Polyuronides in the cell walls of Mucor rouxii

Molecular architecture of chitin and chitosan-dominated cell walls in zygomycetous fungal pathogens by solid-state NMR

Zygomycetous fungal infections pose an emerging medical threat among individuals with compromised immunity and metabolic abnormalities. Our pathophysiological understanding of these infections, particularly the role of fungal cell walls in growth and immune response, remains limited. Here we conducted multidimensional solid-state NMR analysis to examine cell walls in five Mucorales species, including key mucormycosis causative agents like Rhizopus and Mucor species. We show that the rigid core of the cell wall primarily comprises highly polymorphic chitin and chitosan, with minimal quantities of β-glucans linked to a specific chitin subtype. Chitosan emerges as a pivotal molecule preserving hydration and dynamics. Some proteins are entrapped within this semi-crystalline chitin/chitosan layer, stabilized by the sidechains of hydrophobic amino acid residues, and situated distantly from β-glucans. The mobile domain contains galactan- and mannan-based polysaccharides, along with polymeric α-fucoses. Treatment with the chitin synthase inhibitor nikkomycin removes the β-glucan-chitin/chitosan complex, leaving the other chitin and chitosan allomorphs untouched while simultaneously thickening and rigidifying the cell wall. These findings shed light on the organization of Mucorales cell walls and emphasize the necessity for a deeper understanding of the diverse families of chitin synthases and deacetylases as potential targets for novel antifungal therapies.

Study of fungal cell wall evolution through its monosaccharide composition: An insight into fungal species interacting with plants

Every fungal cell is encapsulated in a cell wall, essential for cell viability, morphogenesis, and pathogenesis. Most knowledge of the cell wall composition in fungi has focused on ascomycetes, especially human pathogens, but considerably less is known about early divergent fungal groups, such as species in the Zoopagomycota and Mucoromycota phyla. To shed light on evolutionary changes in the fungal cell wall, we studied the monosaccharide composition of the cell wall of 18 species including early diverging fungi and species in the Basidiomycota and Ascomycota phyla with a focus on those with pathogenic lifestyles and interactions with plants. Our data revealed that chitin is the most characteristic component of the fungal cell wall, and was found to be in a higher proportion in the early divergent groups. The Mucoromycota species possess few glucans, but instead have other monosaccharides such as fucose and glucuronic acid that are almost exclusively found in their cell walls. Additionally, we observed that hexoses (glucose, mannose and galactose) accumulate in much higher proportions in species belonging to Dikarya. Our data demonstrate a clear relationship between phylogenetic position and fungal cell wall carbohydrate composition and lay the foundation for a better understanding of their evolution and their role in plant interactions.

Use of Anionic Polysaccharides in the Development of 3D Bioprinting Technology

Three-dimensional (3D) bioprinting technology is now one of the best ways to generate new biomaterial for potential biomedical applications. Significant progress in this field since two decades ago has pointed the way toward use of natural biopolymers such as polysaccharides. Generally, these biopolymers such as alginate possess specific reactive groups such as carboxylate able to be chemically or enzymatically functionalized to generate very interesting hydrogel structures with biomedical applications in cell generation. This present review gives an overview of the main natural anionic polysaccharides and focuses on the description of the 3D bioprinting concept with the recent development of bioprinting processes using alginate as polysaccharide.

Polysaccharides Cell Wall Architecture of Mucorales

Invasive fungal infections are some of the most life-threatening infectious diseases in the hospital setting. In industrialized countries, the most common fungal species isolated from immunocompromised patients are Candida and Aspergillus spp. However, the number of infections due to Mucorales spp. is constantly increasing and little is known about the virulence factors of these fungi. The fungal cell wall is an important structure protecting fungi from the environment. A better knowledge of its composition should improve our understanding of host-pathogen interactions. Cell wall molecules are involved in tissue adherence, immune escape strategies, and stimulation of host defenses including phagocytosis and mediators of humoral immunity. The fungal cell wall is also a target of choice for the development of diagnostic or therapeutic tools. The present review discusses our current knowledge on the cell wall structure of Mucorales in terms of the polysaccharides and glyco-enzymes involved in its biosynthesis and degradation, with an emphasis on the missing gaps in our knowledge.

