A Bacillus amyloliquefaciens ChbB protein binds beta- and alpha-chitin and has homologues in related strains

Microbial chitinases and their relevance in various industries

Chitin, the second most abundant biopolymer on earth after cellulose, is composed of β-1,4-N-acetylglucosamine (GlcNAc) units. It is widely distributed in nature, especially as a structural polysaccharide in the cell walls of fungi, the exoskeletons of crustaceans, insects, and nematodes. However, the principal commercial source of chitin is the shells of marine or freshwater invertebrates. Microbial chitinases are largely responsible for chitin breakdown in nature, and they play an important role in the ecosystem's carbon and nitrogen balance. Several microbial chitinases have been characterized and are gaining prominence for their applications in various sectors. The current review focuses on chitinases of microbial origin, their diversity, and their characteristics. The applications of chitinases in several industries such as agriculture, food, the environment, and pharmaceutical sectors are also highlighted.

Expression regulation of chitin-binding protein and metal-binding peptide in new Bacillus velezensis: MALDI-TOF MS/MS analysis

Metallothionein and metal-binding peptides are small cysteine-rich proteins produced by different organisms in stress conditions. In this study, the metal-binding peptide was detected in extracellular proteins of a new Bacillus velezensis strain, isolated from metal contaminated soil, and grown on the lead-enriched medium, for the first time. The presence of sulfide peptide was assayed by two simple tests (lead sulfide and Ellman's reagent test) for preliminary, and subsequently confirmed using polyacrylamide gel electrophoresis at media with different lead concentrations that the low-molecular-weight protein fragments (≈10 kDa) were observed while none were detected in the medium containing sodium chloride or calcium salt. The amino acids of the observed fragments were analyzed by matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF MS/MS). Also, the metal adsorption was confirmed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) by staining with chromium solution. The results showed that the putative sulfide peptide is metallothionein, which is induced in stress conditions. It was interesting that in all SDS profiles, one protein fragment (≈18 kDa) was inhibited in lead-enriched media. The data from MALDI-TOF MS/MS analysis showed that this fraction was a chitin-binding protein whose production was regulated by metal contamination. It is anticipated that these two proteins regulate the toxicity of lead.

A Bacillus thuringiensis Chitin-Binding Protein is Involved in Insect Peritrophic Matrix Adhesion and Takes Part in the Infection Process

Bacillus thuringiensis (Bt) is used for insect pest control, and its larvicidal activity is primarily attributed to Cry toxins. Other factors participate in infection, and limited information is available regarding factors acting on the peritrophic matrix (PM). This study aimed to investigate the role of a Bt chitin-binding protein (CBPA) that had been previously shown to be expressed at pH 9 in vitro and could therefore be expressed in the alkaline gut of lepidopteron larvae. A ∆cbpA mutant was generated that was 10-fold less virulent than wild-type Bt HD73 towards Ostrinia furnacalis neonate larvae, indicating its important role in infection. Purified recombinant Escherichia coli CBPA was shown to have a chitin affinity, thus indicating a possible interaction with the chitin-rich PM. A translational GFP-CBPA fusion elucidated the localization of CBPA on the bacterial surface, and the transcriptional activity of the promoter PcbpA was immediately induced and confirmed at pH 9. Next, in order to connect surface expression and possible in vivo gut activity, last instar Galleria mellonella (Gm) larvae (not susceptible to Bt HD-73) were used as a model to follow CBPA in gut expression, bacterial transit, and PM adhesion. CBPA-GFP was quickly expressed in the Gm gut lumen, and more Bt HD73 strain bacteria adhered to the PM than those of the ∆cbpA mutant strain. Therefore, CBPA may help to retain the bacteria, via the PM binding, close to the gut surface and thus takes part in the early steps of Bt gut interactions.

Improving extracellular production of Serratia marcescens lytic polysaccharide monooxygenase CBP21 and Aeromonas veronii B565 chitinase Chi92 in Escherichia coli and their synergism

Lytic polysaccharide monooxygenases (LPMOs) can oxidize recalcitrant polysaccharides and boost the conversion of the second most abundant polysaccharide chitin by chitinase. In this study, we aimed to achieve the efficient extracellular production of Serratia marcescens LPMO CBP21 and Aeromonas veronii B565 chitinase Chi92 by Escherichia coli. Twelve signal peptides reported with high secretion efficiency were screened to assess the extracellular production efficiency of CBP21 and Chi92, with glycine used as a medium supplement. The results showed that PelB was the most productive signal peptide for the extracellular production of CBP21 and Chi92 in E. coli. Furthermore, CBP21 facilitated the degradation of the three chitin substrates (colloidal chitin, β-chitin, and α-chitin) by Chi92. This study will be valuable for the industrial production and application of the two enzymes for chitin degradation.

