demonstrated better performance compared with those with either a shorter linker or a longer linker. A highlight of our findings was the substantial thermostability observed in the serine-rich variant. Importantly, the linker affects thermal unfolding behavior and enzyme stability. In particular, unfolding studies show that the two domains unfold independently when mixed, whereas the full-length enzyme shows one cooperative unfolding transition, meaning that the impact of linkers in biomass-processing enzymes is more complex than mere structural tethering.

Molecular insight into cellulose degradation by the phototrophic green alga Scenedesmus

Lignocellulose is the most abundant natural biopolymer on earth and a potential raw material for the production of fuels and chemicals. However, only some organisms such as bacteria and fungi produce enzymes that metabolize this polymer. In this work we have demonstrated the presence of cellulolytic activity in the supernatant of Scenedesmus quadricauda cultures and we identified the presence of extracellular cellulases in the genome of five Scenedesmus species. Scenedesmus is a green alga which grows in both freshwater and saltwater regions as well as in soils, showing highly flexible metabolic properties. Sequence comparison of the different identified cellulases with hydrolytic enzymes from other organisms using multisequence alignments and phylogenetic trees showed that these proteins belong to the families of glycosyl hydrolases 1, 5, 9, and 10. In addition, most of the Scenedesmus cellulases showed greater sequence similarity with those from invertebrates, fungi, bacteria, and other microalgae than with the plant homologs. Furthermore, the data obtained from the three dimensional structure showed that both, their global structure and the main amino acid residues involved in catalysis and substrate binding are well conserved. Based on our results, we propose that different species of Scenedesmus could act as biocatalysts for the hydrolysis of cellulosic biomass produced from sunlight.

Sas20 is a highly flexible starch-binding protein in the Ruminococcus bromii cell-surface amylosome

Ruminococcus bromii is a keystone species in the human gut that has the rare ability to degrade dietary resistant starch (RS). This bacterium secretes a suite of starch-active proteins that work together within larger complexes called amylosomes that allow R. bromii to bind and degrade RS. Starch adherence system protein 20 (Sas20) is one of the more abundant proteins assembled within amylosomes, but little could be predicted about its molecular features based on amino acid sequence. Here, we performed a structure-function analysis of Sas20 and determined that it features two discrete starch-binding domains separated by a flexible linker. We show that Sas20 domain 1 contains an N-terminal β-sandwich followed by a cluster of α-helices, and the nonreducing end of maltooligosaccharides can be captured between these structural features. Furthermore, the crystal structure of a close homolog of Sas20 domain 2 revealed a unique bilobed starch-binding groove that targets the helical α1,4-linked glycan chains found in amorphous regions of amylopectin and crystalline regions of amylose. Affinity PAGE and isothermal titration calorimetry demonstrated that both domains bind maltoheptaose and soluble starch with relatively high affinity (Kd ≤ 20 μM) but exhibit limited or no binding to cyclodextrins. Finally, small-angle X-ray scattering analysis of the individual and combined domains support that these structures are highly flexible, which may allow the protein to adopt conformations that enhance its starch-targeting efficiency. Taken together, we conclude that Sas20 binds distinct features within the starch granule, facilitating the ability of R. bromii to hydrolyze dietary RS.

A novel accessory protein ArCel5 from cellulose-gelatinizing fungus Arthrobotrys sp. CX1

Improved understanding of cellulose swelling mechanism is beneficial for increasing the hydrolysis efficiency of cellulosic substrates. Here, we report a family 5 glycoside hydrolase ArCel5 isolated from the cellulose-gelatinizing fungus Arthrobotrys sp. CX1. ArCel5 exhibited low specific hydrolysis activity and high cellulose swelling capability, which suggested that this protein might function as an accessory protein. Homology modeling glycosylation detection revealed that ArCel5 is a multi-domain protein including a family 1 carbohydrate-binding module, a glycosylation linker, and a catalytic domain. The adsorption capacity, structural changes and hydrature index of filter paper treated by different ArCel5 mutants demonstrated that CBM1 and linker played an essential role in recognizing, binding and decrystallizing cellulosic substrates, which further encouraged the synergistic action between ArCel5 and cellulases. Notably, glycosylation modification further strengthened the function of the linker region. Overall, our study provides insight into the cellulose decrystallization mechanism by a novel accessory protein ArCel5 that will benefit future applications.

Linkers: A synergistic way for the synthesis of chimeric proteins

In the cell, the protein domains are attached with the short oligopeptide, commonly known as linker peptide. Besides bridging, the linker assists in the domain-domain interaction and protein folding into the peculiar conformations. Linkers allow or control the movement of protein domains in the dynamic cellular environment. The recent advances in the recombinant DNA technology enable the construction of multiple gene constructs in an open reading frame. The express sequences can work in a cascade to cater for myriad functions. This trend has given momentum to incorporating bridge sequences (linker) that essentially separates the independent domains. According to the cellular need, the bridging partner can be spaced at a secure gap or requires attaching or interacting physically. The flexible or rigid linker can help to achieve such conformations in chimeric fusion proteins. The linker can improve solubility, proteolytic resistance and stability of such fusion proteins. Recently, linker aided protein switches and antibody-drug conjugates are gaining the attention of researchers worldwide. Here, we thoroughly reviewed the types of the linker, strategies for linker engineering and the composition of a linker.

Light-stimulated T. thermophilus two-domain LPMO9H: Low-resolution SAXS model and synergy with cellulases

Lytic polysaccharide monooxygenases (LPMOs), monocopper enzymes that oxidatively cleave recalcitrant polysaccharides, have important biotechnological applications. Thermothelomyces thermophilus is a rich source of biomass-active enzymes, including many members from auxiliary activities family 9 LPMOs. Here, we report biochemical and structural characterization of recombinant TtLPMO9H which oxidizes cellulose at the C1 and C4 positions and shows enhanced activity in light-driven catalysis assays. TtLPMO9H also shows activity against xyloglucan. The addition of TtLPMO9H to endoglucanases from four different glucoside hydrolase families (GH5, GH12, GH45 and GH7) revealed that the product formation was remarkably increased when TtLPMO9H was combined with GH7 endoglucanase. Finally, we determind the first low resolution small-angle X-ray scattering model of the two-domain TtLPMO9H in solution that shows relative positions of its two functional domains and a conformation of the linker peptide, which can be relevant for the catalytic oxidation of cellulose and xyloglucan.

