Alpha-amylase starch binding domains: cooperative effects of binding to starch granules of multiple tandemly arranged domains

The Ruminococcus bromii amylosome protein Sas6 binds single and double helical α-glucan structures in starch

Resistant starch is a prebiotic accessed by gut bacteria with specialized amylases and starch-binding proteins. The human gut symbiont Ruminococcus bromii expresses Sas6 (Starch Adherence System member 6), which consists of two starch-specific carbohydrate-binding modules from family 26 (RbCBM26) and family 74 (RbCBM74). Here, we present the crystal structures of Sas6 and of RbCBM74 bound with a double helical dimer of maltodecaose. The RbCBM74 starch-binding groove complements the double helical α-glucan geometry of amylopectin, suggesting that this module selects this feature in starch granules. Isothermal titration calorimetry and native mass spectrometry demonstrate that RbCBM74 recognizes longer single and double helical α-glucans, while RbCBM26 binds short maltooligosaccharides. Bioinformatic analysis supports the conservation of the amylopectin-targeting platform in CBM74s from resistant-starch degrading bacteria. Our results suggest that RbCBM74 and RbCBM26 within Sas6 recognize discrete aspects of the starch granule, providing molecular insight into how this structure is accommodated by gut bacteria.

Digestive exophagy of biofilms by intestinal amoeba and its impact on stress tolerance and cytotoxicity

The human protozoan parasite Entamoeba histolytica is responsible for amebiasis, a disease endemic to developing countries. E. histolytica trophozoites colonize the large intestine, primarily feeding on bacteria. However, in the gastrointestinal tract, bacterial cells form aggregates or structured communities called biofilms too large for phagocytosis. Remarkably, trophozoites are still able to invade and degrade established biofilms, utilizing a mechanism that mimics digestive exophagy. Digestive exophagy refers to the secretion of digestive enzymes that promote the digestion of objects too large for direct phagocytosis by phagocytes. E. histolytica cysteine proteinases (CPs) play a crucial role in the degradation process of Bacillus subtilis biofilm. These proteinases target TasA, a major component of the B. subtilis biofilm matrix, also contributing to the adhesion of the parasite to the biofilm. In addition, they are also involved in the degradation of biofilms formed by Gram-negative and Gram-positive enteric pathogens. Furthermore, biofilms also play an important role in protecting trophozoites against oxidative stress. This specific mechanism suggests that the amoeba has adapted to prey on biofilms, potentially serving as an untapped reservoir for novel therapeutic approaches to treat biofilms. Consistently, products derived from the amoeba have been shown to restore antibiotic sensitivity to biofilm cells. In addition, our findings reveal that probiotic biofilms can act as a protective shield for mammalian cells, hindering the progression of the parasite towards them.

Carbohydrate-binding modules facilitate the enzymatic hydrolysis of lignocellulosic biomass: Releasing reducing sugars and dissociative lignin available for producing biofuels and chemicals

The microbial decomposition and utilization of lignocellulosic biomass present in the plant tissues are driven by a series of carbohydrate active enzymes (CAZymes) acting in concert. As the non-catalytic domains widely found in the modular CAZymes, carbohydrate-binding modules (CBMs) are intimately associated with catalytic domains (CDs) that effect the diverse hydrolytic reactions. The CBMs function as auxiliary components for the recognition, adhesion, and depolymerization of the complex substrate mediated by the associated CDs. Therefore, CBMs are deemed as significant biotools available for enzyme engineering, especially to facilitate the enzymatic hydrolysis of dense and insoluble plant tissues to acquire more fermentable sugars. This review aims at presenting the taxonomies and biological properties of the CBMs currently curated in the CAZy database. The molecular mechanisms that CBMs use in assisting the enzymatic hydrolysis of plant polysaccharides and the regulatory factors of CBM-substrate interactions are outlined in detail. In addition, guidelines for the rational designs of CBM-fused CAZymes are proposed. Furthermore, the potential to harness CBMs for industrial applications, especially in enzymatic pretreatment of the recalcitrant lignocellulose, is evaluated. It is envisaged that the ideas outlined herein will aid in the engineering and production of novel CBM-fused enzymes to facilitate efficient degradation of lignocellulosic biomass to easily fermentable sugars for production of value-added products, including biofuels.

