Diversification of fungal chitinases and their functional differentiation in Histoplasma capsulatum

Chitinases enzymatically hydrolyze chitin, a highly abundant biomolecule with many potential industrial and medical uses in addition to their natural biological roles. Fungi are a rich source of chitinases, however the phylogenetic and functional diversity of fungal chitinases are not well understood. We surveyed fungal chitinases from 373 publicly available genomes, characterized domain architecture, and conducted phylogenetic analyses of the glycoside hydrolase family 18 (GH18) domain. This large-scale analysis does not support the previous division of fungal chitinases into three major clades (A, B, C). The chitinases previously assigned to the "C" clade are not resolved as distinct from the "A" clade in this larger phylogenetic analysis. Fungal chitinase diversity was partly shaped by horizontal gene transfer, and at least one clade of bacterial origin occurs among chitinases previously assigned to the "B" clade. Furthermore, chitin binding domains (CBD) including the LysM domain do not define specific clades but instead are found more broadly across clades of chitinase enzymes. To gain insight into biological function diversity, we characterized all eight chitinases (Cts) from the thermally dimorphic fungus, Histoplasma capsulatum: six A clade (3 A-V, 1 A-IV, and two A-II), one B clade (B-I), and one formerly classified C clade (C-I) chitinases. Expression analyses showed variable induction of chitinase genes in the presence of chitin but preferential expression of CTS3 in the mycelial stage. Activity assays demonstrated that Cts1 (B-I), Cts2 (A-V), Cts3 (A-V), Cts4 (A-V) have endochitinase activities with varying degrees of chitobiosidase function. Cts6 (C-I) has activity consistent with N-acetyl-glucosaminidase exochitinase function and Cts8 (A-II) has chitobiase activity. This suggests chitinase activity is variable even within sub-clades and that predictions of functionality require more sophisticated models.