Abstract
Iron is an essential metal cofactor that is required for many biological processes. Eukaryotic cells have consequently developed different strategies for its acquisition. Until now, Schizosaccharomyces pombe was known to use reductive iron uptake and siderophore-bound iron transport to scavenge iron from the environment. Here, we report the identification of a gene designated shu1(+) that encodes a protein that enables S. pombe to take up extracellular heme for cell growth. When iron levels are low, the transcription of shu1(+) is induced, although its expression is repressed when iron levels rise. The iron-dependent down-regulation of shu1(+) requires the GATA-type transcriptional repressor Fep1, which strongly associates with a proximal promoter region of shu1(+) in vivo in response to iron repletion. HA4-tagged Shu1 localizes to the plasma membrane in cells expressing a functional shu1(+)-HA4 allele. When heme biosynthesis is selectively blocked in mutated S. pombe cells, their ability to acquire exogenous hemin or the fluorescent heme analog zinc mesoporphyrin IX is dependent on the expression of Shu1. Further analysis by absorbance spectroscopy and hemin-agarose pulldown assays showed that Shu1 interacts with hemin, with a KD of ∼2.2 μm. Taken together, results reported here revealed that S. pombe possesses an unexpected pathway for heme assimilation, which may also serve as a source of iron for cell growth.
Highlights
Organisms have evolved diverse strategies to scavenge extracellular iron
Heme is essential for growth of pathogenic fungi such as C. albicans, C. parapsilosis, and C. neoformans, which have evolved mechanisms to acquire iron from this source [12]
In the cases of C. albicans and C. parapsilosis, a group of proteins involved in this process are members of the CFEM family [14, 18]
Summary
Organisms have evolved diverse strategies to scavenge extracellular iron. Results: The iron-regulated cell-surface Shu protein serves as a novel component for heme acquisition. Besides the cell-surface glycosylphosphatidylinositol (GPI)-anchored proteins Rbt and Pga, additional C. albicans proteins are involved in exogenous heme acquisition. These proteins include heme oxygenase Hmx1 [15, 16], vacuolar ATPase Vma, and proteins of the ESCRT (endosomal sorting complex required for transport) system that may be involved in heme trafficking to the vacuole for processing and its utilization as a source of iron [17]. When a hem1⌬ shu1⌬ mutant strain was incubated in the absence of ALA and in the presence of hemin, cells were unable to grow unless an untagged shu1ϩ or HA4-tagged shu1ϩ allele was re-integrated and expressed in this mutant strain This result revealed that Shu allowed the hem1⌬ shu1⌬ mutant to regain its capacity to acquire hemin. These results led to the interpretation that Shu supports iron acquisition from extracellular heme under low iron conditions
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