Resistant starch, microbiome, and precision modulation

Resistant starch, microbiome, and precision modulation. Mounting evidence has positioned the gut microbiome as a nexus of health. Modulating its phylogenetic composition and function has become an attractive therapeutic prospect. Resistant starches (granular amylase-resistant α-glycans) are available as physicochemically and morphologically distinguishable products. Attempts to leverage resistant starch as microbiome-modifying interventions in clinical studies have yielded remarkable inter-individual variation. Consequently, their utility as a potential therapy likely depends predominantly on the selected resistant starch and the subject's baseline microbiome. The purpose of this review is to detail i) the heterogeneity of resistant starches, ii) how resistant starch is sequentially degraded and fermented by specialized gut microbes, and iii) how resistant starch interventions yield variable effects on the gut microbiome.

Differential Evolution of α-Glucan Water Dikinase (GWD) in Plants

The alpha-glucan water dikinase (GWD) enzyme catalyzes starch phosphorylation, an integral step in transitory starch degradation. The high phosphate content in stored starch has great industrial value, due to its physio–chemical properties making it more versatile, although the phosphate content of stored starch varies depending on the botanical source. In this study, we used various computational approaches to gain insights into the evolution of the GWD protein in 48 plant species with possible roles in enzyme function and alteration of phosphate content in their stored starch. Our analyses identified deleterious mutations, particularly in the highly conserved 5 aromatic amino acid residues in the dual tandem carbohydrate binding modules (CBM-45) of GWD protein in C. zofingiensis, G. hirsutum, A. protothecoides, P. miliaceum, and C. reinhardtii. These findings will inform experimental designs for simultaneous repression of genes coding for GWD and the predicted interacting proteins to elucidate the role this enzyme plays in starch degradation. Our results reveal significant diversity in the evolution of GWD enzyme across plant species, which may be evolutionarily advantageous according to the varying needs for phosphorylated stored starch between plants and environments.

New Recombinant Antimicrobial Peptides Confer Resistance to Fungal Pathogens in Tobacco Plants

Antimicrobial peptides have been long known to confer resistance to plant pathogens. In this study, new recombinant peptides constructed from a dermaseptin B1 (DrsB1) peptide fused to a chitin-binding domain (CBD) from Avr4 protein, were used for Agrobacterium tumefaciens-mediated transformation of tobacco plants. Polymerase chain reaction (PCR), semi-quantitative RT-PCR, and western blotting analysis demonstrated the incorporation and expression of transgenes in tobacco genome and transgenic plants, respectively. In vitro experiments with recombinant peptides extracted from transgenic plants demonstrated a significant (P<0.01) inhibitory effect on the growth and development of plant pathogens. The DrsB1-CBD recombinant peptide had the highest antifungal activity against fungal pathogens. The expression of the recombinant peptides greatly protected transgenic plants from Alternaria alternata, Alternaria solani, Fusarium oxysporum, and Fusarium solani fungi, in comparison to Pythium sp. and Pythium aphanidermatum. Expression of new recombinant peptides resulted in a delay in the colonization of fungi and appearance of fungal disease symptoms from 6 days to more than 7 weeks. Scanning electron microscopy images revealed that the structure of the fungal mycelia appeared segmented, cling together, and crushed following the antimicrobial activity of the recombinant peptides. Greenhouse bioassay analysis showed that transgenic plants were more resistant to Fusarium and Pythium infections as compared with the control plants. Due to the high antimicrobial activity of the recombinant peptides against plant pathogens and novelty of recombinant peptides, this report shows the feasibility of this approach to generate disease resistance transgenic plants.

