Functional Roles of a Putative B' Delta Regulatory Subunit and a Catalytic Subunit of Protein Phosphatase 2A in the Cereal Pathogen Fusarium graminearum

Department of Medical Biotechnology, Soonchunhyang University, Asan 336-745, Korea(Received on May 12, 2012; Revised on May 23, 2012; Accepted on May 23, 2012)Protein phosphatase 2A (PP2A), a family of serine/threonine protein phosphatases, plays an important rolein balancing the phosphorylation status of cellularproteins for regulating diverse biological functions ineukaryotic organisms. Despite intensive studies inmammals, limited information on its role is available infilamentous fungi. Here, we investigated the functionalroles of genes for a putative B' delta regulatory subunit(FgPP2AR) and a catalytic subunit ( FgPP2AC) of PP2Ain a filamentous ascomycete, Fusarium graminearum.Molecular characterization of an insertional mutant ofthis plant pathogenic fungus allowed us to identify theroles of FgPP2AR. Targeted gene replacement and com-plementation analyses demonstrated that the deletion ofFgPP2AR, which was constitutively expressed in allgrowth stages, caused drastic changes in hyphal growth,conidia morphology/germination, gene expression formycotoxin production, sexual development and patho-genicity. In particular, overproduction of aberrantcylindrical-shaped conidia is suggestive of arthroconi-dial induction in the ΔFgPP2AR strain, which has neverbeen described in F. graminearum. In contrast, theΔFgPP2AC strain was not significantly different fromits wild-type progenitor in conidiation, trichothecenegene expression, and pathogenicity; however, it showedreduced hyphal growth and no perithecial formation.The double-deletion ΔFgPP2AR;ΔFgPP2AC strain hadmore severe defects than single-deletion strains in allexamined phenotypes. Taken together, our results indi-cate that both the putative regulatory and catalytic sub-units of PP2A are involved in various cellular processesfor fungal development in F. graminearum. Keywords : aberrant conidia morphology, B regulatorysubunit, catalytic subunit, Fusarium graminearum, proteinphosphatase 2A Molecular regulation of many biological functions ineukaryotic organisms is mediated by protein posttranslationalmodifications. Protein phosphorylation is one of the majormechanisms in this regulatory system. In particular, rever-sible phosphorylation of cellular proteins in response toexternal or intracellular stimuli is an essential step for signaltransmission to specific transcription factors that regulatedownstream genes in a specific signal transduction cascade(Liu et al., 2000; Luan, 2003). The phosphorylation statusof target proteins is modulated by the reversible activities ofprotein kinases and protein phosphatases. Serine/threonineprotein phosphatases (PPs), which catalyze the dephos-phorylation of proteins, comprise two different structuralfamilies based on biochemical characteristics: type 1 (PP1)and type 2 (PP2). PP2s are subsequently divided into threegroups (2A, 2B, and 2C). Protein phosphatase 2A (PP2A)holoenzyme, a member of the PP2 family, consists of a coredimer comprising a catalytic C subunit (PP2AC) and ascaffolding A subunit, which can be complexed with a thirdvariable regulatory B subunit (PP2AB) to form a hetero-trimer (Mumby, 2007). The variable B subunits, which candetermine the substrate specificity, catalytic activity, andcellular localization of PP2A, are grouped into four un-related families, namely B (PR55), B' (B56/PR61), B''(PR48/59/72/130) and B''' (PR93/110) based on structuralsimilarities. Among the B regulatory subunits, the B' sub-unit is the most diverse, consisting of five isoforms (alpha,beta, gamma, delta, and epsilon) (McCright et al., 1996;Muneer et al., 2002; Sontag, 2001). The diversity of possiblecombinations of these subunits allows the PP2A holo-enzyme to play roles in the regulation of a wide range ofcellular and metabolic processes in a variety of organisms(Gallego and Virshup, 2005; Lechward et al., 2001). Despiteintensive investigations in human and model organisms,limited information on the role of PP2A is available infilamentous fungi.Fusarium graminearum (teleomorph: Gibberella zeae) isa filamentous ascomycetous fungus that threatens bothplant and animal health not only by causing disease but also