The structural unit of melanin in the cell wall of the fungal pathogen Cryptococcus neoformans

Melaninsare synthesized macromolecules that are found in all biological kingdoms. These pigments have a myriad of roles that range from microbial virulence to key components of the innate immune response in invertebrates. Melaninsalso exhibit unique properties with potential applications in physics and material sciences, ranging from electrical batteriesto novel therapeutics. In the fungi, melaninssuch as eumelanins, are components of the cell wallthat provide protection against biotic and abiotic elements. Elucidation of the smallest fungal cell wall- asociated melaninunit that serves as a building block is critical to understand the architecture of these polymers, its interaction with surrounding components, and their functional versatility. In this study, we used isopycnicgradient sedimentation, NMR, EPR, high-resolution microscopy, and proteomics to analyze the melaninin the cell wallof the human pathogenic fungus Cryptococcus neoformans. We observed that melaninis assembled into the cryptococcal cell wallin spherical structures of ~200 nm in diameter, termed melaningranules, which are in turn composed of nanospheres of ~30 nm in diameter, the fungal melanosomes. We noted that melaningranules are closely associated with proteins that may play critical roles in the fungal melanogenesis and the supramolecular structure of this polymer. Using this structural information, we propose a model for C. neoformans melanizationthat is similar to the process used in animal melanizationand is consistent with the phylogenetic relatedness of the fungal and animal kingdoms.