Physiological and Molecular Understanding of Bacterial Polysaccharide Monooxygenases

Bacteria have long been known to secrete enzymes that degrade cellulose and chitin. The degradation of these two polymers predominantly involves two enzyme families that work synergistically with one another: glycoside hydrolases (GHs) and polysaccharide monooxygenases (PMOs). Although bacterial PMOs are a relatively recent addition to the known biopolymer degradation machinery, there is an extensive amount of literature implicating PMO in numerous physiological roles. This review focuses on these diverse and physiological aspects of bacterial PMOs, including facilitating endosymbiosis, conferring a nutritional advantage, and enhancing virulence in pathogenic organisms. We also discuss the correlation between the presence of PMOs and bacterial lifestyle and speculate on the advantages conferred by PMOs under these conditions. In addition, the molecular aspects of bacterial PMOs, as well as the mechanisms regulating PMO expression and the function of additional domains associated with PMOs, are described. We anticipate that increasing research efforts in this field will continue to expand our understanding of the molecular and physiological roles of bacterial PMOs.

Elucidating biochemical features and biological roles of Streptomyces proteins recognizing crystalline chitin- and cellulose-types and their soluble derivatives

Pioneering biochemical, immunological, physiological and microscopic studies in combination with gene cloning allowed uncovering previously unknown genes encoding proteins of streptomycetes to target crystalline chitin and cellulose as well as their soluble degradation-compounds via binding protein dependent transporters. Complementary analyses provoked an understanding of novel regulators governing transcription of selected genes. These discoveries induced detecting close and distant homologues of former orphan proteins encoded by genes from different bacteria. Grounded on structure-function-relationships, several researchers identified a few of these proteins as novel members of the growing family for lytic polysaccharides monooxygenases. Exemplary, the ecological significance of the characterized proteins including their role to promote interactions among organisms is outlined and discussed.

Activity, stability and 3-D structure of the Cu(ii) form of a chitin-active lytic polysaccharide monooxygenase from Bacillus amyloliquefaciens

The enzymatic deconstruction of recalcitrant polysaccharide biomass is central to the conversion of these substrates for societal benefit, such as in biofuels.

Structural and functional characterization of a conserved pair of bacterial cellulose-oxidizing lytic polysaccharide monooxygenases

For decades, the enzymatic conversion of cellulose was thought to rely on the synergistic action of hydrolytic enzymes, but recent work has shown that lytic polysaccharide monooxygenases (LPMOs) are important contributors to this process. We describe the structural and functional characterization of two functionally coupled cellulose-active LPMOs belonging to auxiliary activity family 10 (AA10) that commonly occur in cellulolytic bacteria. One of these LPMOs cleaves glycosidic bonds by oxidation of the C1 carbon, whereas the other can oxidize both C1 and C4. We thus demonstrate that C4 oxidation is not confined to fungal AA9-type LPMOs. X-ray crystallographic structures were obtained for the enzyme pair from Streptomyces coelicolor, solved at 1.3 Å (ScLPMO10B) and 1.5 Å (CelS2 or ScLPMO10C) resolution. Structural comparisons revealed differences in active site architecture that could relate to the ability to oxidize C4 (and that also seem to apply to AA9-type LPMOs). Despite variation in active site architecture, the two enzymes exhibited similar affinities for Cu(2+) (12-31 nM), redox potentials (242 and 251 mV), and electron paramagnetic resonance spectra, with only the latter clearly different from those of chitin-active AA10-type LPMOs. We conclude that substrate specificity depends not on copper site architecture, but rather on variation in substrate binding and orientation. During cellulose degradation, the members of this LPMO pair act in synergy, indicating different functional roles and providing a rationale for the abundance of these enzymes in biomass-degrading organisms.

