Tyrosine-phosphorylation of WASP alters the association of a 220-kD cellular protein with WASP. WASP was expressed by cotransfection with Btk (K430R) (lane 1) or wild-type Btk (lane 2). Unphosphorylated (−) or phosphorylated (+) WASP proteins were immunopurified and then incubated with ³⁵S-metabolically labeled RAMOS cell lysate. Labeled proteins bound to the beads were detected by autoradiography.