Structure and dynamics analysis of multi-domain putative β-1,4-glucosidase of family 3 glycoside hydrolase (PsGH3) from Pseudopedobacter saltans

Structure and conformational behaviour of a putative β-1,4-glucosidase of glycoside hydrolase family 3 (PsGH3) from Pseudopedobacter saltans was predicted by using in-silico tools. PsGH3 modeled structure constructed using Phyre2 displayed multidomain architecture comprising an N-terminal (β/α)₈-fold domain followed by (α/β)₆-sandwich domain, PA14 domain, and a C-terminal domain resembling an immunoglobulin fold. Ramachandran plot displayed 99.3% of amino acids in the allowed region and 0.7% residues in the disallowed region. Multiple sequence alignment (MSA) and structure superposition of PsGH3 with other homologues from GH3 family revealed the conserved residues, Asp274 and Glu624 present in loops LA and LB, respectively originating from N-terminal domain act as catalytic residues. The volume and area calculated for PsGH3 displayed a deep active-site conformation comparable with its homologues, β-1,4-glucosidases (GH3) of Kluyveromyces marxianus and Streptomyces venezuelae. Molecular dynamic (MD) simulation of PsGH3 structure for 80 ns suggested stable and compact structure. Molecular docking studies revealed deeper active site conformation of PsGH3 that could house larger cellooligosaccharides up to 7° of polymerization (DP7). The amino acid residues, Ala86, Leu88, Cys275, Pro483, Phe493, Asn417, Asn491, Pro492, and Leu495 created a binding pocket near the catalytic cleft, crucial for ligand binding. MD simulation of PsGH3 in the presence of cellooligosaccharides, viz., cellobiose and celloheptaose showed stability in terms of RMSD, R_g, and SASA values till 80 ns. The calculation of average number of hydrogen bond (H-bond), interaction energy, and binding free energy confirmed the stronger binding affinity of the larger cellooligosaccharides such as celloheptaose in the binding cavity of PsGH3.

Pseudo-RNA-Binding Domains Mediate RNA Structure Specificity in Upstream of N-Ras

RNA-binding proteins (RBPs) commonly feature multiple RNA-binding domains (RBDs), which provide these proteins with a modular architecture. Accumulating evidence supports that RBP architectural modularity and adaptability define the specificity of their interactions with RNA. However, how multiple RBDs recognize their cognate single-stranded RNA (ssRNA) sequences in concert remains poorly understood. Here, we use Upstream of N-Ras (Unr) as a model system to address this question. Although reported to contain five ssRNA-binding cold-shock domains (CSDs), we demonstrate that Unr includes an additional four CSDs that do not bind RNA (pseudo-RBDs) but are involved in mediating RNA tertiary structure specificity by reducing the conformational heterogeneity of Unr. Disrupting the interactions between canonical and non-canonical CSDs impacts RNA binding, Unr-mediated translation regulation, and the Unr-dependent RNA interactome. Taken together, our studies reveal a new paradigm in protein-RNA recognition, where interactions between RBDs and pseudo-RBDs select RNA tertiary structures, influence RNP assembly, and define target specificity.

The structural basis of fungal glucuronoyl esterase activity on natural substrates

Structural and functional studies were conducted of the glucuronoyl esterase (GE) from Cerrena unicolor (CuGE), an enzyme catalyzing cleavage of lignin-carbohydrate ester bonds. CuGE is an α/β-hydrolase belonging to carbohydrate esterase family 15 (CE15). The enzyme is modular, comprised of a catalytic and a carbohydrate-binding domain. SAXS data show CuGE as an elongated rigid molecule where the two domains are connected by a rigid linker. Detailed structural information of the catalytic domain in its apo- and inactivated form and complexes with aldouronic acids reveal well-defined binding of the 4-O-methyl-a-D-glucuronoyl moiety, not influenced by the nature of the attached xylo-oligosaccharide. Structural and sequence comparisons within CE15 enzymes reveal two distinct structural subgroups. CuGE belongs to the group of fungal CE15-B enzymes with an open and flat substrate-binding site. The interactions between CuGE and its natural substrates are explained and rationalized by the structural results, microscale thermophoresis and isothermal calorimetry.

Glycosylation effects on the structure and dynamics of a full-length Cel7A cellulase

Fungi cellulases are used to degrade cellulose-containing biomass for bioethanol production. Industrial cellulases such as Cel7A from Trichoderma reesei (TrCel7A) are critical in this process. Thus, the understanding of structure and dynamics is crucial for engineering variants with improved cellulolytic activity. This cellulase consists of two domains connected by a flexible and highly glycosylated linker. However, the linker flexibility has hindered the determination of Cel7A complete structure. Herein, based on atomic and sparse data, we applied integrative modelling to build a model of the complete enzyme structure. Next, through simulations, we studied the glycosylation effects on the structure and dynamics of a solubilized TrCel7A. Essential dynamics analysis showed that O-glycosylation in the linker led to the stabilization of protein overall dynamics. O-linked glycans seem to restrict protein dihedral angles distribution in this region, selecting more elongated conformations. Besides the reduced flexibility, functional interdomain motions occurred in a more concerted way in the glycosylated system. In contrast, in the absence of glycosylation, we observed vast conformational plasticity with the functional domains frequently collapsing. We report here evidence that targeting Cel7A linker flexibility by point mutations including modification of glycosylation sites could be a promising design strategy to improve cellulase activity.