Starch-binding domains as CBM families-history, occurrence, structure, function and evolution

The term "starch-binding domain" (SBD) has been applied to a domain within an amylolytic enzyme that gave the enzyme the ability to bind onto raw, i.e. thermally untreated, granular starch. An SBD is a special case of a carbohydrate-binding domain, which in general, is a structurally and functionally independent protein module exhibiting no enzymatic activity but possessing potential to target the catalytic domain to the carbohydrate substrate to accommodate it and process it at the active site. As so-called families, SBDs together with other carbohydrate-binding modules (CBMs) have become an integral part of the CAZy database (http://www.cazy.org/). The first two well-described SBDs, i.e. the C-terminal Aspergillus-type and the N-terminal Rhizopus-type have been assigned the families CBM20 and CBM21, respectively. Currently, among the 85 established CBM families in CAZy, fifteen can be considered as families having SBD functional characteristics: CBM20, 21, 25, 26, 34, 41, 45, 48, 53, 58, 68, 69, 74, 82 and 83. All known SBDs, with the exception of the extra long CBM74, were recognized as a module consisting of approximately 100 residues, adopting a β-sandwich fold and possessing at least one carbohydrate-binding site. The present review aims to deliver and describe: (i) the SBD identification in different amylolytic and related enzymes (e.g., CAZy GH families) as well as in other relevant enzymes and proteins (e.g., laforin, the β-subunit of AMPK, and others); (ii) information on the position in the polypeptide chain and the number of SBD copies and their CBM family affiliation (if appropriate); (iii) structure/function studies of SBDs with a special focus on solved tertiary structures, in particular, as complexes with α-glucan ligands; and (iv) the evolutionary relationships of SBDs in a tree common to all SBD CBM families (except for the extra long CBM74). Finally, some special cases and novel potential SBDs are also introduced.

Production of a Recombinant Dermaseptin Peptide in Nicotiana tabacum Hairy Roots with Enhanced Antimicrobial Activity

Expression of strong antimicrobial peptides in plants is of great interest to combat a wide range of plant pathogens. To bring the Dermaseptin B1 (DrsB1) peptide to the intimate contact of the plant pathogens cell wall surface, the DrsB1 encoding sequence was fused to the C-terminal part of the two copies of the chitin-binding domain (CBD) of the Avr4 effector protein and used for Agrobacterium rhizogenes-mediated transformation. The expression of the recombinant protein in the tobacco hairy roots (HRs) was confirmed by molecular analysis. Antimicrobial activity analysis of the recombinant protein purified from the transgenic HRs showed that the (CBD)2-DrsB1 recombinant protein had a significant (p < 0.01) antimicrobial effect on the growth of different fungal and bacterial pathogens. The results of this study indicated that the recombinant protein had a higher antifungal activity against chitin-producing Alternaria alternata than Pythium spp. Scanning electron microscopy images demonstrated that the recombinant protein led to fungal hypha deformation, fragmentation, and agglutination of growing hypha, possibly by dissociating fungal cell wall components. In vitro evidences suggest that the expression of the (CBD)2-DrsB1 recombinant protein in plants by generating transgenic lines is a promising approach to produce disease-resistant plants, resistance to chitin-producing pathogenic fungi.

Protein Adsorption to Charged Gold Nanospheres as a Function of Protein Deformability

The corona that forms as protein adsorbs to gold nanospheres (AuNSs) is directly influenced by the surface chemistry of the AuNS. Tools to predict adsorption outcomes are needed for intelligent design of nanomaterials for biological applications. We hypothesized that the denaturation behavior of a protein might be a useful predictor of adsorption behavior to AuNSs, and used this idea to study protein adsorption to anionic citrate-capped AuNSs and to cationic poly(allylamine hydrochloride) (PAH) wrapped AuNSs. Three proteins (α-amylase (A-Amy), β-lactoglobulin (BLG), and bovine serum albumin (BSA)), representing three different classes of acid denaturation behavior, were selected with BLG being the least deformable and BSA being the most deformable. Protein adsorption to AuNSs was monitored via UV-vis spectrophotometry and dynamic light scattering. Changes to the protein structure upon AuNS interaction were monitored via circular dichroism spectroscopy. Binding constants were determined using the Langmuir adsorption isotherm, resulting in BSA > BLG ≫ A-Amy affinities for citrate-capped gold nanospheres. PAH-coated AuNSs displayed little affinity for these proteins at similar concentrations as citrate-coated AuNSs and became agglomerated at high protein concentrations. The enzymatic activity of A-Amy/citrate AuNS conjugates was measured via colorimetric assay, and found to be 11% of free A-Amy, suggesting that binding restricts access to the active site. Across both citrate AuNSs and PAH AuNSs, the changes in secondary structure were greatest for BSA > A-Amy > BLG, which does follow the trends predicted by acid denaturation characteristics.

