Basidiomycetes secretomes: an under-exploited tank of new plant-polysaccharide-depolymerizing enzymes
Fungi are essential for the global carbon cycle as they use plant biomass by producing enzymes that degrade plant cell wall polysaccharides into metabolizable sugars. Plant-polysaccharide-depolymerizing enzymes are of great interest to biotechnology, as the products of their catalysis can be used as precursors in the processes that generate bio-based products, e.g., fuels and chemicals (1). The enzymes degrading or modifying plant polysaccharides are classified as carbohydrate-active enzymes (CAZymes) and are divided into families according to their amino acid sequence and structural similarity (2). The CAZy database is organized into families of glycoside hydrolases, carbohydrate esterases, polysaccharide lyases, glycosyltransferases, and auxiliary activities. Our knowledge of basidiomycetes regarding their ability to decompose plant polysaccharides is limited compared to the wealth of information on ascomycetes, due largely to the traditional and well-established industrial relevance of several ascomycetes. In nature, basidiomycetes are the most efficient degraders of woody biomass and therefore contain a huge potential for applications in various industries, which has so far remained largely unexplored (3,4). The identification of lignocellulose-degrading enzymes is based on their sequence similarity to members of known sequence-based families of CAZymes on the CAZy database. But key enzymes of industrial interest might not be homologous to known CAZymes, preventing their identification and characterization. One of the most promising approaches to discover new catalytic functions is the analysis of the secreted enzymes (secretomes) produced when fungi are cultured on lignocellulosic substrates. This approach undertaken in our group thanks to –omic approaches has led to the identification of several putative new CAZymes among the secretomes of a white-rot basidiomycete fungus, Pycnoporus coccineus (5).