ABCF ATPases Involved in Protein Synthesis, Ribosome Assembly and Antibiotic Resistance: Structural and Functional Diversification across the Tree of Life

Within the larger ABC superfamily of ATPases, ABCF family members eEF3 in Saccharomyces cerevisiae and EttA in Escherichia coli have been found to function as ribosomal translation factors. Several other ABCFs including biochemically characterized VgaA, LsaA and MsrE confer resistance to antibiotics that target the peptidyl transferase center and exit tunnel of the ribosome. However, the diversity of ABCF subfamilies, the relationships among subfamilies and the evolution of antibiotic resistance (ARE) factors from other ABCFs have not been explored. To address this, we analyzed the presence of ABCFs and their domain architectures in 4505 genomes across the tree of life. We find 45 distinct subfamilies of ABCFs that are widespread across bacterial and eukaryotic phyla, suggesting that they were present in the last common ancestor of both. Surprisingly, currently known ARE ABCFs are not confined to a distinct lineage of the ABCF family tree, suggesting that ARE can readily evolve from other ABCF functions. Our data suggest that there are a number of previously unidentified ARE ABCFs in antibiotic producers and important human pathogens. We also find that ATPase-deficient mutants of all four E. coli ABCFs (EttA, YbiT, YheS and Uup) inhibit protein synthesis, indicative of their ribosomal function, and demonstrate a genetic interaction of ABCFs Uup and YheS with translational GTPase BipA involved in assembly of the 50S ribosome subunit. Finally, we show that the ribosome-binding resistance factor VmlR from Bacillus subtilis is localized to the cytoplasm, ruling out a role in antibiotic efflux.

Altering the linker in processive GH5 endoglucanase 1 modulates lignin binding and catalytic properties

BACKGROUND

The non-productive adsorption of cellulases onto lignin in biomass is a key issue for the biofuel process economy. It would be helpful to reduce the inhibitory effect of lignin on enzymatic hydrolysis by engineering weak lignin-binding cellulases. Cellulase linkers are highly divergent in their lengths, compositions, and glycosylations. Numerous studies have revealed that linkers can facilitate optimal interactions between structured domains. Recently, efforts have focused on the contributions and mechanisms of carbohydrate-binding modules and catalytic domains that affect lignin affinity and processivity of cellulases, but our understanding of the effects of the linker regions on lignin adsorption and processivity of GH5 processive endoglucanases is still limited.

RESULTS

Eight GH5 endoglucanase 1 variants of varying length, flexibility, and sequence in the linker region were constructed. Their characteristics were then compared to the wild-type enzyme (EG1). Remarkably, significant differences in the lignin adsorption profiles and processivities were observed for EG1 and other variants. Our studies suggest that either the length or the specific amino acid composition of the linker has a prominent influence on the lignin-binding affinity of the enzymes. Comparatively, the processivity may depend primarily on the length of the linker and less so on the specific amino acid composition. EG1-ApCel5A, a variant with better performance in enzymatic hydrolysis in the presence of lignin, was obtained by replacing a longer, flexible linker. In total, up to between 28.2 and 30.1% more reducing sugars were generated from filter paper by EG1-ApCel5A in the presence of lignin compared to EG1.

CONCLUSIONS

Our results highlight the relevance of the linker region in the lignin adsorption and processivity of a processive endoglucanase. Our findings suggest that the linker region may be used as a target for the design of more active and weaker lignin-binding cellulases.

Predominant Nonproductive Substrate Binding by Fungal Cellobiohydrolase I and Implications for Activity Improvement

Enzymatic conversion of the most abundant renewable source of organic compounds, cellulose to fermentable sugars is attractive for production of green fuels and chemicals. The major component of industrial enzyme systems, cellobiohydrolase I from Hypocrea jecorina (Trichoderma reesei) (HjCel7A) processively splits disaccharide units from the reducing ends of tightly packed cellulose chains. HjCel7A consists of a catalytic domain (CD) and a carbohydrate-binding module (CBM) separated by a linker peptide. A tunnel-shaped substrate-binding site in the CD includes nine subsites for β-d-glucose units, seven of which (-7 to -1) precede the catalytic center. Low catalytic activity of Cel7A is the bottleneck and the primary target for improvement. Here it is shown for the first time that, in spite of much lower apparent k_cat of HjCel7A at the hydrolysis of β-1,4-glucosidic linkages in the fluorogenic cellotetra- and -pentaose compared to the structurally related endoglucanase I (HjCel7B), the specificity constants (catalytic efficiency) k_cat /K_m for both enzymes are almost equal in these reactions. The observed activity difference appears from strong nonproductive substrate binding by HjCel7A, particularly significant for MU-β-cellotetraose (MUG₄ ). Interaction of substrates with the subsites -6 and -5 proximal to the nonconserved Gln101 residue in HjCel7A decreases K_m,ap by >1500 times. HjCel7A can be nonproductively bound onto cellulose surface with K_d ≈2-9 nM via CBM and CD that captures six terminal glucose units of cellulose chain. Decomposition of this nonproductive complex can determine the rate of cellulose conversion. MUG₄ is a promising substrate to select active cellobiohydrolase I variants with reduced nonproductive substrate binding.

Novel Insights from Comparative In Silico Analysis of Green Microalgal Cellulases

The assumption that cellulose degradation and assimilation can only be carried out by heterotrophic organisms was shattered in 2012 when it was discovered that the unicellular green alga, Chlamydomonas reinhardtii (Cr), can utilize cellulose for growth under CO₂-limiting conditions. Publications of genomes/transcriptomes of the colonial microalgae, Gonium pectorale (Gp) and Volvox carteri (Vc), between 2010⁻2016 prompted us to look for cellulase genes in these algae and to compare them to cellulases from bacteria, fungi, lower/higher plants, and invertebrate metazoans. Interestingly, algal catalytic domains (CDs), belonging to the family GH9, clustered separately and showed the highest (33⁻42%) and lowest (17⁻36%) sequence identity with respect to cellulases from invertebrate metazoans and bacteria, respectively, whereas the identity with cellulases from plants was only 27⁻33%. Based on comparative multiple alignments and homology models, the domain arrangement and active-site architecture of algal cellulases are described in detail. It was found that all algal cellulases are modular, consisting of putative novel cysteine-rich carbohydrate-binding modules (CBMs) and proline/serine-(PS) rich linkers. Two genes were found to encode a protein with a putative Ig-like domain and a cellulase with an unknown domain, respectively. A feature observed in one cellulase homolog from Gp and shared by a spinach cellulase is the existence of two CDs separated by linkers and with a C-terminal CBM. Dockerin and Fn-3-like domains, typically found in bacterial cellulases, are absent in algal enzymes. The targeted gene expression analysis shows that two Gp cellulases consisting, respectively, of a single and two CDs were upregulated upon filter paper addition to the medium.

