The bZIP transcription factor HapX is regulated at multiple levels to control iron homeostasis in Aspergillus fumigatus

HapX is a bZIP transcription factor required for maintenance of iron homeostasis in filamentous fungi. HapX-mediated adaptation to iron-limiting conditions was shown to be crucial for virulence of several fungal species in animal and plant hosts, including the pathogenicity of the opportunistic human pathogen Aspergillus fumigatus in animal models for aspergillosis. The differences in iron homeostasis-maintaining mechanisms between fungi and mammals might serve to improve therapy and diagnosis of fungal infections. Previously, expression of hapX was shown to be repressed by iron at the transcriptional level, partially dependent on the GATA factor SreA. Here we show that HapX protein stability is very low during iron starvation and decreases dramatically during a shift from iron depleted to iron-replete conditions, while siderophore biosynthetic enzymes appear to be quite stable. Immunoprecipitation followed by MS/MS analysis identified several phosphorylation sites in HapX and site-directed mutagenesis identified one phosphorylation site that determined the HapX protein stability. Additionally, our studies demonstrate that iron also controls the stability of hapX mRNA. Taken together, our results show that expression of hapX is subject to indicate that regulation of HapX is subject to multiple levels of control in Aspergillus fumigatus.