BRI1 is a Ser/Thr kinase. Immunoprecipitated HA-BRI1, wild type, bri1-101, and bri1-113 were used for an in vitro kinase assay. A, The top presents an autoradiogram of the kinase assay. The bottom presents an immunoblot using anti-HA antibody of the same gel. Lane 1, Untransfected 293T cells; lane 2, wild-type BRI1; lane 3, bri1-101 kinase mutant; lane 4, bri1-113 island domain mutant. B, Autophosphorylation of BRI1 occurs on Ser/Thr residues. Anti-HA-BRI1 autophosphorylation was subjected to phosphoamino acid analysis. The positions of the internal phospho-Ser, phospho-Thr, and phospho-Tyr standards (visualized by ninhydrin staining) are indicated. The origin (+) and the directions of electrophoresis with the pH are as indicated.