Mucor rouxii Rho1 protein; characterization and possible role in polarized growth

We have previously shown that protein kinase A of the medically important zygomycete Mucor rouxii participates in fungal morphology through cytoskeletal organization. As a first step towards finding the link between protein kinase A and cytoskeletal organization we here demonstrate the cloning of the Rho1 gene and the characterization of its protein product. The RHO1 protein primary sequence shows 70-85% identity with fungal RHO1 or mammalian RhoA. Two protein kinase A phosphorylation sequences in adequate context are predicted, Ser73 and Ser135. The peptide IRRNSQKFV, containing Ser135 proved to be a good substrate for M. rouxii protein kinase A catalytic subunit. The over-expressed Rho1 fully complements a Saccharomyces cerevisiae null mutant. The endogenous protein was identified by western blot against a developed antibody and by ADP-ribosylation. Localization in germlings was visualized by immunofluorescence; the protein was localized in patches in the mother cell surface and excluded from the germ tube. Measurement of Rho1 expression during germination indicates that Rho1, at both the mRNA and protein levels, correlates with differentiation and not with growth. Rho1 has been shown to be the regulatory protein of the beta-1,3-glucan synthase complex in fungi in which beta-1,3-glucans are major components of the cell wall. Even though glucans have not been detected in zygomycetes, caspofungin, an echinochandin known to be an inhibitor of beta-1,3-glucan synthase complex, is shown here to have a negative effect on growth and to produce an alteration on morphology when added to M. rouxii growth culture medium. This result has an important impact on the possible participation of beta-1,3-glucans on the regulation of morphology of zygomycetes.

Isolation and partial characterization of uronic acid-containing glycoproteins from Mucor rouxii

Five different fractions containing uronic acids associated with protein were isolated from the cytoplasm of the filamentous form of Mucor rouxii. A single fraction was isolated from the cell wall by hot sodium dodecyl sulfate followed by ion exchange column chromatography. Two cytoplasmic entities (peaks I and II) were not adsorbed to DEAE Bio-Gel A. The molecular mass of peaks I to V ranged from 16.5 to 210 kDa. The protein-uronic acid ratios were different for each fraction. The cell wall fraction showed a molecular mass of 16.5 kDa, similar to that of peak II but with differences in chromatographic behavior and protein-uronic acid ratio. The possible role of these molecules as acceptors of sugar residues during polyuronide chain growth is discussed.

Structural characterization and rheological properties of an extracellular glucuronan produced by a Rhizobium meliloti M5N1 mutant strain

The mutant strain M5N1 C.S. (NCIMB 40472) of Rhizobium meliloti M5N1 is able to produce during fermentation a partially acetylated extracellular (1-->4)-beta-D-glucuronan. At low concentration (1 g.l-1), in the presence of monovalent cations, this new glucuronate behaves as a thickening agent, whereas at higher concentration a thermoreversible gel is obtained. With such divalent cations as Ca2+, a thermally stable gel can be formed.

Formation of antigenic extracellular polysaccharides by selected strains of Mucor spp., Rhizopus spp., Rhizomucor spp., Absidia corymbifera and Syncephalastrum racemosum

In this study, polyclonal IgG antibodies raised against extracellular polysaccharides (EPS) of Mucor racemosus were characterised as almost specific for moulds belonging to the order of Mucorales. Cross-reactivity in the ELISA could be observed only towards the yeast Pichia membranaefaciens. EPS were isolated from various cultures of M. hiemalis growing on six different carbon sources and two nitrogen sources, with ratios varying from 0.13 to 0.44 relative to the amount of biomass. Other strains including Mucor spp., Rhizopus spp., Rhizomucor spp., Absidia corymbifera and Syncephalastrum racemosum also excreted EPS, with ratios varying from 0.05 to 0.23. In all cases, the excreted EPS had similar antigenic properties as determined by ELISA. No enzymatic degradation of the antigenic parts of the polysaccharides could be observed upon prolonged incubation. Considering that all tested strains formed similar amounts of antigenic EPS there might be scope for the specific detection of biomass of Mucoralean moulds using ELISA techniques for example in food.