Advances in molecular engineering of carbohydrate-binding modules

Carbohydrate-binding modules (CBMs) are non-catalytic domains that are generally appended to carbohydrate-active enzymes. CBMs have a broadly conserved structure that allows recognition of a notable variety of carbohydrates, in both their soluble and insoluble forms, as well as in their alpha and beta conformations and with different types of bonds or substitutions. This versatility suggests a high functional plasticity that is not yet clearly understood, in spite of the important number of studies relating protein structure and function. Several studies have explored the flexibility of these systems by changing or improving their specificity toward substrates of interest. In this review, we examine the molecular strategies used to identify CBMs with novel or improved characteristics. The impact of the spatial arrangement of the functional amino acids of CBMs is discussed in terms of unexpected new functions that are not related to the original biological roles of the enzymes. Proteins 2017; 85:1602-1617. © 2017 Wiley Periodicals, Inc.

Expression and comparative characterization of complete and C-terminally truncated forms of saccharifying α-amylase from Lactobacillus plantarum S21

Lactobacillus plantarum S21 α-amylase possesses 475 amino acids at the C-terminal region identified as the starch-binding domain (SBD) and has been previously reported to play a role in raw starch degradation. To understand the specific roles of this SBD, cloning and expression of the complete (AmyL9) and C-terminally truncated (AmyL9Δ_SBD) forms of α-amylase were conducted for enzyme purification and comparative characterization. AmyL9 and AmyL9Δ_SBD were overproduced in Escherichia coli at approximately 10- and 20-times increased values of volumetric productivity when compared to α-amylase produced by the wild type, respectively. AmyL9Δ_SBD was unable to hydrolyze raw starch and exhibited substrate specificity in a similar manner to that of AmyL9, but it was weakly active toward amylopectin and glycogen. The hydrolysis products obtained from the amylaceous substrates of both enzymes were the same. In addition, AmyL9Δ_SBD showed comparatively higher K_m values than AmyL9 when it reacted with starch and amylopectin, and lower values for other kinetic constants namely v_max, k_cat, and k_cat/K_m. The results indicated that the C-terminal SBDs of L. plantarum S21 α-amylase contribute to not only substrate preference but also substrate affinity and the catalytic efficiency of the α-amylase without any changes in the degradation mechanisms of the enzyme.

Bacterial and Archaeal α-Amylases: Diversity and Amelioration of the Desirable Characteristics for Industrial Applications

Industrial enzyme market has been projected to reach US$ 6.2 billion by 2020. Major reasons for continuous rise in the global sales of microbial enzymes are because of increase in the demand for consumer goods and biofuels. Among major industrial enzymes that find applications in baking, alcohol, detergent, and textile industries are α-amylases. These are produced by a variety of microbes, which randomly cleave α-1,4-glycosidic linkages in starch leading to the formation of limit dextrins. α-Amylases from different microbial sources vary in their properties, thus, suit specific applications. This review focuses on the native and recombinant α-amylases from bacteria and archaea, their production and the advancements in the molecular biology, protein engineering and structural studies, which aid in ameliorating their properties to suit the targeted industrial applications.

