ZAP-70 binding specificity to T cell receptor tyrosine-based activation motifs: the tandem SH2 domains of ZAP-70 bind distinct tyrosine-based activation motifs with varying affinity.

Engagement of the T cell antigen receptor (TCR) results in activation of several tyrosine kinases leading to tyrosine phosphorylation of protein substrates and activation of multiple biochemical pathways. TCR-mediated activation of the src-family kinases, Lck and Fyn, results in tyrosine phosphorylation of the TCR zeta and CD3 chains. The site of phosphorylation in these chains is the tyrosine-based activation motif (TAM), a 15-16 amino acid module containing two tyrosine residues. Tyrosine-phosphorylated TAMs serve as targets for binding of the zeta-associated protein (ZAP-70) tyrosine kinase via its tandem SH2 domains. This binding correlates with activation of ZAP-70, a critical event in T cell activation. To further define the structural requirements for ZAP-70 interaction with the TCR, we developed a binding assay using immobilized glutathione S-transferase fusion proteins containing the NH2- and/or COOH-terminal SH2 domains of ZAP-70, and soluble synthetic peptides with the sequence of the cytoplasmic region of the TCR zeta chain (TCR zeta cyt) or individual TCR zeta and CD3 epsilon TAM motifs. Direct binding studies demonstrated that the tandem ZAP-70 SH2 domains bind phosphorylated, but not nonphosphorylated, TCR zeta cyt. The NH2-terminal ZAP-70 SH2 domain also binds to TCR zeta cyt but with 100-fold lower affinity. No binding was observed with the COOH-terminal ZAP-70 SH2 domain. Similar studies demonstrated that the ZAP-70 tandem SH2 domain can bind a TCR zeta 3 TAM peptide in which both tyrosine residues are phosphorylated: Little or no binding was observed with peptides phosphorylated at only one tyrosine residue, or a nonphosphorylated peptide. Binding of the tandem SH2 domains to the other two TCR zeta TAM peptides and to a CD3 epsilon TAM peptide was also observed. All four doubly tyrosine phosphorylated TAM peptides cross-compete with each other for binding to the tandem SH2 domains of ZAP-70. The affinity of these peptides for the tandem SH2 construct demonstrated a hierarchy of TAM zeta 1 > or = TAM zeta 2 > TAM epsilon > or = TAM zeta 3. The results provide further evidence that the ZAP-70 interaction with the TCR requires prior phosphorylation of both tyrosine residues within a TAM motif. Binding of ZAP-70 to phospho-TAMs is notable for the high level of cooperativity between the two SH2 domains, which individually demonstrate low affinity interaction with the ligand. The cooperativity ensures higher affinity for the doubly phosphorylated ligand. Affinity differences of as much as 30-fold indicates a significant specificity of interaction of ZAP-70 SH2 domains for different phospho-TAMs.

Zeta phosphorylation without ZAP-70 activation induced by TCR antagonists or partial agonists.

Small changes in the peptide-major histocompatibility complex (MHC) molecule ligands recognized by antigen-specific T cell receptors (TCRs) can convert fully activating complexes into partially activating or even inhibitory ones. This study examined early TCR-dependent signals induced by such partial agonists or antagonists. In contrast to typical agonist ligands, both an antagonist and several partial agonists stimulated a distinct pattern of zeta chain phosphorylation and failed to activate associated ZAP-70 kinase. These results identify a specific step in the early tyrosine phosphorylation cascade that is altered after TCR engagement with modified peptide-MHC molecule complexes. This finding may explain the different biological responses to TCR occupancy by these variant ligands.

Guanine nucleotide binding regulatory proteins and adenylate cyclase in livers of streptozotocin- and BB/Wor-diabetic rats. Immunodetection of Gs and Gi with antisera prepared against synthetic peptides.

