The regulatory subunit MoB56 of PP2A phosphatase regulates pathogenicity, growth and development in a protein complex with the atypical catalytic subunit Ppg1 in the rice blast fungus Magnaporthe oryzae

Abstract

Protein phosphatase 2A (PP2A) is usually a heterotrimeric enzyme, consisting of a catalytic subunit (C) and a scaffolding subunit (A) associated with a third, variable regulatory subunit (B). Fungi usually carry a single gene for A and C subunits, and three genes for the B subunit. In addition, fungi contain a conserved atypical C subunit named Ppg1, which is essential to the pathogenicity of the rice blast fungus Magnaporthe oryzae. However, it remains largely unknown how the B subunit combinatorically assembles with the A and C subunits or Ppg1 to regulate fungal growth, development and pathogenicity. Here we report and functionally characterize one regulatory subunit of PP2A, named MoB56, in M. oryzae. We generated a MoB56 deletion mutant Δmob56, which was severely defective in vegetative growth, conidiation and septum formation, and had lost pathogenicity. The defects of Δmob56 could be rescued by introducing MoB56 fused with GFP (MoB56-GFP) at its C terminus. Fluorescence microscopic observations revealed that the MoB56-GFP signals were widely distributed in the cytoplasm and formed a dot-like structure at the center of the septum in conidia, appressoria and infection hyphae, supporting its function in septation. Further, we performed co-immunoprecipitation and pull-down assays, indicating that MoB56 forms a protein complex with the A subunit and Ppg1 in mycelial cells. The yeast two-hybrid assay showed that MoB56 could interact with the A subunit of PP2A but not with Ppg1, while Ppg1 could interact with the A subunit, suggesting that the A subunit ties MoB56 with Ppg1 for the protein complex formation. In addition, we revealed that MoB56 has multiple isoforms, which are likely originated from alternative splicing and sumoylation. This is the first report revealing that the regulatory subunit B56 is associated with the PP2A-like phosphatase Ppg1 in fungi. Importantly, this study showed that B56, like Ppg1, is essential to the pathogenicity of M. oryzae, offering a potential new lead to control this devastating fungal pathogen by targeting specific PP2A-like phosphatase. Together, this study provides important information for understanding how the regulatory subunit B56 of PP2A regulates fungal pathogenicity and for the control of rice blast disease.