B lymphocyte activation is accompanied by phosphorylation of a 72-kDa protein-tyrosine kinase.

The cross-linking of membrane IgM on the surface of splenic B lymphocytes or WEHI 231 cells leads to the rapid phosphorylation on tyrosine of a 72-kDa protein as detected in Western blotting experiments using anti-phosphotyrosine antibodies. The 72-kDa phosphoprotein detected in this manner comigrates, in both one- and two-dimensional polyacrylamide gel electrophoresis systems, with PTK72, a 72-kDa protein-tyrosine kinase characterized previously in this laboratory (Zioncheck, T. F., Harrison, M. L., Isaacson, C. C., and Geahlen, R. L. (1988) J. Biol. Chem. 263, 19195-19202). Anti-phosphotyrosine antibodies and anti-PTK72 antibodies immunoprecipitate the same protein-tyrosine kinase from extracts of anti-IgM-activated cells as determined by immune complex kinase assays and one-dimensional phosphopeptide mapping. These results indicate that the tyrosine phosphorylation of a 72-kDa protein-tyrosine kinase is an early event in the activation of B lymphocytes via the antigen receptor.