Immune responses to flagellin have been evolutionarily conserved between plants and animals. In Arabidopsis , FLS2, a leucine-rich repeat (LRR) domain containing serine-threonine transmembrane kinase binds bacterial flagellin and mediates signals, culminating in a defensive response. Gómez-Gómez et al. found that two mutant alleles of fls2 , one in the extracellular LRR domain ( fls2-24 ) and the other in the kinase domain ( fls2-17 ), exhibit reduced binding to flagellin. Further characterization of fls2-17 indicated that it lacked kinase activity and was not phosphorylated in vitro, suggesting that kinase activity was important not only for signaling. FLS2 associated with the protein phosphatase KAPP, and overexpression of KAPP mimicked the phenotype of catalytically inactive FLS2, suggesting that phosphorylation and perhaps phosphorylation-dependent conformational changes transmitted to the extracellular LRR domain are important for flagellin binding. An alternative, but not mutually exclusive, possibility is that, because FLS2 exists in a multiprotein complex, phosphorylation may recruit other essential flagellin binding and signaling proteins. L. Gómez-Gómez, Z. Bauer, T. Boller, Both the extracellular leucine-rich repeat domain and the kinase activity of FLS2 are required for flagellin binding and signaling in Arabidopsis . Plant Cell 13 , 1155-1163 (2001). [Abstract] [Full Text]