The human T3 γ chain is phosphorylated at serine 126 in response to T lymphocyte activation

The γ subunit of the human T lymphocyte T3 antigen is rapidly phosphorylated on serine residues in vivo during the initiation of T cell activation by a polyclonal mitogen (Phaseolus vulgaris phytohemagglutinin), an activator of protein kinase C (phorbol 12,13-dibutyrate), and an elevator of intracellular calcium (ionomycin). The sites of phosphorylation were identified by comparing tryptic peptide analyses of T3 γ chains labeled in vivo with various synthetic peptides, corresponding to portions of the cytoplasmic domain of the γ chain that had been labeled in vitro using purified protein kinase C. Two sites, serines 123 and 126, were phosphorylated in response to ionomycin, whereas a single site, serine 126, was phosphorylated when T lymphocytes were stimulated by P. vulgaris phytohemagglutinin or when protein kinase C was directly activated by phorbol 12,13-dibutyrate. Immune activation of T cells via the protein kinase C pathway thus induces phosphorylation of a single site on the T3 γ chain, namely serine 126.