Background: GPI anchor is essential for virulence of C. albicans. Little is known about its GPI biosynthetic pathway. We explore roles of two GPI-N-acetylglucosaminyltransferase subunits catalyzing the first step. Results: Subunits GPI2 and GPI19 are negatively co-regulated, affecting Ras1 activity and ERG11 levels, respectively. Conclusion: GPI2/GPI19 levels affect morphogenesis and ergosterol biosynthesis. Significance: C. albicans can be targeted by modulation of cross-talk among major pathways.
The ability of 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 We show that the first step of GPI biosynthesis is activated by Ras in This is diametrically opposite to what is reported in Of the two 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--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 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 mutant is also more susceptible to macrophage-mediated killing, and macrophage cells show better survival when co-cultured with .
Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are present ubiquitously at the cell surface in all eukaryotes. They play a crucial role in the interaction of the cell with its external environment, allowing the cell to receive signals, respond to challenges, and mediate adhesion. In yeast and fungi, they also participate in the structural integrity of the cell wall and are often essential for survival. Roughly four decades after the discovery of the first GPI-APs, this review provides an overview of the insights gained from studies of the GPI biosynthetic pathway and the future challenges in the field. In particular, we focus on the biosynthetic pathway in Saccharomyces cerevisiae, which has for long been studied as a model organism. Where available, we also provide information about the GPI biosynthetic steps in other yeast/ fungi. Although the core structure of the GPI anchor is conserved across organisms, several variations are built into the biosynthetic pathway. The present Review specifically highlights these variations and their implications. There is growing evidence to suggest that several phenotypes are common to GPI deficiency and should be expected in GPI biosynthetic mutants. However, it appears that several phenotypes are unique to a specific step in the pathway and may even be species-specific. These could suggest the points at which the GPI biosynthetic pathway intersects with other important cellular pathways and could be points of regulation. They could be of particular significance in the study of pathogenic fungi and in identification of new and specific antifungal drugs/ drug targets. © 2018 IUBMB Life, 70(5):355-383, 2018.
Ras signaling in response to environmental cues is critical for cellular morphogenesis in eukaryotes. This signaling is tightly regulated and its activation involves multiple players. Sometimes Ras signaling may be hyperactivated. In C. albicans, a human pathogenic fungus, we demonstrate that dynamics of hyperactivated Ras1 (Ras1G13V or Ras1 in Hsp90 deficient strains) can be reliably differentiated from that of normal Ras1 at (near) single molecule level using fluorescence correlation spectroscopy (FCS). Ras1 hyperactivation results in significantly slower dynamics due to actin polymerization. Activating actin polymerization by jasplakinolide can produce hyperactivated Ras1 dynamics. In a sterol-deficient hyperfilamentous GPI mutant of C. albicans too, Ras1 hyperactivation results from Hsp90 downregulation and causes actin polymerization. Hyperactivated Ras1 co-localizes with G-actin at the plasma membrane rather than with F-actin. Depolymerizing actin with cytochalasin D results in faster Ras1 dynamics in these and other strains that show Ras1 hyperactivation. Further, ergosterol does not influence Ras1 dynamics.
Glycosylphosphatidylinositol (GPI) anchor biosynthesis is a multi‐step pathway found in all eukaryotes. GPI anchored proteins are involved in a variety of functions like adhesion, host cell attachment, invasion and biofilm formation in Candida albicans, a human pathogenic fungus. The first and committed step of the pathway i.e., transfer of GlcNAc from UDP‐GlcNAc to phosphatidylinositol (PI) to form GlcNAc‐PI, involves an enzyme complex (GPI‐GnT) of six subunits viz. CaGpi1, CaGpi2, CaGpi3, CaGpi15, CaGpi19 and CaEri1. A cross‐talk exists between GPI anchor biosynthesis, Ras signaling and ergosterol biosynthesis in Candida albicans, in which CaGPI2 controls hyphal morphogenesis through CaRAS1 while CaGPI19 seems to regulate azole drug response via co‐regulation of CaERG11. Interestingly, both CaGPI2 and CaGPI19 also negatively regulate each other. Further investigations on how Ras signaling cross‐talks with GPI biosynthesis in C. albicans suggest that Ras activates GPI‐GnT activity, which runs counter to what is reported in Saccharomyces cerevisiae. Out of the two Ras proteins of Candida albicans, CaRas1 alone activates the GPI‐GnT activity. Different forms of CaRas1 too can activate the GPI‐GnT in vivo. This evidence is further strengthened by the fact that CaRas1 physically interacts with CaGpi2, as evidenced by FRET. Furthermore, CaGpi2 appears to be important for virulence since a heterozygous mutant of CaGPI2 is unable to kill macrophage cells and the mutant is more susceptible for macrophage‐mediated killing. Current work is focused on identifying the domains of interaction between CaGpi2 and CaRas1. Understanding this interaction will enable us to target any or both of these molecules to control the virulence of this pathogen. Support or Funding Information SSK received funding from Department of Science and Technology (DST) India Grant SB/SO/BB‐011/2014 and an umbrella grant to the School of Life Sciences from the DST under Promotion of University research and Scientific Excellence Grant DST PURSE No. SR/PURSE/PHASE 2/11 (C), and Department of Biotechnology (DBT) India Grant BT/PR3749/BRB/10/986/2011. SCS received Junior and Senior Research Fellowships from University Grants Commission (UGC), India.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.