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A novel 90‐kDa tyrosine‐phosphorylated protein associated with TCR complex in thymocytes

Ligation of the TCR‐CD3 complex initiates a cascade of tyrosine phosphorylation that results in T cell activation. Initial activation of tyrosine kinases depends on the phosphorylation of activation motifs on CD3 chains. We previously found that a 90‐kDa protein was tyrosine phosphorylated upon TCR cross‐linking and the induction of the phosphorylation was dependent on the structure of the CD3 complex. In this study, we further characterized p90 phosphorylation. Phosphorylation of p90 was induced only by stimulation through the TCR‐CD3 complex but not by other kinds of stimulation including CD28‐ or hydrogen peroxide‐mediated activation and was dynamically regulated. Phosphorylated p90 was associated with the TCR‐CD3 complex upon T cell activation. In a normal T cell population, thymocytes but not splenic T cells induced the tyrosine phosphorylation of p90 upon TCR cross‐linking. These results suggest that p90 is a novel phosphoprotein associated with the TCR‐CD3 complex and may play a role in TCR signaling during thymocyte differentiation.