IP(7)-SPX Domain Interaction Controls Fungal Virulence by Stabilizing Phosphate Signaling Machinery

In the human-pathogenic fungus Cryptococcus neoformans, the inositol polyphosphate signaling pathway is critical for virulence. We recently demonstrated the key role of the inositol pyrophosphate IP(7) (isomer 5-PP-IP(5)) in driving fungal virulence; however, the mechanism of action remains elusive. Using genetic and biochemical approaches, and mouse infection models, we show that IP(7) synthesized by Kcs1 regulates fungal virulence by binding to a conserved lysine surface cluster in the SPX domain of Pho81. Pho81 is the cyclin-dependent kinase (CDK) inhibitor of the phosphate signaling (PHO) pathway. We also provide novel mechanistic insight into the role of IP(7) in PHO pathway regulation by demonstrating that IP(7) functions as an intermolecular “glue” to stabilize Pho81 association with Pho85/Pho80 and, hence, promote PHO pathway activation and phosphate acquisition. Blocking IP(7)-Pho81 interaction using site-directed mutagenesis led to a dramatic loss of fungal virulence in a mouse infection model, and the effect was similar to that observed following PHO81 gene deletion, highlighting the key importance of Pho81 in fungal virulence. Furthermore, our findings provide additional evidence of evolutionary divergence in PHO pathway regulation in fungi by demonstrating that IP(7) isomers have evolved different roles in PHO pathway control in C. neoformans and nonpathogenic yeast.