Cryptic Disorder Out of Disorder: Encounter between Conditionally Disordered CP12 and Glyceraldehyde-3-Phosphate Dehydrogenase

Among intrinsically disordered proteins, conditionally disordered proteins undergo dramatic structural disorder rearrangements upon environmental changes and/or post-translational modifications that directly modulate their function. Quantifying the dynamics of these fluctuating proteins is extremely challenging but paramount to understanding the regulation of their function. The chloroplast protein CP12 is a model of such proteins and acts as a redox switch by formation/disruption of its two disulfide bridges. It regulates the Calvin cycle by forming, in oxidized conditions, a supramolecular complex with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and then phosphoribulokinase. In this complex, both enzymes are inactive. The highly dynamic nature of CP12 has so far hindered structural characterization explaining its mode of action. Thanks to a synergistic combination of small-angle X-ray scattering, nuclear magnetic resonance and circular dichroism that drove the molecular modeling of structural ensembles, we deciphered the structural behavior of Chlamydomonas reinhardtii oxidized CP12 alone and in the presence of GAPDH. Contrary to sequence-based structural predictions, the N-terminal region is unstable, oscillates at the ms timescale between helical and random conformations, and is connected through a disordered linker to its C-terminus, which forms a stable helical turn. Upon binding to GAPDH, oxidized CP12 undergoes an induced unfolding of its N-terminus. This phenomenon called cryptic disorder contributes to decrease the entropy cost and explains CP12 unusual high affinity for its partners.

Genome sequencing and comparative genomics reveal the potential pathogenic mechanism of Cercospora sojina Hara on soybean

Frogeye leaf spot, caused by Cercospora sojina Hara, is a common disease of soybean in most soybean-growing countries of the world. In this study, we report a high-quality genome sequence of C. sojina by Single Molecule Real-Time sequencing method. The 40.8-Mb genome encodes 11,655 predicated genes, and 8,474 genes are revealed by RNA sequencing. Cercospora sojina genome contains large numbers of gene clusters that are involved in synthesis of secondary metabolites, including mycotoxins and pigments. However, much less carbohydrate-binding module protein encoding genes are identified in C. sojina genome, when compared with other phytopathogenic fungi. Bioinformatics analysis reveals that C. sojina harbours about 752 secreted proteins, and 233 of them are effectors. During early infection, the genes for metabolite biosynthesis and effectors are significantly enriched, suggesting that they may play essential roles in pathogenicity. We further identify 13 effectors that can inhibit BAX-induced cell death. Taken together, our results provide insights into the infection mechanisms of C. sojina on soybean.

The influence of different linker modifications on the catalytic activity and cellulose affinity of cellobiohydrolase Cel7A from Hypocrea jecorina

Various cellulases consist of a catalytic domain connected to a carbohydrate-binding module (CBM) by a flexible linker peptide. The linker if often strongly O-glycosylated and typically has a length of 20-50 amino acid residues. Functional roles, other than connecting the two folded domains, of the linker and its glycans, have been widely discussed, but experimental evidence remains sparse. One of the most studied cellulose degrading enzymes is the multi-domain cellobiohydrolase Cel7A from Hypocrea jecorina. Here, we designed variants of Cel7A with mutations in the linker region to elucidate the role of the linker. We found that moderate modification of the linker could result in significant changes in substrate affinity and catalytic efficacy. These changes were quite different for different linker variants. Thus, deletion of six residues near the catalytic domain had essentially no effects on enzyme function. Conversely, a substitution of four glycosylation sites near the middle of the linker reduced substrate affinity and increased maximal turnover. The observation of weaker binding provides some support of recent suggestions that linker glycans may be directly involved in substrate interactions. However, a variant with several inserted glycosylation sites near the CBM also showed lower affinity for the substrate compared to the wild-type, and we suggest that substrate interactions of the glycans depend on their exact location as well as other factors such as changes in structure and dynamics of the linker peptide.

Two degradation strategies for overcoming the recalcitrance of natural lignocellulosic xylan by polysaccharides-binding GH10 and GH11 xylanases of filamentous fungi

The recalcitrance of lignocellulose forms a strong barrier for the bioconversion of lignocellulosic biomass in chemical or biofuel industries. Filamentous fungi are major plant biomass decomposer, and capable of forming all the required enzymes. Here, they characterized the GH10 and GH11 endo-xylanases and a CE1 acetyl-xylan esterase (Axe1) from a superior biomass-degrading strain, Aspergillus fumigatus Z5, and examined how they interact in xylan degradation. Cellulose-binding (CBM1) domain inhibited GH10 xylanase activities for pure xylan, but afforded them an ability to hydrolyze washed corncob particles (WCCP). CBM1-containing GH10 xylanases also showed synergism with CBM1-containing Axe1 in WCCP hydrolysis, and this synergy was strictly dependent on the presence of their CBM1 domains. In contrast, GH11 xylanases had no CBM1, but still could bind xylan and hydrolyzed WCCP; however, no synergism displayed with Axe1. GH10 xylanases and GH11 xylanases showed a pronounced synergism in WCCP hydrolysis, which was dependent on the presence of the CBM1 in GH10 xylanases and absence from GH11 xylanases. They exhibit different mechanisms to bind to cellulose and xylan, and act in synergy when these two structures are intact. These findings will be helpful for the further development of highly efficient enzyme mixtures for lignocellulosic biomass conversion.

Effects of the linker region on the structure and function of modular GH5 cellulases

The association of glycosyl hydrolases with catalytically inactive modules is a successful evolutionary strategy that is commonly used by biomass-degrading microorganisms to digest plant cell walls. The presence of accessory domains in these enzymes is associated with properties such as higher catalytic efficiency, extension of the catalytic interface and targeting of the enzyme to the proper substrate. However, the importance of the linker region in the synergistic action of the catalytic and accessory domains remains poorly understood. Thus, this study examined how the inter-domain region affects the structure and function of modular GH5 endoglucanases, by using cellulase 5A from Bacillus subtilis (BsCel5A) as a model. BsCel5A variants featuring linkers with different stiffnesses or sizes were designed and extensively characterized, revealing that changes in flexibility or rigidity in this region differentially affect kinetic behavior. Regarding the linker length, we found that precise inter-domain spacing is required to enable efficient hydrolysis because excessively long or short linkers were equally detrimental to catalysis. Together, these findings identify molecular and structural features that may contribute to the rational design of chimeric and multimodular glycosyl hydrolases.

