Glycoenzymes: an unusual type of glycoprotein structure for a glucoamylase

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.

Structural analysis of N- and O-glycans using ZIC-HILIC/dialysis coupled to NMR detection

Protein glycosylation, an important and complex post-translational modification (PTM), is involved in various biological processes, including the receptor-ligand and cell-cell interaction, and plays a crucial role in many biological functions. However, little is known about the glycan structures of important biological complex samples, and the conventional glycan enrichment strategy (i.e., size-exclusion column [SEC] separation) prior to nuclear magnetic resonance (NMR) detection is time-consuming and tedious. In this study, we developed a glycan enrichment strategy that couples Zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) with dialysis to enrich the glycans from the pronase E digests of RNase B, followed by NMR analysis of the glycoconjugate. Our results suggest that the ZIC-HILIC enrichment coupled with dialysis is a simple, fast, and efficient sample preparation approach. The approach was thus applied to analysis of a biological complex sample, the pronase E digest of the secreted proteins from the fungus Aspergillus niger. The NMR spectra revealed that the secreted proteins from A. niger contain both N-linked glycans with a high-mannose core similar to the structure of the glycan from RNase B, and O-linked glycans bearing mannose and glucose with 1→3 and 1→6 linkages. In all, our study provides compelling evidence that ZIC-HILIC separation coupled with dialysis is very effective and accessible in preparing glycans for the downstream NMR analysis, which could greatly facilitate the future NMR-based glycoproteomics research.

Protein glycosylation pathways in filamentous fungi

Glycosylation of proteins is important for protein stability, secretion, and localization. In this study, we have investigated the glycan synthesis pathways of 12 filamentous fungi including those of medical/agricultural/industrial importance for which genomes have been recently sequenced. We have adopted a systems biology approach to combine the results from comparative genomics techniques with high confidence information on the enzymes and fungal glycan structures, reported in the literature. From this, we have developed a composite representation of the glycan synthesis pathways in filamentous fungi (both N- and O-linked). The N-glycosylation pathway in the cytoplasm and endoplasmic reticulum was found to be highly conserved evolutionarily across all the filamentous fungi considered in the study. In the final stages of N-glycan synthesis in the Golgi, filamentous fungi follow the high mannose pathway as in Saccharomyces cerevisiae, but the level of glycan mannosylation is reduced. Highly specialized N-glycan structures with galactofuranose residues, phosphodiesters, and other insufficiently trimmed structures have also been identified in the filamentous fungi. O-Linked glycosylation in filamentous fungi was seen to be highly conserved with many mannosyltransferases that are similar to those in S. cerevisiae. However, highly variable and diverse O-linked glycans also exist. We have developed a web resource for presenting the compiled data with user-friendly query options, which can be accessed at www.fungalglycans.org. This resource can assist attempts to remodel glycosylation of recombinant proteins expressed in filamentous fungal hosts.

Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

Protein O-mannosyltransferase A of Aspergillus awamori is involved in O-mannosylation of glucoamylase I

Industrially important extracellular enzymes from filamentous fungi are often O-mannosylated. The structure and function of the pmtA (AapmtA) gene encoding the protein O-D-mannosyltransferase of Aspergillus awamori were characterized. The AapmtA disruptant, designated AaPMTA, was constructed by homologous recombination. The strain AaPMTA exhibited fragile cell morphology with respect to hyphal extension, as well as swollen hyphae formation and conidia formation in potato dextrose medium. Moreover, the AapmtA disruptant showed increased sensitivity to high temperature and Congo red. Thus, the AaPmtA protein is involved in the formation of the normal cell wall. The strain AaPMTA could grow well in liquid synthetic medium and secrete glucoamylase I (GAI-AaPMTA) to a similar extent to the wild-type strain (GAI-WT). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis of the GAIs revealed that approximately 33 mannose moieties of GAI were absent in strain AaPMTA. This result indicates that the AaPmtA protein is responsible for the transfer of mannose to GAI. Structural analysis of the O-linked oligosaccharides of GAI also demonstrated that the AapmtA disruption resulted in a reduction of the amounts of O-linked oligosaccharides, such as D-mannose and alpha-1,2-mannotriose, in GAI-AaPMTA. However, the amount of alpha-1,2-mannobiose was comparable between GAI-WT and GAI-AaPMTA. The result suggests the presence of a compensatory mechanism in the synthetic pathway of O-mannosylation in A. awamori.