α-Amylase: an enzyme specificity found in various families of glycoside hydrolases

α-Amylase (EC 3.2.1.1) represents the best known amylolytic enzyme. It catalyzes the hydrolysis of α-1,4-glucosidic bonds in starch and related α-glucans. In general, the α-amylase is an enzyme with a broad substrate preference and product specificity. In the sequence-based classification system of all carbohydrate-active enzymes, it is one of the most frequently occurring glycoside hydrolases (GH). α-Amylase is the main representative of family GH13, but it is probably also present in the families GH57 and GH119, and possibly even in GH126. Family GH13, known generally as the main α-amylase family, forms clan GH-H together with families GH70 and GH77 that, however, contain no α-amylase. Within the family GH13, the α-amylase specificity is currently present in several subfamilies, such as GH13_1, 5, 6, 7, 15, 24, 27, 28, 36, 37, and, possibly in a few more that are not yet defined. The α-amylases classified in family GH13 employ a reaction mechanism giving retention of configuration, share 4-7 conserved sequence regions (CSRs) and catalytic machinery, and adopt the (β/α)8-barrel catalytic domain. Although the family GH57 α-amylases also employ the retaining reaction mechanism, they possess their own five CSRs and catalytic machinery, and adopt a (β/α)7-barrel fold. These family GH57 attributes are likely to be characteristic of α-amylases from the family GH119, too. With regard to family GH126, confirmation of the unambiguous presence of the α-amylase specificity may need more biochemical investigation because of an obvious, but unexpected, homology with inverting β-glucan-active hydrolases.

The carbohydrate-binding module of Fragaria × ananassa expansin 2 (CBM-FaExp2) binds to cell wall polysaccharides and decreases cell wall enzyme activities "in vitro"

A putative carbohydrate binding module (CBM) from strawberry (Fragaria × ananassa Duch.) expansin 2 (CBM-FaExp2) was cloned and the encoding protein was over-expressed in Escherichia coli and purified in order to evaluate its capacity to bind different cell wall polysaccharides "in vitro". The protein CBM-FaExp2 bound to microcrystalline cellulose, xylan and pectin with different affinities (K(ad) = 33.6 ± 0.44 mL g(-1), K(ad) = 11.37 ± 0.87 mL g(-1), K(ad) = 10.4 ± 0.19 mL g(-1), respectively). According to "in vitro" enzyme assays, this CBM is able to decrease the activity of cell wall degrading enzymes such as polygalacturonase, endo-glucanase, pectinase and xylanase, probably because the binding of CBM-FaExp2 to the different substrates interferes with enzyme activity. The results suggest that expansins would bind not only cellulose but also a wide range of cell wall polymers.

Surface display of heterologous proteins in Bacillus thuringiensis using a peptidoglycan hydrolase anchor

Background

Previous studies have revealed that the lysin motif (LysM) domains of bacterial cell wall-degrading enzymes are able to bind to peptidoglycan moieties of the cell wall. This suggests an approach for a cell surface display system in Gram-positive bacteria using a LysM-containing protein as the anchoring motif. In this study, we developed a new surface display system in B. thuringiensis using a LysM-containing peptidoglycan hydrolase, endo-β-N-acetylglucosaminidase (Mbg), as the anchor protein.

Results

Homology searching in the B. thuringiensis YBT-1520 genome revealed a putative peptidoglycan hydrolase gene. The encoded protein, Mbg, exhibited substantial cell-wall binding capacity. The deduced amino acid sequence of Mbg was structurally distinguished as an N-terminal domain with two tandemly aligned LysMs and a C-terminal catalytic domain. A GFP-fusion protein was expressed and used to verify the surface localization by Western blot, flow cytometry, protease accessibility, SDS sensitivity, immunofluorescence, and electron microscopy assays. Low-level constitutive expression of Mbg was elevated by introducing a sporulation-independent promoter of cry3Aa. Truncated Mbg domains with separate N-terminus (Mbgn), C-terminus (Mbgc), LysM₁, or LysM₂ were further compared for their cell-wall displaying efficiencies. The Mbgn moiety contributed to cell-wall anchoring, while LysM₁ was the active domain. Two tandemly repeated Mbgns exhibited the highest display activity, while the activity of three repeated Mbgns was decreased. A heterologous bacterial multicopper oxidase (WlacD) was successfully displayed onto the surface of B. thuringiensis target cells using the optimum (Mbgn)₂ anchor, without radically altering its catalytic activity.