Cellular automata modeling depicts degradation of cellulosic material by a cellulase system with single-molecule resolution

BACKGROUND

Enzymatic hydrolysis of cellulose involves the spatiotemporally correlated action of distinct polysaccharide chain cleaving activities confined to the surface of an insoluble substrate. Because cellulases differ in preference for attacking crystalline compared to amorphous cellulose, the spatial distribution of structural order across the cellulose surface imposes additional constraints on the dynamic interplay between the enzymes. Reconstruction of total system behavior from single-molecule activity parameters is a longstanding key goal in the field.

RESULTS

We have developed a stochastic, cellular automata-based modeling approach to describe degradation of cellulosic material by a cellulase system at single-molecule resolution. Substrate morphology was modeled to represent the amorphous and crystalline phases as well as the different spatial orientations of the polysaccharide chains. The enzyme system model consisted of an internally chain-cleaving endoglucanase (EG) as well as two processively acting, reducing and non-reducing chain end-cleaving cellobiohydrolases (CBHs). Substrate preference (amorphous: EG, CBH II; crystalline: CBH I) and characteristic frequencies for chain cleavage, processive movement, and dissociation were assigned from biochemical data. Once adsorbed, enzymes were allowed to reach surface-exposed substrate sites through "random-walk" lateral diffusion or processive motion. Simulations revealed that slow dissociation of processive enzymes at obstacles obstructing further movement resulted in local jamming of the cellulases, with consequent delay in the degradation of the surface area affected. Exploiting validation against evidence from atomic force microscopy imaging as a unique opportunity opened up by the modeling approach, we show that spatiotemporal characteristics of cellulose surface degradation by the system of synergizing cellulases were reproduced quantitatively at the nanometer resolution of the experimental data. This in turn gave useful prediction of the soluble sugar release rate.

CONCLUSIONS

Salient dynamic features of cellulose surface degradation by different cellulases acting in synergy were reproduced in simulations in good agreement with evidence from high-resolution visualization experiments. Due to the single-molecule resolution of the modeling approach, the utility of the presented model lies not only in predicting system behavior but also in elucidating inherently complex (e.g., stochastic) phenomena involved in enzymatic cellulose degradation. Thus, it creates synergy with experiment to advance the mechanistic understanding for improved application.

Hydrodynamic and Membrane Binding Properties of Purified Rous Sarcoma Virus Gag Protein

Previously, no retroviral Gag protein has been highly purified in milligram quantities and in a biologically relevant and active form. We have purified Rous sarcoma virus (RSV) Gag protein and in parallel several truncation mutants of Gag and have studied their biophysical properties and membrane interactions in vitro. RSV Gag is unusual in that it is not naturally myristoylated. From its ability to assemble into virus-like particles in vitro, we infer that RSV Gag is biologically active. By size exclusion chromatography and small-angle X-ray scattering, Gag in solution appears extended and flexible, in contrast to previous reports on unmyristoylated HIV-1 Gag, which is compact. However, by neutron reflectometry measurements of RSV Gag bound to a supported bilayer, the protein appears to adopt a more compact, folded-over conformation. At physiological ionic strength, purified Gag binds strongly to liposomes containing acidic lipids. This interaction is stimulated by physiological levels of phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2] and by cholesterol. However, unlike HIV-1 Gag, RSV Gag shows no sensitivity to acyl chain saturation. In contrast with full-length RSV Gag, the purified MA domain of Gag binds to liposomes only weakly. Similarly, both an N-terminally truncated version of Gag that is missing the MA domain and a C-terminally truncated version that is missing the NC domain bind only weakly. These results imply that NC contributes to membrane interaction in vitro, either by directly contacting acidic lipids or by promoting Gag multimerization.

IMPORTANCE

Retroviruses like HIV assemble at and bud from the plasma membrane of cells. Assembly requires the interaction between thousands of Gag molecules to form a lattice. Previous work indicated that lattice formation at the plasma membrane is influenced by the conformation of monomeric HIV. We have extended this work to the more tractable RSV Gag. Our results show that RSV Gag is highly flexible and can adopt a folded-over conformation on a lipid bilayer, implicating both the N and C termini in membrane binding. In addition, binding of Gag to membranes is diminished when either terminal domain is truncated. RSV Gag membrane association is significantly less sensitive than HIV Gag membrane association to lipid acyl chain saturation. These findings shed light on Gag assembly and membrane binding, critical steps in the viral life cycle and an untapped target for antiretroviral drugs.

Linker length and flexibility induces new cellobiohydrolase activity of PoCel6A from Penicillium oxalicum

In a previous study, a novel cellobiohydrolase, PoCel6A, with new enzymatic activity against p-nitrophenyl-β-D-cellobioside (pNPC), was purified from Penicillium oxalicum. The cellulose-binding module and catalytic domain of PoCel6A showed a high degree of sequence similarity with other fungal Cel6As. However, PoCel6A had 11 more amino acids in the linker region than other Cel6As. To evaluate the relationship between the longer linker of PoCel6A and its enzymatic activity, 11 amino acids were deleted from the linker region of PoCel6A. The shortened PoCel6A linker nullified the enzymatic activity against pNPC but dramatically increased the enzyme's capacity for crystalline cellulose degradation. The shortened linker segment appeared to have no effect on the secondary structural conformation of PoCel6A. Another variant (PoCel6A-6pro) with six consecutive proline residues in the interdomain linker had a higher rigid linker, and no enzymatic activity was observed against soluble and insoluble substrate. The flexibility of the linker had an important function in the formation of active cellulase. The length and flexibility of the linker is clearly able to modify the function of PoCel6A and induce new characteristics of Cel6A.