Characterization of cis-acting elements residing in the chitinase promoter of Bacillus pumilus SG2

Bacillus pumilus SG2 is a chitinolytic bacterium that produces two chitinases, namely ChiS and ChiL. The chiS and chiL genes are consecutively expressed under a common promoter. Regulation of the chiS and chiL genes is under the control of carbon catabolite repression (CCR) in B. pumilus. This study aimed to investigate the cis-acting elements of the chitinase promoter. For this purpose, we transferred the chiS gene along with its specific promoter to Bacillus subtilis as a host. Primer extension analysis revealed two transcription start sites located 287 and 65 bp upstream of the chiS start codon. The distal promoter was highly compatible with the consensus sequence of the σ(A)-type promoters in B. subtilis, whereas the proximal promoter sequence showed less similarity to the σ(A)-type consensus sequence. A catabolite responsive element (cre), which is required for CCR in Bacillus species, was found to be 136 to 123 bp upstream of the chiS start codon. Interestingly, this cre site was located upstream of the -35 of the proximal promoter and downstream of the distal promoter. Deletion of this cre site sequence rendered the chiS expression constitutive.

Domain wise docking analyses of the modular chitin binding protein CBP50 from Bacillus thuringiensis serovar konkukian S4

This paper presents an in silico characterization of the chitin binding protein CBP50 from B. thuringiensis serovar konkukian S4 through homology modeling and molecular docking. The CBP50 has shown a modular structure containing an N-terminal CBM33 domain, two consecutive fibronectin-III (Fn-III) like domains and a C-terminal CBM5 domain. The protein presented a unique modular structure which could not be modeled using ordinary procedures. So, domain wise modeling using MODELLER and docking analyses using Autodock Vina were performed. The best conformation for each domain was selected using standard procedure. It was revealed that four amino acid residues Glu-71, Ser-74, Glu-76 and Gln-90 from N-terminal domain are involved in protein-substrate interaction. Similarly, amino acid residues Trp-20, Asn-21, Ser-23 and Val-30 of Fn-III like domains and Glu-15, Ala-17, Ser-18 and Leu-35 of C-terminal domain were involved in substrate binding. Site-directed mutagenesis of these proposed amino acid residues in future will elucidate the key amino acids involved in chitin binding activity of CBP50 protein.

Three dimensional (3D) structure prediction and substrate-protein interaction study of the chitin binding protein CBP24 from B. thuringiensis

Bacillus thuringiensis is an insecticidal bacterium whose chitinolytic system has been exploited to improve insect resistance in crops. In the present study, we studied the CBP24 from B. thuringiensis using homology modeling and molecular docking. The primary and secondary structure analyses showed CBP24 is a positively charged protein and contains single domain that belongs to family CBM33. The 3D model after refinement was used to explore the chitin binding characteristics of CBP24 using AUTODOCK. The docking analyses have shown that the surface exposed hydrophilic amino acid residues Thr-103, Lys-112 and Ser-162 interact with substrate through H-bonding. While, the amino acids resides Glu-39, Tyr-46, Ser-104 and Asn-109 were shown to have polar interactions with the substrate. The binding energy values evaluation of docking depicts a stable intermolecular conformation of the docked complex. The functional characterization of the CBP24 will elucidate the substrate-interaction pathway of the protein in specific and the carbohydrate binding proteins in general leading towards the exploration and exploitation of the prokaryotic substrate utilization pathways.

Recent insights into copper-containing lytic polysaccharide mono-oxygenases

Recently the role of oxidative enzymes in the degradation of polysaccharides by saprophytic bacteria and fungi was uncovered, challenging the classical model of polysaccharide degradation of being solely via a hydrolytic pathway. 3D structural analyses of lytic polysaccharide mono-oxygenases of both bacterial AA10 (formerly CBM33) and fungal AA9 (formerly GH61) enzymes revealed structures with β-sandwich folds containing an active site with a metal coordinated by an N-terminal histidine. Following some initial confusion about the identity of the metal ion it has now been shown that these enzymes are copper-dependent oxygenases. Here we assess recent developments in the academic literature, focussing on the structures of the copper active sites. We provide critical comparisons with known small-molecules studies of copper-oxygen complexes and with copper methane monoxygenase, another of nature's powerful copper oxygenases.

