Ras activates the first step of GPI anchor biosynthesis in Candida albicans

Abstract

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.