The T-cell receptor (TCR) and B-cell receptor (BCR) are multimeric protein complexes consisting of antigen binding subunits (␣Ti for the TCR and membrane immunoglobulin [mIg] for the BCR) and the signal transducing invariant subunits (␥, ␦, and ε subunits of CD3 complex and proteins for the TCR; Ig␣ and Ig for the BCR) (reviewed in reference 48). Engagement of the TCR and BCR initiates a cascade of intracellular processes that lead to cellular response. One of the earliest detectable biochemical events after TCR and BCR stimulation is the tyrosine phosphorylation of multiple cellular protein substrates, including the invariant chains of the TCR and BCR (reviewed in references 3, 4, 12, 29, 35, and 48).At least two families of protein tyrosine kinases (PTKs), the Src family and the Syk/ZAP-70 family, are implicated in regulating this tyrosine phosphorylation process (reviewed in reference 48). Both biochemical and genetic studies have demonstrated that Src family PTKs activated by TCR and BCR stimulation are required to phosphorylate the signal transducing subunits of these receptors (7,19,34,39). This phosphorylation occurs on the two tyrosine residues present in a common signaling motif which exists as one copy in CD3␥, CD3␦, CD3ε, Ig␣, and Ig and as three copies in TCR (33). This motif, the immunoreceptor tyrosine-based activation motif (ITAM), consists of pairs of tyrosine and leucine residues arranged in the consensus sequence YxxL(x) 6-8 YxxL, where x is variable. After TCR stimulation, ZAP-70 is recruited to the receptor complex through the interaction of its two SH2 domains with the doubly phosphorylated ITAMs (19,46). This interaction is believed to be critical for TCR signaling, since phosphopeptides that block the interaction of ZAP-70 with the chain also inhibit TCR signaling events (45). The association of ZAP-70 with the TCR ITAMs facilitates its autophosphorylation and the tyrosine phosphorylation of ZAP-70 mediated by Src family PTKs (19,27). The critical role for ZAP-70 in T-cell, not B-cell, activation and development has been demonstrated in patients with severe combined immunodeficiency who were deficient in 8,15) and in mice which had been made deficient in . Similarly, a critical role for Syk in B-cell, not ␣ T-cell, activation and development has been shown both in chicken B cells (39) and in mice which had been made deficient in Syk (9, 41).Like Syk, ZAP-70 is composed of three easily identifiable domains, a tyrosine kinase domain and two tandemly arranged SH2 domains (N terminal and C terminal) which mediate the association of ZAP-70 with the TCR after its stimulation (7,19,46). Between the two SH2 domains exists a region (60 amino acids [aa] in length; interdomain A) which forms a coiled-coil structure and is likely involved in bringing together the two SH2 domains which bind to the receptor ITAMs (18). Between the second SH2 domain and the kinase domain lies an additional region (84 aa in size; interdomain B) whose structure and function are unclear. Interdomain B contains a prolin...