Stimulation via CD3-Ti but not CD2 induces rapid tyrosine phosphorylation of a 68-kDa protein in the human Jurkat T cell line.

Tyrosine phosphorylation is an early biochemical event associated with surface receptor triggering in many cellular systems. In T lymphocytes, Ag receptor (CD3-Ti) stimulation results in tyrosine phosphorylation of the CD3 zeta subunit. The tyrosine kinase responsible for this modification after CD3-Ti triggering has not been identified. Here we reported that a 68-kDa T cell membrane-associated protein (pp68) in human Jurkat T cells is phosphorylated on tyrosine residues within 1 min after anti-CD3 mAb addition. This induced tyrosine phosphorylation is detected either by in vivo [32P]orthophosphate labeling of the Jurkat T cells or by in vitro [32P]ATP labeling after immunoprecipitation by antiphosphotyrosine antibody. In contrast, mAb stimulation via CD2 and CD4 structures does not induce phosphorylation of pp68. These data are among the first to provide evidence that CD3-Ti and CD2 activation pathways are distinct. Furthermore, they imply that pp68 is itself a tyrosine kinase and/or is a rapidly phosphorylated substrate of a tyrosine kinase.