Identification of a major IP(5) kinase in Cryptococcus neoformans confirms that PP-IP(5)/IP(7), not IP(6), is essential for virulence

Fungal inositol polyphosphate (IP) kinases catalyse phosphorylation of IP(3) to inositol pyrophosphate, PP-IP(5)/IP(7), which is essential for virulence of Cryptococcus neoformans. Cryptococcal Kcs1 converts IP(6) to PP-IP(5)/IP(7), but the kinase converting IP(5) to IP(6) is unknown. Deletion of a putative IP(5) kinase-encoding gene (IPK1) alone (ipk1Δ), and in combination with KCS1 (ipk1Δkcs1Δ), profoundly reduced virulence in mice. However, deletion of KCS1 and IPK1 had a greater impact on virulence attenuation than that of IPK1 alone. ipk1Δkcs1Δ and kcs1Δ lung burdens were also lower than those of ipk1Δ. Unlike ipk1Δ, ipk1Δkcs1Δ and kcs1Δ failed to disseminate to the brain. IP profiling confirmed Ipk1 as the major IP(5) kinase in C. neoformans: ipk1Δ produced no IP(6) or PP-IP(5)/IP(7) and, in contrast to ipk1Δkcs1Δ, accumulated IP(5) and its pyrophosphorylated PP-IP(4) derivative. Kcs1 is therefore a dual specificity (IP(5) and IP(6)) kinase producing PP-IP(4) and PP-IP(5)/IP(7). 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 ipk1Δ, suggesting that PP-IP(4) partially compensates for absent PP-IP(5)/IP(7) in ipk1Δ grown under this condition. In conclusion, PP-IP(5)/IP(7), not IP(6), is essential for fungal virulence.