Tyrosine 319, a newly identified phosphorylation site of ZAP-70, plays a critical role in T cell antigen receptor signaling

Regulation of beta 1 integrin-mediated adhesion by T cell receptor signaling involves ZAP-70 but differs from signaling events that regulate transcriptional activity

Stimulation of the CD3/TCR results within minutes in an increase in T cell adhesion mediated by beta(1) integrins. The biochemical pathways that control CD3-mediated increases in beta(1) integrin-mediated adhesion remain poorly characterized. In this study, the role of the tyrosine kinase ZAP-70 in the regulation of beta(1) integrin activity by the CD3/TCR was investigated. CD3 stimulation did not increase beta(1) integrin-mediated adhesion of the ZAP-70-deficient Jurkat T cell line, P116, to the beta(1) integrin ligand fibronectin. Reintroduction of wild-type ZAP-70, but not a kinase-inactive variant, K369R, corrected the adhesive defect observed in P116 T cells. In addition, the kinase-inactive ZAP-70 mutant inhibited CD3-induced adhesion of primary human T cell blasts. Interestingly, a ZAP-70 mutant with a tyrosine to phenylalanine substitution at position 319 (Y319F) restored the adhesive defect in P116 T cells, even though Y319F ZAP-70 failed to fully reconstitute CD3-initiated NF-AT-dependent transcription and tyrosine phosphorylation of the LAT adapter protein. Finally, expression of mutants of LAT and the SLP-76 adapter protein that modulate CD3-mediated activation of an NF-AT reporter gene failed to block CD3-induced increases in beta(1) integrin-mediated adhesion. These observations support a model in which the tyrosine kinase activity of ZAP-70 kinase is critical for regulation of beta(1) integrin activity by CD3/TCR. However, the signaling events downstream of ZAP-70 that regulate CD3/TCR-mediated activation of beta(1) integrin function exhibit key differences when compared with the signaling pathways that regulate transcriptional events initiated by CD3/TCR stimulation.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

T cell activation and the cytoskeleton

Ligation of the T cell antigen receptor (TCR) stimulates protein tyrosine kinases (PTKs), which regulate intracellular calcium and control the activity of protein kinase C (PKC) isozymes. PTKs activated by antigen receptors and costimulatory molecules also couple to phosphatidylinositol-3 kinase (PI3K) and control the activity of Ras- and Rho-family GTPases. T cell signal transduction is triggered physiologically by antigen in the context of antigen presenting cells (APC). The formation of stable and prolonged contacts between T cells and APCs is not necessary to initiate T cell signaling but is required for effective T cell proliferation and differentiation. The stabilization of the T cell/ APC conjugate is regulated by intracellular signals induced by antigen receptors and costimulators. These coordinate the regulation of the actin and microtubule cytoskeleton and organize a specialized signaling zone that allows sustained TCR signaling.

Signal transduction by the TCR for antigen

The past several years have seen the beginning of a shift in the way that TCR signal transduction is studied. Although many investigators continue to identify new molecules, particularly adaptor proteins, others have attempted to look at signaling events in a larger cellular context. Thus the identification of distinct formations of signaling molecules at junctions between T cells and antigen-presenting cells, the role of the cytoskeleton and the partitioning of molecules into specialized lipid subdomains have been the subjects of many publications. Such concepts are helping to assemble a blueprint of how the myriad adaptors and kinases fit together to effect T cell activation.

Role of the Src homology 2 domains and interdomain regions in ZAP-70 phosphorylation and enzymatic activity

The protein tyrosine kinase ZAP-70, which mediates T-cell antigen receptor (TCR) signalling, contains three distinct functional modules, two tandemly arranged SH2 domains, a kinase domain and a linker region (interdomain B) that connects them. ZAP-70 enzymatic activation is strictly dependent on the binding, via its SH2 domains, to the triggered TCR and on tyrosine phosphorylation. Here we utilized recombinant ZAP-70 and carried out a mutational analysis to understand the structural requirements for its activation. We show that deletion of both SH2 domains corresponding to the first 254 residues moderately increases ZAP-70 enzymatic activity on an exogenous substrate in vitro, results in increased tyrosine phosphorylation and produces subtle conformational changes, as judged by altered SDS/PAGE migration. Mutation of Tyr292, 315 and 319 to Phe in the interdomain B region, which constitute the major phosphorylation sites both in vitro and in vivo, did not affect ZAP-70 enzymatic activity. Moreover, deletion analysis of the interdomain B region established residues 320-619 as a minimal region endowed with full kinase activity. We propose that binding of ZAP-70 to the TCR promotes, through conformational changes, its extensive phosphorylation on tyrosine. However, Tyr292, 315 and 319 do not affect ZAP-70 enzymatic activity and may influence ZAP-70 signalling only indirectly by mediating its association with intracellular transducers.

Tyrosine 319 in the interdomain B of ZAP-70 is a binding site for the Src homology 2 domain of Lck

T-cell antigen receptor-induced signaling requires both ZAP-70 and Lck protein-tyrosine kinases. One essential function of Lck in this process is to phosphorylate ZAP-70 and up-regulate its catalytic activity. We have previously shown that after T-cell antigen receptor stimulation, Lck binds to ZAP-70 via its Src homology 2 (SH2) domain (LckSH2) and, more recently, that Tyr319 of ZAP-70 is phosphorylated in vivo and plays a positive regulatory role. Here, we investigated the possibility that Tyr319 mediates the SH2-dependent interaction between Lck and ZAP-70. We show that a phosphopeptide encompassing the motif harboring Tyr319, YSDP, interacted with LckSH2, although with a lower affinity compared with a phosphopeptide containing the optimal binding motif, YEEI. Moreover, mutation of Tyr319 to phenylalanine prevented the interaction of ZAP-70 with LckSH2. Based on these results, a gain-of-function mutant of ZAP-70 was generated by changing the sequence Y319SDP into Y319EEI. As a result of its increased ability to bind LckSH2, this mutant induced a dramatic increase in NFAT activity in Jurkat T-cells, was hyperphosphorylated, and displayed a higher catalytic activity compared with wild-type ZAP-70. Collectively, our findings indicate that Tyr319-mediated binding of the SH2 domain of Lck is crucial for ZAP-70 activation and consequently for the propagation of the signaling cascade leading to T-cell activation.