Molecular characterization of protein O-mannosyltransferase and its involvement in cell-wall synthesis in Aspergillus nidulans

ProteinO-glycosylation is essential for protein modification and plays important roles in eukaryotic cells.O-Mannosylation of proteins occurs in the filamentous fungusAspergillus. The structure and function of thepmtAgene, encoding proteinO-d-mannosyltransferase, which is responsible for the initialO-mannosylation reaction inAspergillus nidulans, was characterized. Disruption of thepmtAgene resulted in the reduction ofin vitroproteinO-d-mannosyltransferase activity to 6 % of that of the wild-type strain and led to underglycosylation of an extracellular glucoamylase. ThepmtAdisruptant exhibited abnormal cell morphology and alteration in carbohydrate composition, particularly reduction in the skeletal polysaccharides in the cell wall. The results indicate that PmtA is required for the formation of a normal cell wall inA. nidulans.

Influence of mannan epitopes in glycoproteins–Concanavalin A interaction. Comparison of natural and synthetic glycosylated proteins

Two natural glycoproteins/glycoenzymes, invertase and glucoamylase, and two neoglycoconjugates, synthetized from Saccharomyces cerevisiae mannan, bovine serum albumin and penicillin G acylase were tested for interaction with lectin Concanavalin A (Con A). The interaction of natural and synthetic glycoproteins with Con A was studied using three different experimental methods: (i). quantitative precipitation in solution (ii). sorption to Con A immobilized on bead cellulose; and (iii). kinetic measurement of the interaction by surface plasmon resonance. Prepared neoglycoproteins were further characterized: saccharide content, molecular weight, polydispersion, kinetic and equilibrium association constants with Con A were determined. It can be concluded that the used conjugation method proved to be able to produce neoglycoproteins with similar properties like natural glycoproteins, i.e. enzymatic activity (protein part) and lectin binding activity (mannan part) were preserved and the neoglycoconjugates interact with Con A similarly as natural mannan-type glycoproteins.

Evidence that the glucoamylases and alpha-amylase secreted by Aspergillus niger are proteolytically processed products of a precursor enzyme

A 125-kDa starch hydrolysing enzyme of Aspergillus niger characterised by its ability to dextrinise and saccharify starch [Suresh et al. (1999) Appl. Microbiol. Biotechnol. 51, 673-675] was also found to possess activity towards raw starch. Segregation of these activities in the 71-kDa glucoamylase and a 53-kDa alpha-amylase-like enzyme supported by antibody cross-reactivity studies and the isolation of mutants based on assay screens for the secretion of particular enzyme forms revealed the 125-kDa starch hydrolysing enzyme as their precursor. N-terminal sequence analysis further revealed that the 71-kDa glucoamylase was the N-terminal product of the precursor enzyme. Immunological cross reactivity of the 53-kDa amylase with antibodies raised against the precursor enzyme but not with the 71- and 61-kDa glucoamylase antibodies suggested that this enzyme activity is represented by the C-terminal fragment of the precursor. The N-terminal sequence of the 53-kDa protein showed similarity to the reported Taka amylase of Aspergillus oryzae. Antibody cross-reactivity to a 10-kDa non-enzymic peptide and a 61-kDa glucoamylase described these proteins as products of the 71-kDa glucoamylase. Identification of only the precursor starch hydrolysing enzyme in the protein extracts of fungal protoplasts suggested proteolytic processing in the cellular periplasmic space as the cause for the secretion of multiple forms of amylases by A. niger.