The copper active site of CBM33 polysaccharide oxygenases

The capacity of metal-dependent fungal and bacterial polysaccharide oxygenases, termed GH61 and CBM33, respectively, to potentiate the enzymatic degradation of cellulose opens new possibilities for the conversion of recalcitrant biomass to biofuels. GH61s have already been shown to be unique metalloenzymes containing an active site with a mononuclear copper ion coordinated by two histidines, one of which is an unusual τ-N-methylated N-terminal histidine. We now report the structural and spectroscopic characterization of the corresponding copper CBM33 enzymes. CBM33 binds copper with high affinity at a mononuclear site, significantly stabilizing the enzyme. X-band EPR spectroscopy of Cu(II)-CBM33 shows a mononuclear type 2 copper site with the copper ion in a distorted axial coordination sphere, into which azide will coordinate as evidenced by the concomitant formation of a new absorption band in the UV/vis spectrum at 390 nm. The enzyme's three-dimensional structure contains copper, which has been photoreduced to Cu(I) by the incident X-rays, confirmed by X-ray absorption/fluorescence studies of both aqueous solution and intact crystals of Cu-CBM33. The single copper(I) ion is ligated in a T-shaped configuration by three nitrogen atoms from two histidine side chains and the amino terminus, similar to the endogenous copper coordination geometry found in fungal GH61.

Bacterial chitin binding proteins show differential substrate binding and synergy with chitinases

Glycosyl hydrolase (GH) family 18 chitinases (Chi) and family 33 chitin binding proteins (CBPs) from Bacillus thuringiensis serovar kurstaki (BtChi and BtCBP), B. licheniformis DSM13 (BliChi and BliCBP) and Serratia proteamaculans 568 (SpChiB and SpCBP21) were used to study the efficiency and synergistic action of BtChi, BliChi and SpChiB individually with BtCBP, BliCBP or SpCBP21. Chitinase assay revealed that only BtChi and SpChiB showed synergism in hydrolysis of chitin, while there was no increase in products generated by BliChi, in the presence of the three above mentioned CBPs. This suggests that some (specific) CBPs are able to exert a synergistic effect on (specific) chitinases. A mutant of BliChi, designated as BliGH, was constructed by deleting the C-terminal fibronectin III (FnIII) and carbohydrate binding module 5 (CBM5) to assess the contribution of FnIII and CBM5 domains in the synergistic interactions of GH18 chitinases with CBPs. Chitinase assay with BliGH revealed that the accessory domains play a major role in making BliChi an efficient enzyme. We studied binding of BtCBP and BliCBP to α- and β-chitin. The BtCBP, BliCBP or SpCBP21 did not act synergistically with chitinases in hydrolysis of the chitin, interspersed with other polymers, present in fungal cell walls.

Chitin binding proteins act synergistically with chitinases in Serratia proteamaculans 568

Genome sequence of Serratia proteamaculans 568 revealed the presence of three family 33 chitin binding proteins (CBPs). The three Sp CBPs (Sp CBP21, Sp CBP28 and Sp CBP50) were heterologously expressed and purified. Sp CBP21 and Sp CBP50 showed binding preference to β-chitin, while Sp CBP28 did not bind to chitin and cellulose substrates. Both Sp CBP21 and Sp CBP50 were synergistic with four chitinases from S. proteamaculans 568 (Sp ChiA, Sp ChiB, Sp ChiC and Sp ChiD) in degradation of α- and β-chitin, especially in the presence of external electron donor (reduced glutathione). Sp ChiD benefited most from Sp CBP21 or Sp CBP50 on α-chitin, while Sp ChiB and Sp ChiD had major advantage with these Sp CBPs on β-chitin. Dose responsive studies indicated that both the Sp CBPs exhibit synergism ≥0.2 µM. The addition of both Sp CBP21 and Sp CBP50 in different ratios to a synergistic mixture did not significantly increase the activity. Highly conserved polar residues, important in binding and activity of CBP21 from S. marcescens (Sm CBP21), were present in Sp CBP21 and Sp CBP50, while Sp CBP28 had only one such polar residue. The inability of Sp CBP28 to bind to the test substrates could be attributed to the absence of important polar residues.

