Identification of a major IP5 kinase in Cryptococcus neoformans confirms that PP-IP5/IP7, not IP6, is essential for virulence

Fungal inositol polyphosphate (IP) kinases catalyse phosphorylation of IP3 to inositol pyrophosphate, PP-IP5/IP7, which is essential for virulence of Cryptococcus neoformans. Cryptococcal Kcs1 converts IP6 to PP-IP5/IP7, but the kinase converting IP5 to IP6 is unknown. Deletion of a putative IP5 kinase-encoding gene (IPK1) alone (ipk1 Delta), and in combination with KCS1 (ipk1 Delta kcs1 Delta), profoundly reduced virulence in mice. However, deletion of KCS1 and IPK1 had a greater impact on virulence attenuation than that of IPK1 alone. ipk1 Delta kcs1 Delta and kcs1 Delta lung burdens were also lower than those of ipk1 Delta. Unlike ipk1 Delta, ipk1 Delta kcs1 Delta and kcs1 Delta failed to disseminate to the brain. IP profiling confirmed Ipk1 as the major IP5 kinase in C. neoformans: ipk1 Delta produced no IP6 or PP-IP5/IP7 and, in contrast to ipk1 Delta kcs1 Delta, accumulated IP5 and its pyrophosphorylated PP-IP4 derivative. Kcs1 is therefore a dual specificity (IP5 and IP6) kinase producing PP-IP4 and PP-IP5/IP7. All mutants were similarly attenuated in virulence phenotypes including laccase, urease and growth under oxidative/nitrosative stress. Alternative carbon source utilisation was also reduced significantly in all mutants except ipk1 Delta, suggesting that PP-IP4 partially compensates for absent PP-IP5/IP7 in ipk1 Delta grown under this condition. In conclusion, PP-IP5/IP7, not IP6, is essential for fungal virulence.