Surface exposed and charged residues drive thermostability in fungi

Fungi, though mesophilic, include thermophilic and thermostable species, as well. The thermostability of proteins observed in these fungi is most likely to be attributed to several molecular factors, such as the presence of salt bridges and hydrogen bond interactions between side chains. These factors cannot be generalized for all fungi. Factors impacting thermostability can guide how fungal thermophilic proteins gain thermostability. We curated a dataset of proteins for 14 thermophilic fungi and their evolutionarily closer mesophiles. Additionally, the proteome of Chaetomium thermophilum and its evolutionarily related mesophile Chaetomium globosum was analyzed. Using eggNOG, we categorized the proteomes into clusters of orthologous groups (COGs). While the individual count of proteins is over-represented in mesophiles (for COGs S, G, L, and Q), there are certain features that are significantly enriched in thermophiles (such as charged residues, exposed residues, polar residues, etc.). Since fungi are known to be cellulolytic and chitinolytic by nature, we selected 37 existing carbohydrate-active enzymes (CAZyme) families in Eurotiales, Mucorales, and Sordariales. We looked at closely similar sequences and their modeled structures for further comparison. Comparing solvent accessibilities of thermophilic and mesophilic proteins, exposed and intermediate residues are observed higher in thermophiles whereas buried residues are observed higher in mesophiles. For specific five CAZYme families (GH7, GH11, GH18, GH45, and CBM1) we looked at position-specific substitutions between thermophiles and mesophiles. We also found that there are relatively more intramolecular interactions in thermophiles compared to mesophiles. Thus, we found factors such as surface exposed residues and charged residues that are highly likely to impart thermostability in fungi, and this study sets the stage for further studies in the area of fungal thermostability.