Unconventional Constituents and Shared Molecular Architecture of the Melanized Cell Wall of C. neoformans and Spore Wall of S. cerevisiae

Abstract

The fungal cell wall serves as the interface between the cell and the environment. Fungal cell walls are composed largely of polysaccharides, primarily glucans and chitin, though in many fungi stress-resistant cell types elaborate additional cell wall structures. Here, we use solid-state nuclear magnetic resonance spectroscopy to compare the architecture of cell wall fractions isolated from Saccharomyces cerevisiae spores and Cryptococcus neoformans melanized cells. The specialized cell walls of these two divergent fungi are highly similar in composition. Both use chitosan, the deacetylated derivative of chitin, as a scaffold on which a polyaromatic polymer, dityrosine and melanin, respectively, is assembled. Additionally, we demonstrate that a previously identified but uncharacterized component of the S. cerevisiae spore wall is composed of triglycerides, which are also present in the C. neoformans melanized cell wall. Moreover, we identify a tyrosine-derived constituent in the C. neoformans wall that, although it is not dityrosine, is a non-pigment constituent of the cell wall. The similar composition of the walls of these two phylogenetically distant species suggests that triglycerides, polyaromatics, and chitosan are basic building blocks used to assemble highly stress-resistant cell walls and the use of these constituents may be broadly conserved in other fungal species.

A Conserved Machinery Underlies the Synthesis of a Chitosan Layer in the Candida Chlamydospore Cell Wall

“…It is possible that a similar mechanism occurs during chlamydospore formation. An alternative possibility was recently suggested by nuclear magnetic resonance (NMR) studies of chitosan-containing cell wall preparations from both S. cerevisiae and Cryptococcus neoformans that revealed neutral lipids are directly incorporated into the cell wall ( 16 , 60 ). Thus, the Rrt8, Srt1, and Mum3 proteins may be involved in the synthesis of some lipid component that is then transferred from the lipid droplet to play a structural role during chitosan layer assembly.…”

“…For example, in the pathogen Cryptococcus neoformans chitosan in the wall dampens the host inflammatory response, and Cryptococcus strains unable to synthesize chitosan are avirulent ( 13 – 15 ). Chitosan is often found in conjunction with polyphenolic compounds, which has led to the proposal that chitosan-polyphenol complexes are a conserved architectural motif in fungal walls ( 16 ).…”

A conserved machinery underlies the synthesis of a chitosan layer in theCandidachlamydospore cell wall

“…It is possible that a similar mechanism occurs during chlamydospore formation. An alternative possibility was recently suggested by nuclear magnetic resonance (NMR) studies of chitosan-containing cell wall preparations from both S. cerevisiae and Cryptococcus neoformans that revealed neutral lipids are directly incorporated into the cell wall (16, 58). Thus, the Rrt8, Srt1, and Mum3 proteins may be involved in the synthesis of some lipid component that is then transferred from the lipid droplet to play a structural role during chitosan layer assembly.…”

“…Cryptococcus strains unable to synthesize chitosan are avirulent (13)(14)(15). Chitosan is often found in conjunction with polyphenolic compounds, which has led to the proposal that chitosanpolyphenol complexes are a conserved architectural motif in fungal walls (16).…”

“…For example, in the pathogen Cryptococcus neoformans , chitosan in the wall dampens the host inflammatory response, and Cryptococcus strains unable to synthesize chitosan are avirulent (13-15). Chitosan is often found in conjunction with polyphenolic compounds, which has led to the proposal that chitosan-polyphenol complexes are a conserved architectural motif in fungal walls (16).…”