Repurposing a bacterial quality control mechanism to enhance enzyme production in living cells

Heterologous expression of many proteins in bacteria, yeasts, and plants is often limited by low titers of functional protein. To address this problem, we have created a two-tiered directed evolution strategy in Escherichia coli that enables optimization of protein production while maintaining high biological activity. The first tier involves a genetic selection for intracellular protein stability that is based on the folding quality control mechanism inherent to the twin-arginine translocation pathway, while the second is a semi-high-throughput screen for protein function. To demonstrate the utility of this strategy, we isolated variants of the endoglucanase Cel5A, from the plant-pathogenic fungus Fusarium graminearum, whose production was increased by as much as 30-fold over the parental enzyme. This gain in production was attributed to just two amino acid substitutions, and it was isolated after two iterations through the two-tiered approach. There was no significant tradeoff in activity on soluble or insoluble cellulose substrates. Importantly, by combining the folding filter afforded by the twin-arginine translocation quality control mechanism with a function-based screen, we show enrichment for variants with increased protein abundance in a manner that does not compromise catalytic activity, providing a highly soluble parent for engineering of improved or new function.

Glycosylation of Cellulases: Engineering Better Enzymes for Biofuels

Cellulose in plant cell walls is the largest reservoir of renewable carbon on Earth. The saccharification of cellulose from plant biomass into soluble sugars can be achieved using fungal and bacterial cellulolytic enzymes, cellulases, and further converted into fuels and chemicals. Most fungal cellulases are both N- and O-glycosylated in their native form, yet the consequences of glycosylation on activity and structure are not fully understood. Studying protein glycosylation is challenging as glycans are extremely heterogeneous, stereochemically complex, and glycosylation is not under direct genetic control. Despite these limitations, many studies have begun to unveil the role of cellulase glycosylation, especially in the industrially relevant cellobiohydrolase from Trichoderma reesei, Cel7A. Glycosylation confers many beneficial properties to cellulases including enhanced activity, thermal and proteolytic stability, and structural stabilization. However, glycosylation must be controlled carefully as such positive effects can be dampened or reversed. Encouragingly, methods for the manipulation of glycan structures have been recently reported that employ genetic tuning of glycan-active enzymes expressed from homogeneous and heterologous fungal hosts. Taken together, these studies have enabled new strategies for the exploitation of protein glycosylation for the production of enhanced cellulases for biofuel production.

Challenges and advances in the heterologous expression of cellulolytic enzymes: a review

Second generation biofuel development is increasingly reliant on the recombinant expression of cellulases. Designing or identifying successful expression systems is thus of preeminent importance to industrial progress in the field. Recombinant production of cellulases has been performed using a wide range of expression systems in bacteria, yeasts and plants. In a number of these systems, particularly when using bacteria and plants, significant challenges have been experienced in expressing full-length proteins or proteins at high yield. Further difficulties have been encountered in designing recombinant systems for surface-display of cellulases and for use in consolidated bioprocessing in bacteria and yeast. For establishing cellulase expression in plants, various strategies are utilized to overcome problems, such as the auto-hydrolysis of developing plant cell walls. In this review, we investigate the major challenges, as well as the major advances made to date in the recombinant expression of cellulases across the commonly used bacterial, plant and yeast systems. We review some of the critical aspects to be considered for industrial-scale cellulase production.

Structure, dynamics, and specificity of endoglucanase D from Clostridium cellulovorans

The enzymatic degradation of cellulose is a critical step in the biological conversion of plant biomass into an abundant renewable energy source. An understanding of the structural and dynamic features that cellulases utilize to bind a single strand of crystalline cellulose and hydrolyze the β-1,4-glycosidic bonds of cellulose to produce fermentable sugars would greatly facilitate the engineering of improved cellulases for the large-scale conversion of plant biomass. Endoglucanase D (EngD) from Clostridium cellulovorans is a modular enzyme comprising an N-terminal catalytic domain and a C-terminal carbohydrate-binding module, which is attached via a flexible linker. Here, we present the 2.1-Å-resolution crystal structures of full-length EngD with and without cellotriose bound, solution small-angle X-ray scattering (SAXS) studies of the full-length enzyme, the characterization of the active cleft glucose binding subsites, and substrate specificity of EngD on soluble and insoluble polymeric carbohydrates. SAXS data support a model in which the linker is flexible, allowing EngD to adopt an extended conformation in solution. The cellotriose-bound EngD structure revealed an extended active-site cleft that contains seven glucose-binding subsites, but unlike the majority of structurally determined endocellulases, the active-site cleft of EngD is partially enclosed by Trp162 and Tyr232. EngD variants, which lack Trp162, showed a significant reduction in activity and an alteration in the distribution of cellohexaose degradation products, suggesting that Trp162 plays a direct role in substrate binding.

Aspergillus niger β-glucosidase has a cellulase-like tadpole molecular shape: insights into glycoside hydrolase family 3 (GH3) β-glucosidase structure and function

Aspergillus niger is known to secrete large amounts of β-glucosidases, which have a variety of biotechnological and industrial applications. Here, we purified an A. niger β-glucosidase (AnBgl1) and conducted its biochemical and biophysical analyses. Purified enzyme with an apparent molecular mass of 116 kDa forms monomers in solution as judged by native gel electrophoresis and has a pI value of 4.55, as found for most of the fungi of β-glucosidases. Surprisingly, the small angle x-ray experiments reveal that AnBgl1 has a tadpole-like structure, with the N-terminal catalytic domain and C-terminal fibronectin III-like domain (FnIII) connected by the long linker peptide (∼100 amino acid residues) in an extended conformation. This molecular organization resembles the one adopted by other cellulases (such as cellobiohydrolases, for example) that frequently contain a catalytic domain linked to the cellulose-binding module that mediates their binding to insoluble and polymeric cellulose. The reasons why AnBgl1, which acts on the small soluble substrates, has a tadpole molecular shape are not entirely clear. However, our enzyme pulldown assays with different polymeric substrates suggest that AnBgl1 has little or no capacity to bind to and to adsorb cellulose, xylan, and starch, but it has high affinity to lignin. Molecular dynamics simulations suggested that clusters of residues located in the C-terminal FnIII domain interact strongly with lignin fragments. The simulations showed that numerous arginine residues scattered throughout the FnIII surface play an important role in the interaction with lignin by means of cation-π stacking with the lignin aromatic rings. These results indicate that the C-terminal FnIII domain could be operational for immobilization of the enzyme on the cell wall and for the prevention of unproductive binding of cellulase to the biomass lignin.

