A 20-kDa protein associated with the murine T-cell antigen receptor is phosphorylated in response to activation by antigen or concanavalin A.

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Abstract
Antigen or concanavalin A activation of a murine T-cell hybrid specific for pigeon cytochrome c and restricted to the .**GRAPHIC**. immune response-associated (Ia) molecule resulted in phosphorylation of a 20-kDa [kilodalton] protein that was specifically coprecipitated by a monoclonal antibody binding the T cell antigen receptor. There was no evidence for phosphorylation of the antigen receptor itself. The phosphorylation of the 20-kDa polypeptide was dependent on the concentration of antigen or lectin used to activate the T cell hybrid and reached a maximum 40 min after the addition of antigen. The phosphorylation induced by antigen in the presence of Ia molecule-bearing B cells was specifically blocked by the addition of appropriate anti-Ia molecule monoclonal antibodies. The 20-kDa protein was also radioiodinated with a hydrophobic photoactivatable labeling reagent. The amount of iodinated 20-kDa protein immunoprecipitable with the anti-receptor antibody did not increase with T cell activation, indicating that the phosphorylation occurred on a molecule that was constitutively associated with the antigen receptor. Concanavalin A also induced phosphorylation of a 20-kDa polypeptide in a 2nd antigen-specific major histocompatibility complex-restricted T cell hybrid. The phosphorylated polypeptide was precipitated only by a monoclonal antibody specific for the antigen receptor on this hybrid. The antigen or concanavalin A-induced activation of T-cell hybrids results in the rapid phosphorylation of a 20-kDa protein that is associated with the T cell antigen receptor.