positive contribution of E3 ubiquitin ligases in regulation of MAP kinase cascade involved in plant immunity

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A mitogen-activated protein kinase(MAPK) cascade has an important role in innate immunity in plants. Arabidopsis MEKK1, a MAPK kinase kinase(MAPKKK) is an important factor in the upstream of MPK4 which has at least two functions, negative regulation of SA-mediated defense responses and positive regulation of JA-induced gene expression, however regulatory mechanism of MEKK1 remains to be elucidated. To identify possible regulator of MEKK1, we performed yeast two-hybrid screening using MEKK1 protein as bait. We found two putative E3 ligases which contain U-box domain, named PUB25 and PUB26 as novel MEKK1-binding proteins. Characterization using pub25/ 26 double mutant suggested that PUB25 and PUB26 positively contribute MEKK1 function.