Conclusion

Mbg can be a functional anchor protein to target different heterologous proteins onto the surface of B. thuringiensis cells. Since the LysM domain appears to be universal in Gram-positive bacteria, the strategy presented here could be applicable in other bacteria for developing this type of system.

The carbohydrate-binding module family 20--diversity, structure, and function

Starch-active enzymes often possess starch-binding domains (SBDs) mediating attachment to starch granules and other high molecular weight substrates. SBDs are divided into nine carbohydrate-binding module (CBM) families, and CBM20 is the earliest-assigned and best characterized family. High diversity characterizes CBM20s, which occur in starch-active glycoside hydrolase families 13, 14, 15, and 77, and enzymes involved in starch or glycogen metabolism, exemplified by the starch-phosphorylating enzyme glucan, water dikinase 3 from Arabidopsis thaliana and the mammalian glycogen phosphatases, laforins. The clear evolutionary relatedness of CBM20s to CBM21s, CBM48s and CBM53s suggests a common clan hosting most of the known SBDs. This review surveys the diversity within the CBM20 family, and makes an evolutionary comparison with CBM21s, CBM48s and CBM53s, discussing intrafamily and interfamily relationships. Data on binding to and enzymatic activity towards soluble ligands and starch granules are summarized for wild-type, mutant and chimeric fusion proteins involving CBM20s. Noticeably, whereas CBM20s in amylolytic enzymes confer moderate binding affinities, with dissociation constants in the low micromolar range for the starch mimic beta-cyclodextrin, recent findings indicate that CBM20s in regulatory enzymes have weaker, low millimolar affinities, presumably facilitating dynamic regulation. Structures of CBM20s, including the first example of a full-length glucoamylase featuring both the catalytic domain and the SBD, are summarized, and distinct architectural and functional features of the two SBDs and roles of pivotal amino acids in binding are described. Finally, some applications of SBDs as affinity or immobilization tags and, recently, in biofuel and in planta bioengineering are presented.

Characterization and application of carbohydrate-binding modules of beta-1,3-xylanase XYL4

beta-1,3-Xylanase from Vibrio sp. strain AX-4 (XYL4) is a modular enzyme composed of an N-terminal catalytic module belonging to glycoside hydrolase family 26 and two putative carbohydrate-binding modules (CBMs) belonging to family 31 in the C-terminal region. To investigate the functions of these three modules, five deletion mutants lacking individual modules were constructed. The binding assay of these mutants showed that a repeating unit of the CBM was a non-catalytic beta-1,3-xylan-binding module, while the catalytic module per se was not likely to contribute to the binding activity when insoluble beta-1,3-xylan was used for the assay. The repeating CBMs were found to specifically bind to insoluble beta-1,3-xylan, but not to beta-1,4-xylan, Avicel, beta-1,4-mannan, curdlan, chitin or soluble glycol-beta-1,3-xylan. Both the enzyme and the binding activities for insoluble beta-1,3-xylan but not soluble glycol-beta-1,3-xylan were enhanced by NaCl in a concentration-dependent manner, indicating that the CBMs of XYL4 bound to beta-1,3-xylan through hydrophobic interaction. This property of the CBMs was successfully applied to the purification of a recombinant XYL4 from the cell extracts of Escherichia coli transformed with the xyl4 gene and the detection of beta-1,3-xylan-binding proteins including beta-1,3-xylanase from the extract of a turban shell, Turbo cornutus.