Glycosylated linkers in multimodular lignocellulose-degrading enzymes dynamically bind to cellulose

Plant cell-wall polysaccharides represent a vast source of food in nature. To depolymerize polysaccharides to soluble sugars, many organisms use multifunctional enzyme mixtures consisting of glycoside hydrolases, lytic polysaccharide mono-oxygenases, polysaccharide lyases, and carbohydrate esterases, as well as accessory, redox-active enzymes for lignin depolymerization. Many of these enzymes that degrade lignocellulose are multimodular with carbohydrate-binding modules (CBMs) and catalytic domains connected by flexible, glycosylated linkers. These linkers have long been thought to simply serve as a tether between structured domains or to act in an inchworm-like fashion during catalytic action. To examine linker function, we performed molecular dynamics (MD) simulations of the Trichoderma reesei Family 6 and Family 7 cellobiohydrolases (TrCel6A and TrCel7A, respectively) bound to cellulose. During these simulations, the glycosylated linkers bind directly to cellulose, suggesting a previously unknown role in enzyme action. The prediction from the MD simulations was examined experimentally by measuring the binding affinity of the Cel7A CBM and the natively glycosylated Cel7A CBM-linker. On crystalline cellulose, the glycosylated linker enhances the binding affinity over the CBM alone by an order of magnitude. The MD simulations before and after binding of the linker also suggest that the bound linker may affect enzyme action due to significant damping in the enzyme fluctuations. Together, these results suggest that glycosylated linkers in carbohydrate-active enzymes, which are intrinsically disordered proteins in solution, aid in dynamic binding during the enzymatic deconstruction of plant cell walls.

Cellulase linkers are optimized based on domain type and function: insights from sequence analysis, biophysical measurements, and molecular simulation

Cellulase enzymes deconstruct cellulose to glucose, and are often comprised of glycosylated linkers connecting glycoside hydrolases (GHs) to carbohydrate-binding modules (CBMs). Although linker modifications can alter cellulase activity, the functional role of linkers beyond domain connectivity remains unknown. Here we investigate cellulase linkers connecting GH Family 6 or 7 catalytic domains to Family 1 or 2 CBMs, from both bacterial and eukaryotic cellulases to identify conserved characteristics potentially related to function. Sequence analysis suggests that the linker lengths between structured domains are optimized based on the GH domain and CBM type, such that linker length may be important for activity. Longer linkers are observed in eukaryotic GH Family 6 cellulases compared to GH Family 7 cellulases. Bacterial GH Family 6 cellulases are found with structured domains in either N to C terminal order, and similar linker lengths suggest there is no effect of domain order on length. O-glycosylation is uniformly distributed across linkers, suggesting that glycans are required along entire linker lengths for proteolysis protection and, as suggested by simulation, for extension. Sequence comparisons show that proline content for bacterial linkers is more than double that observed in eukaryotic linkers, but with fewer putative O-glycan sites, suggesting alternative methods for extension. Conversely, near linker termini where linkers connect to structured domains, O-glycosylation sites are observed less frequently, whereas glycines are more prevalent, suggesting the need for flexibility to achieve proper domain orientations. Putative N-glycosylation sites are quite rare in cellulase linkers, while an N-P motif, which strongly disfavors the attachment of N-glycans, is commonly observed. These results suggest that linkers exhibit features that are likely tailored for optimal function, despite possessing low sequence identity. This study suggests that cellulase linkers may exhibit function in enzyme action, and highlights the need for additional studies to elucidate cellulase linker functions.

How random are intrinsically disordered proteins? A small angle scattering perspective

While the crucial role of intrinsically disordered proteins (IDPs) in the cell cycle is now recognized, deciphering their molecular mode of action at the structural level still remains highly challenging and requires a combination of many biophysical approaches. Among them, small angle X-ray scattering (SAXS) has been extremely successful in the last decade and has become an indispensable technique for addressing many of the fundamental questions regarding the activities of IDPs. After introducing some experimental issues specific to IDPs and in relation to the latest technical developments, this article presents the interest of the theory of polymer physics to evaluate the flexibility of fully disordered proteins. The different strategies to obtain 3-dimensional models of IDPs, free in solution and associated in a complex, are then reviewed. Indeed, recent computational advances have made it possible to readily extract maximum information from the scattering curve with a special emphasis on highly flexible systems, such as multidomain proteins and IDPs. Furthermore, integrated computational approaches now enable the generation of ensembles of conformers to translate the unique flexible characteristics of IDPs by taking into consideration the constraints of more and more various complementary experiment. In particular, a combination of SAXS with high-resolution techniques, such as x-ray crystallography and NMR, allows us to provide reliable models and to gain unique structural insights about the protein over multiple structural scales. The latest neutron scattering experiments also promise new advances in the study of the conformational changes of macromolecules involving more complex systems.

X-ray crystal structure of the streptococcal specific phage lysin PlyC

Bacteriophages deploy lysins that degrade the bacterial cell wall and facilitate virus egress from the host. When applied exogenously, these enzymes destroy susceptible microbes and, accordingly, have potential as therapeutic agents. The most potent lysin identified to date is PlyC, an enzyme assembled from two components (PlyCA and PlyCB) that is specific for streptococcal species. Here the structure of the PlyC holoenzyme reveals that a single PlyCA moiety is tethered to a ring-shaped assembly of eight PlyCB molecules. Structure-guided mutagenesis reveals that the bacterial cell wall binding is achieved through a cleft on PlyCB. Unexpectedly, our structural data reveal that PlyCA contains a glycoside hydrolase domain in addition to the previously recognized cysteine, histidine-dependent amidohydrolases/peptidases catalytic domain. The presence of eight cell wall-binding domains together with two catalytic domains may explain the extraordinary potency of the PlyC holoenyzme toward target bacteria.

