%D 2006 %0 ARTICLE %T Rice XA21 Binding Protein 3 Is a Ubiquitin Ligase Required for Full Xa21-Mediated Disease Resistance . %J %V %P %A Wang YS %A Pi LY %A Chen X %A Chakrabarty PK %A Jiang J %A De Leon %A AL Liu %A GZ Li %A L Benny %A U Oard %A J Ronald %A PC Song %M pub17172358 %X XA21 is a receptor-like kinase protein in rice ( Oryza sativa ) that confers gene-for-gene resistance to specific races of the causal agent of bacterial blight disease , Xanthomonas oryzae pv oryzae . We identified XA21 binding protein 3 ( XB3 ) , an E3 ubiquitin ligase , as a substrate for the XA21 Ser and Thr kinase . The interaction between XB3 and the kinase domain of XA21 has been shown in yeast and in vitro , and the physical association between XB3 and XA21 in vivo has also been confirmed by coimmunoprecipitation assays . XB3 contains an ankyrin repeat domain and a RING finger motif that is sufficient for its interaction with the kinase domain of XA21 and for its E3 ubiquitin ligase activity , respectively . Transgenic plants with reduced expression of the Xb3 gene are compromised in resistance to the avirulent race of X oryzae pv oryzae . Furthermore , reduced levels of Xb3 lead to decreased levels of the XA21 protein . These results indicate that Xb3 is necessary for full accumulation of the XA21 protein and for Xa21-mediated resistance .