Department of Microbiology, Ohio State University, Columbus, OH.
Department of Plant Pathology, Ohio State University, Columbus, OH.
Mol Biol Evol. 2021 Apr 13;38(4):1339-1355. doi: 10.1093/molbev/msaa293.
Chitinases enzymatically hydrolyze chitin, a highly abundant and utilized polymer of N-acetyl-glucosamine. 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 (GH18) domain. This large-scale analysis does not support the previous division of fungal chitinases into three major clades (A, B, C) as chitinases previously assigned to the "C" clade are not resolved as distinct from the "A" clade. 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 (including the LysM domain) do not define specific clades, but instead are found more broadly across clades of chitinases. To gain insight into biological function diversity, we characterized all eight chitinases (Cts) from the thermally dimorphic fungus, Histoplasma capsulatum: six A clade, one B clade, and one formerly classified C clade 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. These results suggest chitinase activity is variable even within subclades and that predictions of functionality require more sophisticated models.
几丁质酶通过酶促水解作用分解几丁质，几丁质是一种高度丰富和利用的 N-乙酰葡萄糖胺聚合物。真菌是几丁质酶的丰富来源；然而，真菌几丁质酶的系统发育和功能多样性尚未得到很好的理解。我们从 373 个公开可用的基因组中调查了真菌几丁质酶，对其结构域结构进行了表征，并对糖苷水解酶 (GH18) 结构域进行了系统发育分析。这项大规模分析不支持以前将真菌几丁质酶分为三大类群 (A、B、C) 的观点，因为以前被归类为“C”类群的几丁质酶与“A”类群没有明显区分。真菌几丁质酶的多样性部分是由水平基因转移形成的，至少有一个起源于细菌的类群存在于以前被归类为“B”类群的几丁质酶中。此外，几丁质结合结构域（包括 LysM 结构域）不能定义特定的类群，而是更广泛地存在于几丁质酶的各个类群中。为了深入了解生物功能的多样性，我们对嗜热双相真菌荚膜组织胞浆菌中的 8 种几丁质酶（Cts）进行了特征描述：6 种 A 类群、1 种 B 类群和 1 种以前归类为 C 类群的几丁质酶。表达分析表明，在几丁质存在的情况下，几丁质酶基因的诱导表达存在差异，但在菌丝体阶段 CTS3 优先表达。活性测定表明，Cts1（B-I）、Cts2（A-V）、Cts3（A-V）、Cts4（A-V）具有不同程度的壳二糖酶功能的内切几丁质酶活性。Cts6（C-I）具有与 N-乙酰氨基葡萄糖苷酶外切几丁质酶功能一致的活性，而 Cts8（A-II）具有壳二糖酶活性。这些结果表明，即使在亚类群内，几丁质酶的活性也存在差异，并且功能预测需要更复杂的模型。