Functional assignment of YvgO, a novel set of purified and chemically characterized proteinaceous antifungal variants produced by Bacillus thuringiensis SF361

This study reports a novel class of antifungal protein derived from bacterial origin. Bacillus thuringiensis SF361, the strain also responsible for producing the novel bacteriocin thurincin H, exhibits broad antifungal activity against select members of several fungal genera, including Aspergillus, Byssochlamys, and Penicillium, as well as the pathogenic yeast Candida albicans. Optimal antifungal production and secretion were observed after-log phase growth when incubated at 37°C in a carbohydrate-free growth medium. High-performance liquid chromatography purification was performed after pH-selective ammonium sulfate precipitation and size-exclusion chromatography. Intact mass analysis and peptide mass fingerprinting identified the 13,484-Da protein to be a mass homolog to the YvgO protein construct sequenced from Bacillus cereus AH 1134. Further analysis via amino-terminal sequencing also revealed the existence of four distinct yet equally efficacious YvgO variants differing only within the first four N-terminal residues. YvgO was found to be remarkably stable, maintaining its antifungal activity under a wide pH and temperature range. When assayed against the toxigenic species Byssochlamys fulva H25, the selected primary filamentous fungal indicator, the MIC was estimated to be 1.5 ppm. Candida albicans 3153 was more resistant, exhibiting MICs between 25 and 800 ppm, depending on growth conditions. YvgO is unique among antifungals, showing no known sequential or functional homology to the typical classes of antifungal proteins, including common membrane-acting agents such as cellulases and glucanases. Due to its activity against an array of pathogenic and spoilage fungi, the potentials for clinical, agricultural, and food-processing applications are encouraging.

Molecular characterization of the modular chitin binding protein Cbp50 from Bacillus thuringiensis serovar konkukian

Bacillus thuringiensis is an insecticidal bacterium whose chitinolytic system may be exploited to improve the insecticidal system of Bt-crops. A nucleotide fragment of 1368 bp from B. thuringiensis serovar konkukian S4, containing the complete coding sequence of the chitin binding protein Cbp50, was cloned and sequenced. Analyses have shown the protein to contain a modular structure consisting of an N-terminal CBM33 domain, two copies of a fibronectin-like domain and a C-terminal chitin binding domain classified as CBM5. The Cbp50 protein was heterologously expressed in Escherichia coli, purified and assessed for chitin binding activity. A deletion mutant (CBD-N; containing only the N-terminal CBM33 domain) of Cbp50 was produced to determine the role of C-terminal domains in the binding activity of the protein. The full-length Cbp50 was shown to bind β-chitin most efficiently followed by α-chitin, colloidal chitin and cellulose. The polysaccharide binding activity of CBD-N was drastically decreased. The data demonstrate that both the N-terminal and C-terminal domains of Cbp50 are essential for the efficient binding of chitin. The purified Cbp50 showed antifungal activity against the phytopathogenic fungus Fusarium oxysporum and the opportunistic human pathogen Aspergillus niger. This is the first report of a modular chitin binding protein in bacteria.

A selective assay to detect chitin and biologically active nano-machineries for chitin-biosynthesis with their intrinsic chitin-synthase molecules

A new assay system for chitin has been developed. It comprises the chitin-binding protein ChbB in fusion with a His-tag as well as with a Strep-tag, the latter of which was chemically coupled to horseradish peroxidase. With the resulting complex, minimal quantities of chitin are photometrically detectable. In addition, the assay allows rapid scoring of the activity of chitin-synthases. As a result, a refined procedure for the rapid purification of yeast chitosomes (nano-machineries for chitin biosynthesis) has been established. Immuno-electronmicroscopical studies of purified chitosomes, gained from a yeast strain carrying a chitin-synthase gene fused to that for GFP (green-fluorescence protein), has led to the in situ localization of chitin-synthase-GFP molecules within chitosomes.

Protein purification involving a unique auto-cleavage feature of a repeated EAAAK peptide

Protein purification generally requires many steps of column chromatography that typically involve ion-exchange, hydrophobic-interaction and gel-filtration separations. More sophisticated purification of protein might be achieved through an application of affinity binding on a functionalized gel such as a nickel column, glutathione-modified column, maltose-modified gel column or others. Of several drawbacks existing in these methods, fusion proteins are commonly obtained, protease digestion might be necessary to remove the fusion moiety; a costly gel is employed for affinity binding, etc. Here we report that an expression vector derived from pREST was constructed to compose the gene of the chitin-binding protein (CBP) and the nucleotide sequence of the (EAAAK)(5) peptide linker following restriction sites for target gene insertion. Fusion proteins were expressed with E. coli and purified with a chitin column. The (EAAAK)(5) linker is shown to possess a pH-dependent auto-cleavage feature. In the range pH 6-7, the target protein becomes automatically released from the fusion protein without proteolytic treatment. Although the mechanism of this auto-cleavage property of an (EAAAK)(5) linker is unclear, this feature has been successfully employed for many cases of protein purification without the tag of a fusion protein.

