Developmental regulation of the TCR ζ-chain: Differential expression and tyrosine phosphorylation of the TCR ζ-chain in resting immature and mature T lymphocytes

The ζ subunit of the TCR complex targets receptor surface expression, is phosphorylated on tyrosine residues upon T cell activation, and is implicated in signal transduction after TCR ligation. Here we show that, although intrathymic expression of the murine TCR-associated ζ-chain relative to the other chains of the Ag receptor complex remains unchanged during early thymocyte development, there is a doubling of TCR-associated ζ-chain surface expression upon thymocyte maturation. The ratio of tyrosine-phosphorylated relative to nonphosphorylated TCR-associated ζ-chain also changes with thymocyte development. This ratio was quantified after the purification and detergent extraction of receptor complexes from freshly isolated immature or mature thymocytes. Immunoprecipitation of the ζ-chain released from the complex allowed for the isolation of the tyrosine-phosphorylated and nonphosphorylated forms of TCR-associated ζ-chain. Intracellular free ζ- chain was characterized by immunoprecipitation after clearing the cell lysate of intact TCR complexes. Densitometric analysis of immunoblots indicated that surface phosphorylated ζ-chain is more abundant in immature relative to mature T cell populations, whereas the inverse is true of intracellular phosphorylated ζ-chain. Surface phosphorylated ζ-chain also migrated at a higher m.w. than its cytoplasmic counterpart, suggesting that it is more highly modified on some or all of its available tyrosines. These findings demonstrate that the stoichiometry and post-translational modification of the TCR complex are regulated, in vivo, and may determine the functional maturation of T cell signaling, selection, and activation.