The polymorphic species Candida albicans is the major cause of candidiasis in humans. The secreted aspartyl proteinases (Saps) of C. albicans, encoded by a family of 10 SAP genes, have been investigated as the virulent factors during candidiasis. However, the biological functions of most Sap proteins are still uncertain. In this study, we applied co-culture system of C. albicans and THP-1 human monocytes to explore the pathogenic roles and biological functions of Sap proteinases. After 1 hr of co-culture of C. albicans strains and THP-1 human monocytes at 37°C, more than 60% of the THP-1-engulfed wild type and Δsap5 Candida cells were developing long hyphae. However, about 50% of THP-1-engulfed Δsap6 Candida cells were generating short hyphae, and more dead Candida cells were found in Δsap6 strain that was ingested by THP-1 cells (about 15% in Δsap6 strain vs. 2 ~ 2.5% in SC5314 and Δsap5 strains). The immunofluorescence staining demonstrated that the Sap6 is the major hyphal tip located Sap protein under THP-1 phagocytosis. The sap6-deleted strains (Δsap6, Δsap4/6, and Δsap5/6) appeared slower growth on Congo red containing solid medium at 25°C, and the growth defect was exacerbated when cultured at 37°C in Congo red or SDS containing medium. In addition, more proteins were secreted from Δsap6 strain and the β-mercaptoethanol (β-ME) extractable surface proteins from Δsap6 mutant were more abundant than that of extracted from wild type strain, which included the plasma membrane protein (Pma1p), the ER-chaperone protein (Kar2p), the protein transport-related protein (Arf1p), the cytoskeleton protein (Act1), and the mitochondrial outer membrane protein (porin 1). Moreover, the cell surface accessibility was increased in sap6-deleted strains. From these results, we speculated that the cell surface constitution of C. albicans Δsap6 strain was defect. This may cause the more accessible of β-ME to disulfide-bridged cell surface components and may weaken the resistance of Δsap6 strain encountering phagocytosis of THP-1 cells. Sap6 protein displays a significant function involving in maintenance the cell surface integrity.
多形性物种白色念珠菌是人念珠菌病的主要病因。白色念珠菌分泌的
天冬氨酸蛋白酶(
SAPs)由一组10个
SAP基因编码,已被研究作为念珠菌病的毒力因子。然而,大多数
SAP蛋白的
生物学功能仍然不确定。在本研究中,我们采用了白色念珠菌和THP-1人单核细胞的共培养系统来探索
SAP蛋白酶的致病作用和
生物学功能。在37°C下共培养白色念珠菌株和THP-1人单核细胞1小时后,超过60%的THP-1吞噬的野生型和Δ
SAP5白色念珠菌细胞正在发育长菌丝。然而,大约50%的THP-1吞噬的Δ
SAP6白色念珠菌细胞正在产生短菌丝,并且在被THP-1细胞吞噬的Δ
SAP6菌株中发现了更多的死白色念珠菌细胞(Δ
SAP6菌株中约15%,而SC5314和Δ
SAP5菌株中约2~2.5%)。
免疫荧光染色证明,
SAP6是THP-1吞噬作用下位于菌丝顶端的主要
SAP蛋白。
SAP6缺失菌株(Δ
SAP6,Δ
SAP4/6和Δ
SAP5/6)在含有刚果红的固体
培养基上在25°C下生长较慢,并且在含有刚果红或
SDS的
培养基中在37°C下培养时生长缺陷加剧。此外,从Δ
SAP6菌株分泌的蛋白更多,并且从Δ
SAP6突变体提取的可溶于β-巯基
乙醇(β-ME)的表面蛋白比从野生型菌株提取的更丰富,其中包括质膜蛋白(Pma1p)、内质网伴侣蛋白(Kar2p)、蛋白质运输相关蛋白(Arf1p)、细胞骨架蛋白(Act1)和线粒体外膜蛋白(孔蛋白1)。此外,
SAP6缺失菌株的细胞表面可及性增加。从这些结果中,我们推测Δ
SAP6白色念珠菌菌株的细胞表层结构存在缺陷。这可能导致β-ME更容易接近二
硫键连接的细胞表面成分,并且可能削弱Δ
SAP6菌株在遇到THP-1细胞的吞噬作用时的抵抗力。
SAP6蛋白在维持细胞表面完整性方面显示出重要的作用。