The chitinolytic system of Lactococcus lactis ssp. lactis comprises a nonprocessive chitinase and a chitin-binding protein that promotes the degradation of alpha- and beta-chitin

It has recently been shown that the Gram-negative bacterium Serratia marcescens produces an accessory nonhydrolytic chitin-binding protein that acts in synergy with chitinases. This provided the first example of the production of dedicated helper proteins for the turnover of recalcitrant polysaccharides. Chitin-binding proteins belong to family 33 of the carbohydrate-binding modules, and genes putatively encoding these proteins occur in many microorganisms. To obtain an impression of the functional conservation of these proteins, we studied the chitinolytic system of the Gram-positive Lactococcus lactis ssp. lactis IL1403. The genome of this lactic acid bacterium harbours a simple chitinolytic machinery, consisting of one family 18 chitinase (named LlChi18A), one family 33 chitin-binding protein (named LlCBP33A) and one family 20 N-acetylhexosaminidase. We cloned, overexpressed and characterized LlChi18A and LlCBP33A. Sequence alignments and structural modelling indicated that LlChi18A has a shallow substrate-binding groove characteristic of nonprocessive endochitinases. Enzymology showed that LlChi18A was able to hydrolyse both chitin oligomers and artificial substrates, with no sign of processivity. Although the chitin-binding protein from S. marcescens only bound to beta-chitin, LlCBP33A was found to bind to both alpha- and beta-chitin. LlCBP33A increased the hydrolytic efficiency of LlChi18A to both alpha- and beta-chitin. These results show the general importance of chitin-binding proteins in chitin turnover, and provide the first example of a family 33 chitin-binding protein that increases chitinase efficiency towards alpha-chitin.

Regulation and characterization of Thermobifida fusca carbohydrate-binding module proteins E7 and E8

E7, a single domain Family 33 cellulose binding module (CBM) protein, and E8, a non-catalytic, three-domain protein consisting of a Family 33 CBM, a FNIII domain, followed by a Family 2 CBM, were cloned, expressed, purified, and characterized. Western blots showed that E7 and E8 were induced and secreted when Thermobifida fusca was grown on cellobiose, Solka floc, switchgrass, or alfalfa as well as on beta-1,3 linked glucose molecules such as laminaribiose or pachyman. E8 bound well to alpha- and beta-chitin and bacterial microcrystalline cellulose (BMCC) at all pHs tested. E7 bound strongly to beta-chitin, less well to alpha-chitin and more weakly to BMCC than E8. Filter paper binding assays showed that E7 was 28% bound, E8 was 39% bound, a purified CBM2 binding domain from Cel6B was 88% bound, and only 5% of the Cel5A catalytic domain was bound. A C-terminal 6xHis tag influenced binding of both E7 and E8 to these substrates. Filter paper activity assays showed enhanced activity of T. fusca cellulases when E7 or E8 was present. This effect was observed at very low concentrations of cellulases or at very long times into the reaction and was mainly independent of the type of cellulase and the number of cellulases in the mixture. E8, and to a lesser extent E7, significantly enhanced the activity of Serratia marscescens Chitinase C on beta-chitin.

Concerted responses between the chitin-binding protein secreting Streptomyces olivaceoviridis and Aspergillus proliferans

Streptomycetes belong to the ecologically important bacterial population within soil, which is also inhabited by many fungi. The highly chitinolytic Streptomyces olivaceoviridis and the ascomycete Aspergillus proliferans were chosen as models to test for interactions among bacteria and fungi. In medium lacking a soluble carbon source, individually cultivated spores of the bacterium S. olivaceoviridis and the fungus A. proliferans do not germinate. However, as shown by viability tests, cultivation of a mixture of both spore types provokes successive events: (i) stimulation of the germination of S. olivaceoviridis spores, (ii) initiation of the outgrowth of some fungal spores to which the S. olivaceoviridis chitinase ChiO1 adheres, (iii) massive extension of viable networks of S. olivaceoviridis hyphae at the expense of fungal hyphae and (iv) balanced proliferation of closely interacting fungal and S. olivaceoviridis hyphae. The replacement of the S. olivaceoviridis wild-type strain by a chromosomal disruption mutant (DeltaC), lacking production of the extracellular chitin-binding protein CHB1 but still secreting the chitinase ChiO1, provokes (v) germination of each spore type, (vi) retarded development of both partners, followed by (vii) preferential proliferation of the fungus. Together with biochemical and immunomicroscopy studies, the data support the conclusion that CHB1 molecules aggregate to an extracellular matrix, maintaining a close contact, followed by several concerted responses of the bacterium and the fungus.

