A relay network of extracellular heme-binding proteins drives C. albicans iron acquisition from hemoglobin

PLoS Pathog. 2014 Oct 2;10(10):e1004407. doi: 10.1371/journal.ppat.1004407. eCollection 2014 Oct.

Abstract

Iron scavenging constitutes a crucial challenge for survival of pathogenic microorganisms in the iron-poor host environment. Candida albicans, like many microbial pathogens, is able to utilize iron from hemoglobin, the largest iron pool in the host's body. Rbt5 is an extracellular glycosylphosphatidylinositol (GPI)-anchored heme-binding protein of the CFEM family that facilitates heme-iron uptake by an unknown mechanism. Here, we characterize an additional C. albicans CFEM protein gene, PGA7, deletion of which elicits a more severe heme-iron utilization phenotype than deletion of RBT5. The virulence of the pga7-/- mutant is reduced in a mouse model of systemic infection, consistent with a requirement for heme-iron utilization for C. albicans pathogenicity. The Pga7 and Rbt5 proteins exhibit distinct cell wall attachment, and discrete localization within the cell envelope, with Rbt5 being more exposed than Pga7. Both proteins are shown here to efficiently extract heme from hemoglobin. Surprisingly, while Pga7 has a higher affinity for heme in vitro, we find that heme transfer can occur bi-directionally between Pga7 and Rbt5, supporting a model in which they cooperate in a heme-acquisition relay. Together, our data delineate the roles of Pga7 and Rbt5 in a cell surface protein network that transfers heme from extracellular hemoglobin to the endocytic pathway, and provide a paradigm for how receptors embedded in the cell wall matrix can mediate nutrient uptake across the fungal cell envelope.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Candida albicans / metabolism*
  • Candida albicans / pathogenicity
  • Carrier Proteins / metabolism*
  • Cell Wall / metabolism
  • Extracellular Space / metabolism
  • Glycosylphosphatidylinositols / metabolism
  • Heme / metabolism*
  • Heme-Binding Proteins
  • Hemeproteins / metabolism*
  • Hemoglobins / metabolism*
  • Humans
  • Iron / metabolism*
  • Mice
  • Virulence / immunology

Substances

  • Carrier Proteins
  • Glycosylphosphatidylinositols
  • Heme-Binding Proteins
  • Hemeproteins
  • Hemoglobins
  • Heme
  • Iron

Grant support

This research project was supported by a grant from the Israeli Ministry of Health's Chief Scientist Office (DK). The NMR characterization was conducted through the FP7 Access to Research Infrastructures (Bio-NMR contract 261863) (PT) and by Instruct, which is part of the European Strategy Forum on Research Infrastructures (ESFRI) and supported by national member subscriptions. Specifically, we thank the Instruct Centre CERM (Italy). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.