Abstract
The ability of Candida albicans to switch between yeast to hyphal form is a property that is primarily associated with the invasion and virulence of this human pathogenic fungus. Several glycosylphosphatidylinositol (GPI)-anchored proteins are expressed only during hyphal morphogenesis. One of the major pathways that controls hyphal morphogenesis is the Ras-signaling pathway. We examine the cross-talk between GPI anchor biosynthesis and Ras signaling in C. albicans. We show that the first step of GPI biosynthesis is activated by Ras in C. albicans This is diametrically opposite to what is reported in Saccharomyces cerevisiae Of the two C. albicans Ras proteins, CaRas1 alone activates GPI-GnT activity; activity is further stimulated by constitutively activated CaRas1. CaRas1 localized to the cytoplasm or endoplasmic reticulum (ER) is sufficient for GPI-GnT activation. Of the six subunits of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) that catalyze the first step of GPI biosynthesis, CaGpi2 is the key player involved in activating Ras signaling and hyphal morphogenesis. Activation of Ras signaling is independent of the catalytic competence of GPI-GnT. This too is unlike what is observed in S. cerevisiae where multiple subunits were identified as inhibiting Ras2. Fluorescence resonance energy transfer (FRET) studies indicate a specific physical interaction between CaRas1 and CaGpi2 in the ER, which would explain the ability of CaRas1 to activate GPI-GnT. CaGpi2, in turn, promotes activation of the Ras-signaling pathway and hyphal morphogenesis. The Cagpi2 mutant is also more susceptible to macrophage-mediated killing, and macrophage cells show better survival when co-cultured with Cagpi2.
Highlights
The ability of Candida albicans to switch between yeast to hyphal form is a property that is primarily associated with the invasion and virulence of this human pathogenic fungus
We examine the cross-talk between GPI anchor biosynthesis and Ras signaling in C. albicans
Of the six subunits of the GPI–N-acetylglucosaminyltransferase (GPI–GnT) that catalyze the first step of GPI biosynthesis, CaGpi2 is the key player involved in activating Ras signaling and hyphal morphogenesis
Summary
The GPI–GnT enzyme in C. albicans is made up of six putative subunits, based on sequence homology with S. cerevisiae. The expression levels of all the GPI–GnT genes are up-regulated in the CaGPI2Hz strain, whereas that of CaGPI2 is reduced (Fig. 5C, panel ii) It appears that a certain optimal stoichiometry of the subunits is required for constitution of an active GPI–GnT enzyme complex. Because it was observed that CaRas best stimulates the GPI–GnT complex when it is present in the cytoplasm or at the ER, and because CaGpi was identified as the only subunit capable of activating hyphal growth via the Ras-signaling pathway, we wondered whether a physical interaction exists between CaRas and CaGpi2 To this end, we first examined the localization of the two proteins. The survival rates for MH-S cells co-cultured with CaGPI2Hz cells are higher, suggesting that the CaGPI2Hz strain is likely to be less virulent than the WT strain
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