Species Context Reverses PPIP5K Control of Fungal Morphogenesis and Actin Organization
Inositol pyrophosphates are conserved signaling molecules synthesized by the bifunctional PPIP5K enzymes, but how their cellular functions diversify across species remains poorly understood. Here, we compared the PPIP5K enzyme Asp1 in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus and in the distantly related fungus Ustilago maydis. All three homologs retained a conserved kinase-phosphatase architecture and catalytic activity. However, whereas Asp1 produced broadly similar effects on actin organization and morphogenesis in S. pombe and U. maydis, its regulatory output was reversed in S. japonicus. In S. pombe and U. maydis, Asp1 positively supported Arp2/3-dependent actin functions, as loss of Asp1 increased sensitivity to the Arp2/3 inhibitor CK666. In contrast, deletion of asp1 in S. japonicus conferred strong CK666 resistance and caused excessive, spatially deregulated actin-patch organization. This opposing cytoskeletal phenotype was mirrored at the level of morphogenesis: Asp1 restricted the yeast-to-hypha transition in S. japonicus, whereas Asp1 was required for pseudohyphal growth in S. pombe and for filamentous development in U. maydis. However, the negative regulatory activity observed in S. japonicus was not an intrinsic property of the SjAsp1 protein. When expressed in S. pombe, SjAsp1 promoted invasive pseudohyphal growth, reproducing the regulatory output of the S. pombe Asp1 morphogenesis pathway rather than that of its native species. Similarly, SjAsp1 supported Arp2/3 functions when expressed in S. pombe. Thus, SjAsp1 adopted the functional behavior imposed by the host cellular environment. S. pombe Asp1 was originally identified as a suppressor of Arp2/3-complex mutant phenotypes, establishing a genetic connection between Asp1 and the actin nucleator. Extending this link, affinity enrichment with inositol pyrophosphates reagents recovered all seven subunits of the S. pombe Arp2/3 complex, providing biochemical support for a potential association between inositol pyrophosphate and Arp2/3. Together, these findings identify S. japonicus as a functional outlier in which a conserved PPIP5K pathway produces an opposing biological output. They further demonstrate that this divergence is determined primarily by species-specific cellular networks rather than by intrinsic differences in the Asp1 protein.