The carbohydrate moiety of alpha-galactosidase from Trichoderma reesei

Alpha-galactosidase from Trichoderma reesei is a glycoprotein that contains O- and N-linked carbohydrate chains. There are 6 O-linked glycans per protein molecule that are linked to serine and threonine and can be released by beta-elimination. Among these are monomers: D-glucose, D-mannose, and D-galactose; dimers: alpha1-6 D-mannopyranosyl-alpha-D-glycopyranoside and alpha1-6 D-glucopyranosyl-alpha-D-galactopyranoside and one trimer: alpha-D-glucopyranosyl-alpha1-2 D-mannopyranosyl-alpha1-6 D-galactopyranoside. N-linked glycans are of the mannose-rich type and may be released by treating the protein with Endo-beta-N-acetyl glycosaminidase F or by hydrozinolysis. The enzyme was deglycosylated with Endo-beta-N-acetyl glycosaminidase F as well as with a number of exoglycosidases that partially remove the terminal residues of O-linked glycans. The effect of enzymatic deglycosylation on the properties of alpha-galactosidase has been considered. The effects of tunicamycin and 2-deoxyglucose on the secretion and glycosylation of the enzyme during culture growth have been analysed. The presence of two glycoforms of alpha-galactosidase differing in the number of N-linked carbohydrate chains and the microheterogeneity of the carbohydrate moiety of the enzyme are described.

The action of alpha-mannosidase from Oerskovia sp. on the mannose-rich O-linked sugar chains of glycoproteins

Alpha-mannosidase was isolated from the culture liquid of Oerskovia sp. The purified enzyme had a molecular mass of 480 kDa and comprises four identical subunits. The enzyme cleaves bonds in side chains of yeast mannan (Km = 0.08 mM, k(cat) = 1.02 micromol x min(-1) x mg(-1)) and reveals a low activity towards p-nitrophenyl alpha-D-mannopyranoside. The alpha-mannosidase is a Ca2+-dependent enzyme and is inhibited by EDTA. The enzyme possess no endo-mannosidase activity releasing only mannose in the reaction with the inversion of anomeric configuration and could be classified as exo-alpha-mannanase. The enzyme revealed a high deglycosylating activity towards the short mannose-rich O-linked carbohydrate chains of glycoproteins.

A model for cleavage of O-glycosidic bonds in glycoproteins

The present work investigated the possibility of cleavage of alpha-linkages between mannose or galactose and serine/threonine residues by alpha-mannosidase and alpha-galactosidase. The study was carried out initially with model synthetic compounds imitating the O-glycosidic bond in glycoproteins, and further with glucoamylase. It was shown that alpha-mannosidase and alpha-galactosidase can hydrolyse these linkages after proteolytic digestion of glucoamylase.

Crystallization and preliminary X-ray study of minor glucoamylase from Aspergillus awamori variant X-100/D27

Crystals of the reduced form of glucoamylase were obtained from polyethylene glycol 6000 solution by the hanging-drop method. The protein was treated with alpha-mannosidase to partly remove the sugar component. The crystals belong to the space group P2(1)2(1)2(1) with cell dimensions a = 116.7 A, b = 104.3 A, c = 48.5 A and diffract beyond 2.5 A resolution.

Binding of isomaltose and maltose to the glucoamylase from Aspergillus niger, as studied by fluorescence spectrophotometry and steady-state kinetics

The binding of maltose, isomaltose, and D-glucono-1,5-lactone to the glucoamylase [E.C.3.2.1.3] from Aspergillus niger was monitored by the fluorescence-intensity change (delta F) based on the tryptophan residues of the enzyme, and the binding parameters (Kd and delta Fmax) were evaluated from the dependence of delta F on the concentration of substrate and analogue. Maltose caused the fluorescence-intensity change, but isomaltose did not, although it is hydrolyzed by the enzyme. Both substrates bind to the glucoamylase of Rhizopus niveus and cause delta F, suggesting that some difference exists in the conformation of the isomaltose-binding subsites between the two glucoamylases.