Biomimetic engineering of modular bispecific antibodies for biomolecule immobilization

Modular bispecific antibodies (BsAb's) that interact directly with a gold surface were engineered for immobilization on biosensing devices. The BsAb's consist of the variable fragments of antigold and antilysozyme antibodies connected via one of three linkers derived from naturally occurring proteins. The BsAb's were bound tightly to both the gold surface and to lysozyme, thus functioning as interface molecules between lysozyme and the gold surface without a substantial loss of antigen-binding activity. The antigen-binding capacity (the ratio of the amount of immobilized lysozyme to the amount of immobilized BsAb) on the gold surface reached 82%. An analysis of the correlation between binding capacity and linker characteristics indicated that the presence of a long, rigid linker sequence derived from a cellulase resulted in a higher antigen-binding capacity than did the presence of a long but relatively flexible glycine-rich linker. This result suggests a strategy for designing linkers suitable for BsAb-based biomolecular immobilization.

Characterizing flexible and intrinsically unstructured biological macromolecules by SAS using the Porod-Debye law

Unstructured proteins, RNA or DNA components provide functionally important flexibility that is key to many macromolecular assemblies throughout cell biology. As objective, quantitative experimental measures of flexibility and disorder in solution are limited, small angle scattering (SAS), and in particular small angle X-ray scattering (SAXS), provides a critical technology to assess macromolecular flexibility as well as shape and assembly. Here, we consider the Porod-Debye law as a powerful tool for detecting biopolymer flexibility in SAS experiments. We show that the Porod-Debye region fundamentally describes the nature of the scattering intensity decay by capturing the information needed for distinguishing between folded and flexible particles. Particularly for comparative SAS experiments, application of the law, as described here, can distinguish between discrete conformational changes and localized flexibility relevant to molecular recognition and interaction networks. This approach aids insightful analyses of fully and partly flexible macromolecules that is more robust and conclusive than traditional Kratky analyses. Furthermore, we demonstrate for prototypic SAXS data that the ability to calculate particle density by the Porod-Debye criteria, as shown here, provides an objective quality assurance parameter that may prove of general use for SAXS modeling and validation.

Small angle neutron scattering reveals pH-dependent conformational changes in Trichoderma reesei cellobiohydrolase I: implications for enzymatic activity

Cellobiohydrolase I (Cel7A) of the fungus Trichoderma reesei (now classified as an anamorph of Hypocrea jecorina) hydrolyzes crystalline cellulose to soluble sugars, making it of key interest for producing fermentable sugars from biomass for biofuel production. The activity of the enzyme is pH-dependent, with its highest activity occurring at pH 4-5. To probe the response of the solution structure of Cel7A to changes in pH, we measured small angle neutron scattering of it in a series of solutions having pH values of 7.0, 6.0, 5.3, and 4.2. As the pH decreases from 7.0 to 5.3, the enzyme structure remains well defined, possessing a spatial differentiation between the cellulose binding domain and the catalytic core that only changes subtly. At pH 4.2, the solution conformation of the enzyme changes to a structure that is intermediate between a properly folded enzyme and a denatured, unfolded state, yet the secondary structure of the enzyme is essentially unaltered. The results indicate that at the pH of optimal activity, the catalytic core of the enzyme adopts a structure in which the compact packing typical of a fully folded polypeptide chain is disrupted and suggest that the increased range of structures afforded by this disordered state plays an important role in the increased activity of Cel7A through conformational selection.

Comparative genomics yields insights into niche adaptation of plant vascular wilt pathogens

The vascular wilt fungi Verticillium dahliae and V. albo-atrum infect over 200 plant species, causing billions of dollars in annual crop losses. The characteristic wilt symptoms are a result of colonization and proliferation of the pathogens in the xylem vessels, which undergo fluctuations in osmolarity. To gain insights into the mechanisms that confer the organisms' pathogenicity and enable them to proliferate in the unique ecological niche of the plant vascular system, we sequenced the genomes of V. dahliae and V. albo-atrum and compared them to each other, and to the genome of Fusarium oxysporum, another fungal wilt pathogen. Our analyses identified a set of proteins that are shared among all three wilt pathogens, and present in few other fungal species. One of these is a homolog of a bacterial glucosyltransferase that synthesizes virulence-related osmoregulated periplasmic glucans in bacteria. Pathogenicity tests of the corresponding V. dahliae glucosyltransferase gene deletion mutants indicate that the gene is required for full virulence in the Australian tobacco species Nicotiana benthamiana. Compared to other fungi, the two sequenced Verticillium genomes encode more pectin-degrading enzymes and other carbohydrate-active enzymes, suggesting an extraordinary capacity to degrade plant pectin barricades. The high level of synteny between the two Verticillium assemblies highlighted four flexible genomic islands in V. dahliae that are enriched for transposable elements, and contain duplicated genes and genes that are important in signaling/transcriptional regulation and iron/lipid metabolism. Coupled with an enhanced capacity to degrade plant materials, these genomic islands may contribute to the expanded genetic diversity and virulence of V. dahliae, the primary causal agent of Verticillium wilts. Significantly, our study reveals insights into the genetic mechanisms of niche adaptation of fungal wilt pathogens, advances our understanding of the evolution and development of their pathogenesis, and sheds light on potential avenues for the development of novel disease management strategies to combat destructive wilt diseases.

The O-glycosylated linker from the Trichoderma reesei Family 7 cellulase is a flexible, disordered protein

Fungi and bacteria secrete glycoprotein cocktails to deconstruct cellulose. Cellulose-degrading enzymes (cellulases) are often modular, with catalytic domains for cellulose hydrolysis and carbohydrate-binding modules connected by linkers rich in serine and threonine with O-glycosylation. Few studies have probed the role that the linker and O-glycans play in catalysis. Since different expression and growth conditions produce different glycosylation patterns that affect enzyme activity, the structure-function relationships that glycosylation imparts to linkers are relevant for understanding cellulase mechanisms. Here, the linker of the Trichoderma reesei Family 7 cellobiohydrolase (Cel7A) is examined by simulation. Our results suggest that the Cel7A linker is an intrinsically disordered protein with and without glycosylation. Contrary to the predominant view, the O-glycosylation does not change the stiffness of the linker, as measured by the relative fluctuations in the end-to-end distance; rather, it provides a 16 Å extension, thus expanding the operating range of Cel7A. We explain observations from previous biochemical experiments in the light of results obtained here, and compare the Cel7A linker with linkers from other cellulases with sequence-based tools to predict disorder. This preliminary screen indicates that linkers from Family 7 enzymes from other genera and other cellulases within T. reesei may not be as disordered, warranting further study.