Mucor dimorphism

Mucor dimorphism has interested microbiologists since the time of Pasteur. When deprived of oxygen, these fungi grow as spherical, multipolar budding yeasts. In the presence of oxygen, they propagate as branching coenocytic hyphae. The ease with which these morphologies can be manipulated in the laboratory, the diverse array of morphopoietic agents available, and the alternative developmental fates that can be elicited from a single cell type (the sporangiospore) make Mucor spp. a highly propitious system in which to study eukaryotic cellular morphogenesis. The composition and organization of the cell wall differ greatly in Mucor yeasts and hyphae. The deposition of new wall polymers is isodiametric in yeasts and apically polarized in hyphae. Current research has focused on the identity and control of enzymes participating in wall synthesis. An understanding of how the chitosome interacts with appropriate effectors, specific enzymes, and the plasma membrane to assemble chitin-chitosan microfibrils and to deposit them at the proper sites on the cell exterior will be critical to elucidating dimorphism. Several biochemical and physiological parameters have been reported to fluctuate in a manner that correlates with Mucor morphogenesis. The literature describing these has been reviewed critically with the intent of distinguishing between causal and casual connections. The advancement of molecular genetics has afforded powerful new tools that researchers have begun to exploit in the study of Mucor dimorphism. Several genes, some encoding products known to correlate with development in Mucor spp. or other fungi, have been cloned, sequenced, and examined for transcriptional activity during morphogenesis. Most have appeared in multiple copies displaying independent transcriptional control. Selective translation of stored mRNA molecules occurs during sporangiospore germination. Many other correlates of Mucor morphogenesis, presently described but not yet explained, should prove amenable to analysis by the emerging molecular technology.

The poly-alpha- and -beta-1,4-glucuronic acid moiety of teichuronopeptide from the cell wall of the alkalophilic Bacillus strain C-125

Teichuronopeptide is a structural component of the cell wall of alkalophilic Bacillus strain C-125 and is a complex composed of polyglutamate and polyglucuronate. A structural analysis of the polyglucuronic acid moiety was carried out. Periodate oxidation and Smith degradation of the moiety, and enzymic analysis after reduction of glucuronic acid to glucose, revealed that glucuronic acid bound together with alternately alpha- and beta-1,4-linkages.

Two distinct classes of polyuronide from the cell walls of a dimorphic fungus, Mucor rouxii

Polyuronides were extracted from purified yeast and mycelial walls of Mucor rouxii by sequential treatments with lithium chloride and potassium hydroxide and were fractionated by ion-exchange chromatography on DEAE-Sephadex. Two polymers (I and II) of different acidity were found in both wall types. Polymer I contained D-glucuronic acid, L-fucose, D-mannose, and much smaller amounts of D-galactose. Yeast and mycelial polymer I had similar uronic acid contents but differed in their neutral sugar compositions and molecular weights. Polymer II from both cell types contained largely D-glucuronic acid and had similar molecular weights. On partial acid hydrolysis, both polymers I and II gave rise to insoluble glucuronans which appeared to be homopolymeric. One-third of the total uronosyl residues of polymer I, and almost all of the uronosyl residues of polymer II, were present in homopolymeric segments. However, homopolymers derived from polymers I and II may not be identical.

Role of membranes of mycelial Mucor rouxii in synthesis and secretion of cell wall matrix polymers

Membrane fractions of the mycelial form of Mucor rouxii contained endogenous polyuronides and glycoproteins with sugar compositions similar to the cell wall polyuronide (mucoran) and extracellular (wall and filtrate) glycoproteins, respectively. The polyuronide pool was rapidly labeled with D-[U-14C]glucose and on pulse-chase experiments appeared to turn over, suggesting that these polymers were precursors of the wall mucoran. In contrast, the glycoproteins appeared to accumulate. Although the membrane-associated glycoproteins had molecular weights similar to those of the extracellular glycoproteins, the bulk of the membrane-associated polyuronides were of a molecular weight lower than that of the wall polyuronide.

