Cryptococcus neoformans is an opportunistic fungal pathogen that infects ∼280,000 people every year, causing >180,000 deaths. The human immune system recognizes chitin as one of the major cell-wall components of invading fungi, but C. neoformans can circumvent this immunosurveillance mechanism by instead exposing chitosan, the partly or fully deacetylated form of chitin. The natural production of chitosans involves the sequential action of chitin synthases (CHSs) and chitin deacetylases (CDAs). C. neoformans expresses four putative CDAs, three of which have been confirmed as functional enzymes that act on chitin in the cell wall. The fourth (CnCda4/Fpd1) is a secreted enzyme with exceptional specificity for d-glucosamine at its -1 subsite, thus preferring chitosan over chitin as a substrate. We used site-specific mutagenesis to reduce the subsite specificity of CnCda4 by converting an atypical isoleucine residue in a flexible loop region to the bulkier or charged residues tyrosine, histidine, and glutamic acid. We also investigated the effect of CnCda4 deacetylation products on human peripheral blood-derived macrophages, leading to a model explaining the function of CnCda4 during infection. We propose that CnCda4 is used for the further deacetylation of chitosans already exposed on the C. neoformans cell wall (originally produced by CnChs3 and CnCda1 to 3) or released from the cell wall as elicitors by human chitinases, thus making the fungus less susceptible to host immunosurveillance. The absence of CnCda4 during infection could therefore promote the faster recognition and elimination of this pathogen.

译文

新型隐球菌是一种机会性真菌病原体,每年感染约280,000人,导致> 180,000人死亡。人类免疫系统将几丁质识别为入侵真菌的主要细胞壁成分之一,但是新生梭菌可以通过暴露几丁质 (几丁质的部分或完全脱乙酰化形式) 来规避这种免疫监视机制。壳聚糖的自然产生涉及几丁质合酶 (CHSs) 和几丁质脱乙酰基酶 (CDAs) 的顺序作用。新孢子虫表达四种假定的cda,其中三种已被确认为作用于细胞壁中几丁质的功能酶。第四种 (CnCda4/Fpd1) 是一种分泌酶,在its -1亚位点上对d-氨基葡萄糖具有特殊的特异性,因此比几丁质更喜欢壳聚糖作为底物。我们使用位点特异性诱变通过将柔性环区中的非典型异亮氨酸残基转化为更大或带电荷的残基酪氨酸,组氨酸和谷氨酸来降低CnCda4的亚位点特异性。我们还研究了CnCda4去乙酰化产物对人外周血来源的巨噬细胞的影响,从而建立了解释CnCda4在感染过程中的功能的模型。我们建议将CnCda4用于已经暴露在新生梭菌细胞壁上的壳聚糖 (最初由CnChs3和CnCda1至3产生) 或由人几丁质酶作为引发剂从细胞壁释放的进一步去乙酰化,从而使真菌不易受到宿主免疫监视。因此,在感染过程中不存在CnCda4可以促进更快地识别和消除该病原体。

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