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An ATPase activity is associated with XB24 and effects XA21 auto- phosphorylation. (A) ATPase activity assay on purified Ntap-XB24 and Ntap- XB24S154Aprotein from transgenic plants. The same amount of proteins was used. (Left) Representative autoradiogram. (Right) Quantitative results of three independent experiments. ATP hydrolysis was quantified based on radioactivity of the reaction product Pi. Error bars indicate SDs. (B) Effects of XB24 on XA21 autophosphorylation. In vitro autophosphorylation assays were performed on GST-XA21K668 and GST-XA21K668K736E, respectively, in the presence of the purified His-XB24 protein. (C) Effects of XB24 ATPase on XA21 autophosphorylation. An in vitro autophosphorylation assay was performed on GST-XA21K668 in the presence of the same amount of rice- expressed Ntap-XB24 or Ntap-XB24S154A. (Upper) Representative auto- radiogram. (Lower) Quantitative results (mean + SD) from three independ- ent experiments. CK, control provided using autophosphorylation assay on GST-XA21K668 in the absence of XB24. The autophosphorylation level from CK was arbitrarily set as “1.”