ZAP-70 Protein Promotes Tyrosine Phosphorylation of T Cell Receptor Signaling Motifs (ITAMs) in Immature CD4⁺8⁺ Thymocytes with Limiting p56^lck

As a result of interaction with epithelial cells in the thymic cortex, immature CD4⁺8⁺ (double positive, DP) thymocytes express relatively few T cell receptors (TCRs) and contain diminished numbers of coreceptor-associated p56^lck (lck) PTK molecules. As a result, TCR signal transduction in DP thymocytes is significantly impaired, despite its importance for repertoire selection. We report here that, in DP thymocytes, tyrosine phosphorylation of TCR signaling motifs (ITAMs) by lck, an early event in TCR signal transduction, is dependent upon ZAP-70 protein independent of ZAP-70's kinase activity. Furthermore, the dependence on ZAP-70 protein for ITAM phosphorylation diminishes as available lck increases. Importantly, ZAP-70's role in ITAM phosphorylation in DP thymocytes is not limited to protecting phosphorylated ITAMs from dephosphorylation. Rather, this study indicates that ZAP-70 protein augments ITAM phosphorylation in DP thymocytes and so compensates in part for the relative deficiency of coreceptor-associated lck.