Abstract
Deacetylation of chitin by chitin deacetylases (Cda) results in the formation of chitosan. Chitosan, a polymer of β1,4 linked glucosamine, plays multiple roles in the function of the fungal cell wall, including virulence and evasion of host immune responses. In this study, the roles of chitosan and putative CDAs in cell wall structure and virulence of Aspergillus fumigatus were investigated. Low levels of chitosan were found in the conidial and cell wall of A. fumigatus. Seven putative CDA genes were identified, disrupted and the phenotype of the single mutants and the septuple mutants were investigated. No alterations in fungal cell wall chitosan levels, changes in fungal growth or alterations in virulence were detected in the single or septuple Δcda1-7 mutant strains. Collectively, these results suggest that chitosan is a minority component of the A. fumigatus cell wall, and that the seven candidate Cda proteins do not play major roles in fungal cell wall synthesis or virulence. However, Cda2 is involved in conidiation, suggesting that this enzyme may play a role in N-acetyl-glucosamine metabolism.
Highlights
The fungal cell wall is a complex exoskeleton that protects the cell from osmotic pressure changes and other environmental stresses, yet allows the cell to interact with its environment
Based on early studies in filamentous phytopathogenic ascomycetes, we hypothesized that chitosan might play a role in cell wall morphogenesis and in host-pathogen interactions in A. fumigatus. We explored this hypothesis with the following three specific questions: (i) is chitosan present in the mycelial or conidial cell wall of A. fumigatus? (ii) how many CDAs genes are present in A. fumigatus genome and (iii) what is the role of these CDAs in A. fumigatus? To answer these questions, we characterized the percentage of deacetylation of the chitosan in the mycelium and conidia
Glucosamine polymer was found in the AI and AS fractions of the conidial cell wall whereas it was only found in the AI fraction of the mycelial cell wall
Summary
The fungal cell wall is a complex exoskeleton that protects the cell from osmotic pressure changes and other environmental stresses, yet allows the cell to interact with its environment. The fungal cell wall constitutes the first interface between pathogenic fungi and their host. Polymers with 50% or more acetylation are generally termed chitin, while those with acetylation levels less than this are commonly referred to as chitosan (Kasaai, 2009). Deacetylation alters the physical properties of the polymer to enhance solubility, flexibility, and confers a positive charge at neutral pH (Wang et al, 2016). A dynamic equilibrium between the quantity of chitin and chitosan exist in the cell wall but the environmental factors and regulatory elements governing this equilibrium are largely unknown
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