Primary structure of an N-linked sugar chain derived from glucoamylase of Rhizopus niveus

The primary structure of the N-linked sugar chain of Rhizopus niveus glucoamylase (major component) was investigated. The carbohydrate moiety was released from the polypeptide backbone by Flavobacterium sp. endo-beta-N-acetylglucosaminidase digestion. Studies using the method of exoglycosidase digestion of the fluorescent pyridylamino derivative, gel-permeation chromatography on Bio-Gel P-4 and 400-MHz 1H-NMR spectroscopy revealed that the most abundant structure is (Man)8-GlcNac-ol.

Purification and properties of an extracellular glucoamylase from a diastatic strain of Saccharomyces cerevisiae

The extracellular glucoamylase from certain strains of Saccharomyces cerevisiae can be purified from culture medium by a simple chromatographic procedure. The native enzyme is heavily glycosylated and has an Mr of about 250,000, but gel filtration indicates the existence of oligomers of larger size. Dissociation yields a form of Mr about 70,000. The glucoamylase is rich in serine and threonine and in aspartic acid plus asparagine, and has a pI of 4.62 and a pH optimum of 4.5-6.5. The thermostability and resistance to denaturants of the yeast enzyme is compared with those of two other fungal glucoamylases. Kinetic data for the yeast enzyme and a variety of substrates is presented; the enzyme is particularly ineffective in cleaving alpha-(1----6)-glycosidic bonds.

The glycoprotein nature and antigenicity of a fungal D-glucosyltransferase

D-Glucosyltransferase (EC 2.4.1.24) from Aspergillus niger has been prepared in pure form by chromatography on DEAE-cellulose. The enzyme transfers D-glucosyl units from maltose and other alpha-linked D-glucosyl oligosaccharides to glucosyl co-substrates resulting in the synthesis of new types of oligosaccharides. The glucosyltransferase has been found to be a glycoprotein containing 20% of carbohydrate consisting of mannose, glucose, and galactose. The carbohydrate residues are attached as either single units or as short oligosaccharide chains by O-glycosyl linkages to the serine and threonine residues of the protein. Antibodies directed against glucosyltransferase have been induced in animals by appropriate immunization regimes. These antibodies combine with the carbohydrate components of the enzyme and, therefore, the carbohydrate residues are the immunodeterminant groups of the glucosyltransferase.

Expression, Glycosylation, and Secretion of an Aspergillus Glucoamylase by Saccharomyces cerevisiae

A strain of Saccharomyces cerevisiae capable of simultaneous hydrolysis and fermentation of highly polymerized starch oligosaccharides was constructed. The Aspergillus awamori glucoamylase enzyme, form GAI, was expressed in Saccharomyces cerevisiae by means of the promoter and termination regions from a yeast enolase gene. Yeast transformed with plasmids containing an intron-free recombinant glucoamylase gene efficiently secreted glucoamylase into the medium, permitting growth of the transformants on starch as the sole carbon source. The natural leader sequence of the precursor of glucoamylase (preglucoamylase) was processed correctly by yeast, and the secreted enzyme was glycosylated through both N- and O-linkages at levels comparable to the native Aspergillus enzyme. The data provide evidence for the utility of yeast as an organism for the production, glycosylation, and secretion of heterologous proteins.

Structural studies on the O-glycosidically linked carbohydrate chains of glucoamylase G1 from Aspergillus niger

Glucoamylase G1 from Aspergillus niger contains an unusual type of carbohydrate-protein linkage, involving mannose O-glycosidically linked to serine and threonine. The majority of the neutral oligosaccharides of glucoamylase G1 are located in a region of about 70 amino acid residues which carries about 35 oligosaccharide units [(1983) Carlsberg Res. Commun. 48, 517-527]. Structural analysis was performed on the O-linked carbohydrates of a tryptic fragment from glucoamylase G1 comprising the segment characterized by a high degree of glycosylation. The carbohydrate structures released by trifluoroacetolysis were elucidated using sugar analysis, methylation analysis, mass spectrometry, chromium trioxide oxidation, digestion with alpha-mannosidase and 1H-NMR spectroscopy. The following structures could be identified. (formula; see text)