Oligoglucuronide production in Mucor rouxii: evidence for a role for endohydrolases in hyphal extension

Culture filtrates of Mucor rouxii contained oligomers of glucuronic acid which were labeled rapidly during pulses with D-[U-14C]glucose. These oligomers were probably derived by enzymatic lysis of acidic polymers in the cell wall. The kinetics of the incorporation of label into oligouronides and cell wall polymers suggested that lysis of the wall was required for active hyphal extension. Experiments with cycloheximide, which inhibited hyphal extension, suggested that wall lysis was also required for the subapical cell wall synthesis which probably occurred under these conditions.

The hyphal wall of Mucor mucedo. 1. Polyanionic polymers

Treatment of isolated hyphal walls of Mucor mucedo with nitrous acid resulted in the release of two water-soluble polyanions: (a) a glycuronan, containing all the neutral sugars and uronic acid present in the hyphal wall and (b) an inorganic polyphosphate. The glycuronan could also be extracted quantitatively with salt solutions of high ionic strength and partially with a solution of potassium hydroxide. This is presented as evidence that the glycuronan is a genuine constituent of the cell wall, non-covalently bound to glucosamine-containing polymers which are susceptible to depolymerization by nitrous acid. By treatment with acid the glycuronan was partly converted to crystalline poly(glucuronic acid) with the properties of mucoric acid. This strongly suggests that mucoric acid, which can be extracted from the walls of M. mucedo by alkali after acid treatment, is not a genuine wall component but arises by partial acid hydrolysis of the heteropolymeric glycuronan.

A pathway of chitosan formation in Mucor rouxii. Enzymatic deacetylation of chitin

1. An enzyme that catalyzes hydrolysis of acetamido groups of chitin derivatives was found in the supernatant fraction of Mucor rouxii. 2. Partially O-hydroxyethylated chitin (glycol chitin) was used as a substrate in the purification and characterization of this enzyme. A 140-fold purification was obtained by means of ammonium sulfate fractionation followed by chromatography on carboxymethylcellulose and DEAE-cellulose. 3. The enzyme releases about 30% of the acetyl groups of glycol chitin, giving a product with a decreased sensitivity to lysozyme. The enzyme also deacetylates chitin and N-acetylchitooligoses, whereas it is inactive toward bacterial cell wall peptidoglycan, N-acetylated heparin, a polymer of N-acetylgalactosamine, di-N-acetylchitobiose and monomeric N-acetylglucosamine derivatives. 4. This enzyme shows a pH optimum of 5.5. The Km value for glycol chitin is 0.87 g/l or 2.6 mM with respect to monosaccharide residues. 5. The occurrence of this enzyme accounts for the formation of chitosan in fungi.

Presence of binding site for alpha-amylase and of masking protein for this site on mycelial cell wall of Aspergillus oryzae

Mycelial cell wall of Aspergillus oryzae M-13 grown in an alpha-amylase-forming medium could not bind alpha-amylase (Taka-amylase A, EC 3.2.1.1). However, by treatment with 1.0 n NaOH at 100 C for 30 min, the wall gained the ability to bind alpha-amylase. This phenomenon was caused by removal of a factor (designated as masking factor) which masked the binding site for alpha-amylase. The masking factor was purified as a preparation giving a single peak in both ultracentrifugation (1.6S) and by gel electrophoresis (M(BPB), 1.0). Approximately 20 mug of the purified factor, bound to 10 mg of the alkali-treated mycelial cell wall, prevented the binding of approximately 100 mug of alpha-amylase or released approximately 100 mug of alpha-amylase which previously was bound to the alkali-treated wall. These findings indicate that the factor has much higher affinity than alpha-amylase for the binding site on the mycelial wall. The masking factor was inducibly formed accompanying the secretion of alpha-amylase.