In the absence of antibody IgGsorb precipitates human placenta phosphotyrosine-containing proteins.

Lectin-purified human placenta plasma membrane proteins were phosphorylated in vitro. Mixing the reaction mixture with IgGsorb and incubation of the resultant pellet with p-nitrophenyl phosphate demonstrated the presence of phosphorylated-insulin receptor beta-subunit and a phosphorylated-180 kDa protein in acrylamide gel electrophoresis. The same two proteins were detected in the electrophoretic analyses of anti-phosphotyrosine immunoprecipitated phosphorylation reaction mixtures. In the absence of antibody, the amount of phosphorprotein in the IgGsorb pellet was dependent on the amount of IgGsorb added. IgGsorb did not precipitate 125I-labeled lectin-purified human placenta protein. Further, 10 mM O-phosphotyrosine completely inhibited the precipitation of phosphorylated human placenta proteins. These data suggest that IgGsorb specifically bound and precipitated phosphotyrosine-containing proteins in soluble human placenta plasma membranes.