Elucidating the Biosynthesis of Chitin Filaments and their Configuration with Specific Proteins and Electron Microscopy

To deepen the knowledge of chitin synthesis, a yeast mutant has been used as a model. Purified chitin synthase I-containing vesicles (chitosomes) with a diameter of 85 to 120 nm are identified by electron microscopy to eject tiny fibers upon addition of UDP-N-acetylglucosamine. The filigree of extruded filaments fused gradually into a large three-dimensional network, which is degradable by a chitinase. The network is targeted and restructured by the Streptomyces chitin-binding protein CHB1, which has a very high affinity only for alpha-chitin. Within the chitosomes, filaments are found to be highly condensed within consecutive oval fibroids, which are specifically targeted by the alpha-chitin-binding protein. The presented data give new insights to the generation of chitin filaments with an antiparallel (alpha) configuration. [image: see text]

The Non-catalytic Chitin-binding Protein CBP21 from Serratia marcescens Is Essential for Chitin Degradation

The Gram-negative soil bacterium Serratia marcescens uses three different family 18 chitinases to degrade chitin, an abundant insoluble carbohydrate polymer composed of beta(1,4)-linked units of N-acetylglucosamine. We show that efficient chitin degradation additionally depends on the action of a small non-catalytic protein, CBP21, which binds to the insoluble crystalline substrate, leading to structural changes in the substrate and increased substrate accessibility. CBP21 strongly promoted hydrolysis of crystalline beta-chitin by chitinases A and C, while it was essential for full degradation by chitinase B. CBP21 variants with single mutations on the largely polar binding surface lost their ability to promote chitin degradation, while retaining considerable affinity for the polymer. Thus, binding alone is not sufficient for CBP21 functionality, which seems to depend on specific, mostly polar interactions between the protein and crystalline chitin. This is the first time a secreted binding protein is shown to assist in the enzymatic degradation of an insoluble carbohydrate via non-hydrolytic disruption of the substrate. Interestingly, homologues of CBP21 occur in most chitin-degrading microorganisms, suggesting a general mechanism by which chitin-binding proteins enhance chitinolytic activity. Homologues also occur in chitinase-containing insect viruses, whose infectiousness is known to depend on chitinase efficiency.

Crystal Structure and Binding Properties of the Serratia marcescens Chitin-binding Protein CBP21

Chitin proteins are commonly found in bacteria that utilize chitin as a source of energy. CBP21 is a chitin-binding protein from Serratia marcescens, a Gram-negative soil bacterium capable of efficient chitin degradation. When grown on chitin, S. marcescens secretes large amounts of CBP21, along with chitin-degrading enzymes. In an attempt to understand the molecular mechanism of CBP21 action, we have determined its crystal structure at 1.55 angstroms resolution. This is the first structure to be solved of a family 33 carbohydrate-binding module. The structure reveals a "budded" fibronectin type III fold consisting of two beta-sheets, arranged as a beta-sheet sandwich, with a 65-residue "bud" consisting of three short helices, located between beta-strands 1 and 2. Remarkably, conserved aromatic residues that have been suggested previously to play a role in chitin binding were mainly found in the interior of the protein, seemingly incapable of interacting with chitin, whereas the structure revealed a surface patch of highly conserved, mainly hydrophilic residues. The roles of six of these conserved surface-exposed residues (Tyr-54, Glu-55, Glu-60, His-114, Asp-182, and Asn-185) were probed by site-directed mutagenesis and subsequent binding studies. All single point mutations lowered the affinity of CBP21 for beta-chitin, as shown by 3-8-fold increases in the apparent binding constant. Thus, binding of CBP21 to chitin seems to be mediated primarily by conserved, solvent-exposed, polar side chains.