Physical interaction between Sit1 and Aft1 upregulates FOB uptake activity by inhibiting protein degradation of Sit1 in Saccharomyces cerevisiae

The polyHIS Tract of Yeast AMPK Coordinates Carbon Metabolism with Iron Availability

Energy status in all eukaryotic cells is sensed by AMP-kinases. We have previously found that the poly-histidine tract at the N-terminus of S. cerevisiae AMPK (Snf1) inhibits its function in the presence of glucose via a pH-regulated mechanism. We show here that in the absence of glucose, the poly-histidine tract has a second function, linking together carbon and iron metabolism. Under conditions of iron deprivation, when different iron-intense cellular systems compete for this scarce resource, Snf1 is inhibited. The inhibition is via an interaction of the poly-histidine tract with the low-iron transcription factor Aft1. Aft1 inhibition of Snf1 occurs in the nucleus at the nuclear membrane, and only inhibits nuclear Snf1, without affecting cytosolic Snf1 activities. Thus, the temporal and spatial regulation of Snf1 activity enables a differential response to iron depending upon the type of carbon source. The linkage of nuclear Snf1 activity to iron sufficiency ensures that sufficient clusters are available to support respiratory enzymatic activity and tests mitochondrial competency prior to activation of nuclear Snf1.

Iron Assimilation during Emerging Infections Caused by Opportunistic Fungi with emphasis on Mucorales and the Development of Antifungal Resistance

Iron is a key transition metal required by most microorganisms and is prominently utilised in the transfer of electrons during metabolic reactions. The acquisition of iron is essential and becomes a crucial pathogenic event for opportunistic fungi. Iron is not readily available in the natural environment as it exists in its insoluble ferric form, i.e., in oxides and hydroxides. During infection, the host iron is bound to proteins such as transferrin, ferritin, and haemoglobin. As such, access to iron is one of the major hurdles that fungal pathogens must overcome in an immunocompromised host. Thus, these opportunistic fungi utilise three major iron acquisition systems to overcome this limiting factor for growth and proliferation. To date, numerous iron acquisition pathways have been fully characterised, with key components of these systems having major roles in virulence. Most recently, proteins involved in these pathways have been linked to the development of antifungal resistance. Here, we provide a detailed review of our current knowledge of iron acquisition in opportunistic fungi, and the role iron may have on the development of resistance to antifungals with emphasis on species of the fungal basal lineage order Mucorales, the causative agents of mucormycosis.

Adaptation to iron deficiency in human pathogenic fungi

Iron is an essential micronutrient for virtually all eukaryotic organisms and plays a central role during microbial infections. Invasive fungal diseases are associated with strikingly high rates of mortality, but their impact on human health is usually underestimated. Upon a fungal infection, hosts restrict iron availability in order to limit the growth and virulence of the pathogen. Here, we use two model yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, to delve into the response to iron deficiency of human fungal pathogens, such as Candida glabrata, Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. Fungi possess common and species-specific mechanisms to acquire iron and to control the response to iron limitation. Upon iron scarcity, fungi activate a wide range of elegant strategies to capture and import exogenous iron, mobilize iron from intracellular stores, and modulate their metabolism to economize and prioritize iron utilization. Hence, iron homeostasis genes represent remarkable virulence factors that can be used as targets for the development of novel antifungal treatments.

Iron acquisition in fungal pathogens of humans

The devastating infections that fungal pathogens cause in humans are underappreciated relative to viral, bacterial and parasitic diseases. In recent years, the contributions to virulence of reductive iron uptake, siderophore-mediated uptake and heme acquisition have been identified in the best studied and most life-threatening fungal pathogens: Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. In particular, exciting new work illustrates the importance of iron acquisition from heme and hemoglobin in the virulence of pathogenic yeasts. However, the challenge of establishing how these fungi gain access to hemoglobin in blood and to other sources of heme remains to be fully addressed. Recent studies are also expanding our knowledge of iron uptake in less-well studied fungal pathogens, including dimorphic fungi where new information reveals an integration of iron acquisition with morphogenesis and cell-surface properties for adhesion to host cells. Overall, the accumulating information provides opportunities to exploit iron acquisition for antifungal therapy, and new work highlights the development of specific inhibitors of siderophore biosynthesis and metal chelators for therapeutic use alone or in conjunction with existing antifungal drugs. It is clear that iron-related therapies will need to be customized for specific diseases because the emerging view is that fungal pathogens use different combinations of strategies for iron acquisition in the varied niches of vertebrate hosts.