Dephosphorylation of the stress-activated MAP kinase P38/Hog1 as a signal in a fungal pathogen of maize

Protein phosphorylation cascades are universal cell signaling routes. The diversity of the kinome allows specific phosphorylation, while relatively few, and abundant, phosphatases dephosphorylate key signaling proteins. Specificity of dephosphorylation may arise from co-expression, localization and protein interactions. Several families of B subunits recruits PP2C heterotrimers, which share the same catalytic subunit, to specific substrates in mammalian cells. MAP kinases of fungal cells, in contrast to their mammalian counterparts, often show detectable basal phosphorylation levels. Dephosphorylation, therefore, could act as a signal. In our experimental model – Cochliobolus heterostrophus, the Dothideomycete causing Southern corn leaf blight, we have shown that ferulic acid (FA), an abundant phenolic found in plant host cell walls, acts as a signal to rapidly dephosphorylate the stress-activated MAP kinase Hog1. To identify the protein phosphatases responsible, we are constructing mutants in Hog1 phosphatases predicted from the C. heterostrophus genome, based on knowledge from yeast and other genetic model fungi. Mutants lacking a member of the PP2C family, ChPtcB, show attenuated dephosphorylation in response to FA. Also, PTP1/2/3 Protein tyrosine phosphatases (PTPs), known Hog1 modulators, could dephosphorylate Hog1. This PTPs are susceptible to effect Hog1 in C. heterostrophus upon FA induction, since they have a dual-specificity phosphatase domain (DSPn)-matching the dual phospho site in Hog1. With the help of these mutants, we can now test whether the sensitivity to FA-induced cell death is altered when FA-induced Hog1 dephosphorylation is compromised.