Immunodetection of myosin phosphatase in human platelets. Platelet lysates were immunoprecipitated with the antibody against MBS as described in the Materials and Methods. (A) Immunoprecipitates with anti-MBS antibody (a) or the preimmune serum (b) were immunoblotted with anti-MBS antibody. Immunoprecipitates with anti-MBS antibody were immunoblotted with antibodies against PP1δ catalytic subunit and the 20-kD subunit, respectively. The immunoprecipitate with the antibody against PP1δ catalytic subunit was immunoblotted with anti-MBS antibody. IP, immunoprecipitation antibody used; IB, immunoblotting antibody used; Ig, cross-reacted immunoglobulin. (B) Immunoprecipitates with anti-MBS antibody were assayed for phosphatase activity, using [³²P]-MLC as substrate. Each phosphatase activity was determined in triplicate as described in Materials and Methods.