Expression of the bacterial type III effector DspA/E in Saccharomyces cerevisiae downregulates the sphingolipid biosynthetic pathway leading to growth-arrest

Erwinia amylovora, the bacterium responsible for fire blight, relies on a type III secretion system and a single injected effector, DspA/E, to induce disease in host plants. DspA/E belongs to the widespread AvrE family of type III effectors which suppress plant defense responses and promote bacterial growth followinginfection. Ectopic expression of DspA/E in plant or in Saccharomyces cerevisiae is toxic indicating that DspA/E likely targets a cellular process conserved between yeast and plant. To unravel the mode of action of DspA/E, we screened the Euroscarf, S. cerevisiae library for mutants resistant toDspA/E-induced growth arrest. The most resistant mutants (Δsur4, Δfen1, Δipt1,Δskn1, Δcsg1, Δcsg2, Δorm1, Δorm2) were impaired in the sphingolipid biosynthetic pathway. Exogenously supplied sphingolipid precursors such as the long chain bases(LCBs) phytosphingosine and dihydrosphingosine also suppressed DspA/E-induced yeast growth defect. Expression of DspA/E in yeast downregulated LCBs biosynthesis and induced a rapid decrease in LCB levels,indicating that SPT, the first and rate limiting enzyme of the sphingolipid biosynthetic pathway was repressed. SPT downregulation was mediated by dephosphorylation and activation of Orm proteins that negatively regulate SPT. A Δcdc55 mutation, affecting Cdc55-PP2A protein phosphatase activity, prevented Orm dephosphorylation and suppressed DspA/E-induced growth arrest.