Ustilago maydis is a dimorphic basidiomycete and the causal agent of corn smut disease.It serves as a genetic model for understanding dimorphism, pathogenicity, and mating response in filamentous fungi.Previous studies indicated the importance of regulated cAMP-dependent protein kinase A (PKA) for Pin and Rack Service Kit filamentous growth and pathogenicity in U.maydis.The roles of two protein phosphatases that potentially act antagonistically to PKA were assessed.
A reverse genetics approach to mutate the catalytic subunits of calcineurin (CN, protein phosphatase [PP]2B) and PP2A in U.maydis was employed.A mutation in the CN catalytic subunit ucn1 caused Screen Protectors a dramatic multiple-budding phenotype and mating between two ucn1 mutants was severely reduced.The pathogenicity of ucn1 mutant strains was also severely reduced, even in a solopathogenic haploid strain.Importantly, mutations disrupting protein phosphorylation by PKA were epistatic to ucn1 mutation, indicating a major role of ucn1 as a PKA antagonistic phosphatase.
Genetic and inhibitor studies indicated that the U.maydis PP2A catalytic subunit gene (upa2) was essential.