Liu Peng, Stajich Jason E
Department of Plant Pathology & Microbiology and Institute for Integrative Genome Biology, University of California, Riverside, CA 92521, USA.
Department of Plant Pathology & Microbiology and Institute for Integrative Genome Biology, University of California, Riverside, CA 92521, USA.
Fungal Genet Biol. 2015 Apr;77:31-9. doi: 10.1016/j.fgb.2015.03.003. Epub 2015 Mar 25.
Batrachochytrium dendrobatidis (Bd) is the causative agent of chytridiomycosis responsible for worldwide decline in amphibian populations. Previous analysis of the Bd genome revealed a unique expansion of the carbohydrate-binding module family 18 (CBM18) predicted to be a sub-class of chitin recognition domains. CBM expansions have been linked to the evolution of pathogenicity in a variety of fungal species by protecting the fungus from the host. Based on phylogenetic analysis and presence of additional protein domains, the gene family can be classified into 3 classes: Tyrosinase-, Deacetylase-, and Lectin-like. Examination of the mRNA expression levels from sporangia and zoospores of nine of the cbm18 genes found that the Lectin-like genes had the highest expression while the Tyrosinase-like genes showed little expression, especially in zoospores. Heterologous expression of GFP-tagged copies of four CBM18 genes in Saccharomyces cerevisiae demonstrated that two copies containing secretion signal peptides are trafficked to the cell boundary. The Lectin-like genes cbm18-ll1 and cbm18-ll2 co-localized with the chitinous cell boundaries visualized by staining with calcofluor white. In vitro assays of the full length and single domain copies from CBM18-LL1 demonstrated chitin binding and no binding to cellulose or xylan. Expressed CBM18 domain proteins were demonstrated to protect the fungus, Trichoderma reeseii, in vitro against hydrolysis from exogenously added chitinase, likely by binding and limiting exposure of fungal chitin. These results demonstrate that cbm18 genes can play a role in fungal defense and expansion of their copy number may be an important pathogenicity factor of this emerging infectious disease of amphibians.
蛙壶菌（Batrachochytrium dendrobatidis，Bd）是壶菌病的病原体，它导致了全球两栖动物数量的减少。先前对Bd基因组的分析显示，碳水化合物结合模块家族18（CBM18）有独特的扩增，预计它是几丁质识别域的一个亚类。CBM的扩增通过保护真菌免受宿主影响，与多种真菌物种致病性的进化相关联。基于系统发育分析和其他蛋白质结构域的存在，该基因家族可分为3类：酪氨酸酶样、脱乙酰酶样和凝集素样。对9个cbm18基因的孢子囊和游动孢子的mRNA表达水平进行检测发现，凝集素样基因的表达最高，而酪氨酸酶样基因的表达很少，尤其是在游动孢子中。在酿酒酵母中对4个CBM18基因的绿色荧光蛋白标记拷贝进行异源表达，结果表明，两个含有分泌信号肽的拷贝被运输到细胞边界。凝集素样基因cbm18-ll1和cbm18-ll2与用荧光增白剂染色显示的几丁质细胞边界共定位。对CBM18-LL1的全长和单结构域拷贝进行的体外分析表明，它们能结合几丁质，而不结合纤维素或木聚糖。表达的CBM18结构域蛋白在体外可保护里氏木霉免受外源添加的几丁质酶的水解，可能是通过结合并限制真菌几丁质的暴露来实现的。这些结果表明，cbm18基因可在真菌防御中发挥作用，其拷贝数的增加可能是这种两栖动物新出现的传染病的一个重要致病因素。
