Sap6, a secreted aspartyl proteinase, participates in maintenance the cell surface integrity of Candida albicans

Leh-Miauh Buu,Yee-Chun Chen

doi:10.1186/1423-0127-20-101

Abstract

BackgroundThe 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.ResultsAfter 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.ConclusionFrom 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.

Highlights

The polymorphic species Candida albicans is the major cause of candidiasis in humans
Three types of covalently bound cell wall proteins in C. albicans have been described, including: the proteins bound to β-1,6-glucans through a glycosylphosphatidylinositol (GPI) moiety, the Pir proteins attached to β-1,3-glucan by unknown alkali-sensitive bonds, and the proteins retained by disulfide bridges which can be extracted by treatment with reducing agents such as β-mercaptoethanol (β-ME) or dithiothreitol [10,11,12,13]
The characteristics of engulfed C. albicans in THP-1 human monocytes The hypha-associated expression of SAP4-6 genes has been investigated as the potent virulent factors in mouse model of systemic candidiasis [34,41]

Summary

Introduction

The polymorphic species Candida albicans is the major cause of candidiasis in humans. The biological functions of most Sap proteins are still uncertain. We applied co-culture system of C. albicans and THP-1 human monocytes to explore the pathogenic roles and biological functions of Sap proteinases. Depending on the physiological status of the hosts, Candida species may convert from the commensally state to the pathogenic one and may cause disease from mucocutaneous superficial infection to systemic. Cell wall proteins of C. albicans are in general highly mannosylated (mannoproteins) and enriched in the outer surface. They are attached mostly to short chains of β-1,6-glucan, to chitin via β-1,6-glucan, or directly to chitin, and to β-1,3-glucan in a lesser extent. Three types of covalently bound cell wall proteins in C. albicans have been described, including: the proteins bound to β-1,6-glucans through a glycosylphosphatidylinositol (GPI) moiety, the Pir (proteins with internal repeats) proteins attached to β-1,3-glucan by unknown alkali-sensitive bonds (possibly O-glycosidic linkages), and the proteins retained by disulfide bridges which can be extracted by treatment with reducing agents such as β-mercaptoethanol (β-ME) or dithiothreitol [10,11,12,13]

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Conclusion