Autoradiogram showing phosphoproteins generated in K562 cell membrane immunoprecipitates and effects of okadaic acid. K562 membrane proteins were immunoprecipitated using biotin-conjugated antiphosphotyrosine (RC20) in the presence of low ionic strength buffers. Immunoreactive proteins were immobilized using agarose-linked streptavidin. Immobilized immunoprecipitates were incubated with protein kinase buffer augmented with 1.0 μmol/L [γ-³²P]ATP (4.4 × 10⁶ dpm) and 10 mmol/L magnesium acetate and varied concentrations of okadaic acid (OA). After labeling, reaction mixtures were applied to 12.5% SDS polyacrylamide gels and autoradiograms were subsequently prepared. Lane 1, OA absent; lane 2, 0.02 nmol/L OA; lane 3, 0.2 nmol/L OA; lane 4, 20 nmol/L OA. Phosphoproteins identified are indicated on the right. Molecular mass markers (17 to 204 kD) are indicated on the left.