Enhancing the receptor affinity of the sialic acid-binding domain of Vibrio cholerae sialidase through multivalency

Many glycoside hydrolases possess carbohydrate-binding modules (CBMs) that help target these enzymes to appropriate substrates and increase their catalytic efficiency. The Vibrio cholerae sialidase contains two CBMs, one of which is designated as a family CBM40 module and has been shown through structural and calorimetry studies to recognize the alpha-anomer of sialic acid with a KD of approximately 30 microM at 37 degrees C. The affinity of this V. cholerae CBM40 module for sialic acid is one of the highest reported for recognition of a monosaccharide by a CBM. As Nature often increases a weak substrate affinity through multivalency, we have explored the potential of developing reagents with an increased affinity for sialic acid receptors through linking CBM40 modules together. The V. cholerae CBM40 was subcloned and crystallized in the presence of sialyllactose confirming its ability to recognize sialic acid. Calorimetry revealed that this CBM40 demonstrated specificity to alpha(2,3)-, alpha(2,6)-, and alpha(2,8)-linked sialosides. Polypeptides containing up to four CBM40 modules in tandem were created to determine if an increase in affinity to sialic acid could be achieved through an avidity effect. Using SPR and a multivalent alpha(2,3)-sialyllactose ligand, we show that increasing the number of linked modules does increase the affinity for sialic acid. The four-CBM40 module protein has a 700- to 1500-fold increase in affinity compared with the single-CBM40 module. Varying the linker length of amino acids between each CBM40 module had little effect on the binding of these polypeptides. Finally, fluorescence-activated cell sorting analysis demonstrated that a green fluorescent protein fused to three CBM40 modules bound to subpopulations of human leukocytes. These studies lay the foundation for creating high affinity, multivalent CBMs that could have broad application in glycobiology.

Role of the N-terminal starch-binding domains in the kinetic properties of starch synthase III from Arabidopsis thaliana

Starch synthase III (SSIII), one of the SS isoforms involved in plant starch synthesis, has been reported to play a regulatory role in the synthesis of transient starch. SSIII from Arabidopsis thaliana contains 1025 amino acid residues and has an N-terminal transit peptide for chloroplast localization which is followed by three repeated starch-binding domains (SBDs; SSIII residues 22-591) and a C-terminal catalytic domain (residues 592-1025) similar to bacterial glycogen synthase. In this work, we constructed recombinant full-length and truncated isoforms of SSIII, lacking one, two, or three SBDs, and recombinant proteins, containing three, two, or one SBD, to investigate the role of these domains in enzyme activity. Results revealed that SSIII uses preferentially ADPGlc, although UDPGlc can also be used as a sugar donor substrate. When ADPGlc was used, the presence of the SBDs confers particular properties to each isoform, increasing the apparent affinity and the V max for the oligosaccharide acceptor substrate. However, no substantial changes in the kinetic parameters for glycogen were observed when UDPGlc was the donor substrate. Under glycogen saturating conditions, the presence of SBDs increases progressively the apparent affinity and V max for ADPGlc but not for UDPGlc. Adsorption assays showed that the N-terminal region of SSIII, containing three, two, or one SBD module have increased capacity to bind starch depending on the number of SBD modules, with the D23 protein (containing the second and third SBD module) being the one that makes the greatest contribution to binding. The results presented here suggest that the N-terminal SBDs have a regulatory role, showing a starch binding capacity and modulating the catalytic properties of SSIII.

System using tandem repeats of the cA peptidoglycan-binding domain from Lactococcus lactis for display of both N- and C-terminal fusions on cell surfaces of lactic acid bacteria

Here, we established a system for displaying heterologous protein to the C terminus of the peptidoglycan-binding domain (cA domain) of AcmA (a major autolysin from Lactococcus lactis). Western blot and flow cytometric analyses revealed that the fusion proteins (cA-AmyA) of the cA domain and alpha-amylase from Streptococcus bovis 148 (AmyA) are efficiently expressed and successfully displayed on the surfaces of L. lactis cells. AmyA was also displayed on the cell surface while retaining its activity. Moreover, with an increase in the number of cA domains, the quantity of cA-AmyA fusion proteins displayed on the cell surface increased. When three repeats of the cA domain were used as an anchor protein, 82% of alpha-amylase activity was detected on the cells. The raw starch-degrading activity of AmyA was significantly higher when AmyA was fused to the C terminus of the cA domain than when it was fused to the N terminus. In addition, cA-AmyA fusion proteins were successfully displayed on the cell surfaces of Lactobacillus plantarum and Lactobacillus casei.