Structural Basis for flg22-Induced Activation of the Arabidopsis FLS2-BAK1 Immune Complex 
Yadong Sun,Lei Li,Alberto P. Macho,Zhifu Han,Zehan Hu,Cyril Zipfel,Jian-min Zhou,Jijie Chai

Science 
DOI:10.1126/science.1243825 

Abstract 
Flagellin perception in Arabidopsis is through recognition of its highly conserved N-terminal epitope (flg22) by flagellin-sensitive 2 (FLS2). Flg22 binding induces FLS2 heteromerization with BRI1-associated kinase 1 (BAK1) and their reciprocal activation followed by plant immunity. Here, we report the crystal structure of FLS2 and BAK1 ectodomains complexed with flg22 at 3.06 angstroms. A conserved and a nonconserved site from the inner surface of the FLS2 solenoid recognize the C- and N-terminal segment of flg22, respectively, without oligomerization or conformational changes in the FLS2 ectodomain. Besides directly interacting with FLS2, BAK1 acts as a coreceptor by recognizing the C terminus of the FLS2-bound flg22. Our data reveal the molecular mechanisms underlying FLS2-BAK1 complex recognition of flg22 and provide insight into the immune receptor complex activation.