Adenylate cyclase in liver plasma membranes from streptozotocin-diabetic (STZ) or BB/Wor spontaneously diabetic rats showed increased responsiveness to GTP, glucagon, fluoroaluminate, and cholera toxin. Basal or forskolin-stimulated activity was unchanged in STZ rats, but increased in BB/Wor rats. No change in the alpha-subunit of Gi (alpha i) was observed in STZ or BB/Wor rats using pertussis toxin-stimulated [32P]ADP-ribosylation. Immunodetection using antibodies against the COOH-terminal decapeptides of alpha T and alpha i-3 showed no change in alpha i in STZ rats and a slight decrease in BB/Wor rats. Angiotensin II inhibition of hepatic adenylate cyclase was not altered in either diabetic rat. In both models of diabetes, Gs alpha-subunits were increased as measured by cholera toxin-stimulated [32P]-ADP-ribosylation of 43-47.5-kD peptides, reconstitution with membranes from S49 cyc- cells or immunoreactivity using antibodies against the COOH-terminal decapeptide of alpha s. These data indicate that STZ-diabetes increases hepatic Gs but does not change Gi or adenylate cyclase catalytic activity. In contrast, BB/Wor rats show increased hepatic Gs and adenylate cyclase. These changes could explain the increase in hepatic cAMP and related dysfunctions observed in diabetes.

Deficiency of PTEN in Jurkat T cells causes constitutive localization of Itk to the plasma membrane and hyperresponsiveness to CD3 stimulation.

Pleckstrin homology (PH) domain binding to D3-phosphorylated phosphatidylinositides (PI) provides a reversible means of recruiting proteins to the plasma membrane, with the resultant change in subcellular localization playing a key role in the activation of multiple intracellular signaling pathways. Previously we found that the T-cell-specific PH domain-containing kinase Itk is constitutively membrane associated in Jurkat T cells. This distribution was unexpected given that the closely related B-cell kinase, Btk, is almost exclusively cytosolic. In addition to constitutive membrane association of Itk, unstimulated JTAg T cells also exhibited constitutive phosphorylation of Akt on Ser-473, an indication of elevated basal levels of the phosphatidylinositol 3-kinase (PI3K) products PI-3,4-P(2) and PI-3,4,5-P(3) in the plasma membrane. Here we describe a defect in expression of the D3 phosphoinositide phosphatase, PTEN, in Jurkat and JTAg T cells that leads to unregulated PH domain interactions with the plasma membrane. Inhibition of D3 phosphorylation by PI3K inhibitors, or by expression of PTEN, blocked constitutive phosphorylation of Akt on Ser-473 and caused Itk to redistribute to the cytosol. The PTEN-deficient cells were also hyperresponsive to T-cell receptor (TCR) stimulation, as measured by Itk kinase activity, tyrosine phosphorylation of phospholipase C-gamma1, and activation of Erk compared to those in PTEN-replete cells. These data support the idea that PH domain-mediated association with the plasma membrane is required for Itk activation, provide evidence for a negative regulatory role of PTEN in TCR stimulation, and suggest that signaling models based on results from Jurkat T-cell lines may underestimate the role of PI3K in TCR signaling.

Zap-70-independent Ca(2+) mobilization and Erk activation in Jurkat T cells in response to T-cell antigen receptor ligation.

The tyrosine kinase ZAP-70 has been implicated as a critical intermediary between T-cell antigen receptor (TCR) stimulation and Erk activation on the basis of the ability of dominant negative ZAP-70 to inhibit TCR-stimulated Erk activation, and the reported inability of anti-CD3 antibodies to activate Erk in ZAP-70-negative Jurkat cells. However, Erk is activated in T cells receiving a partial agonist signal, despite failing to activate ZAP-70. This discrepancy led us to reanalyze the ZAP-70-negative Jurkat T-cell line P116 for its ability to support Erk activation in response to TCR/CD3 stimulation. Erk was activated by CD3 cross-linking in P116 cells. However, this response required a higher concentration of anti-CD3 antibody and was delayed and transient compared to that in Jurkat T cells. Activation of Raf-1 and MEK-1 was coincident with Erk activation. Remarkably, the time course of Ras activation was comparable in the two cell lines, despite proceeding in the absence of LAT tyrosine phosphorylation in the P116 cells. CD3 stimulation of P116 cells also induced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) and increased the intracellular Ca(2+) concentration. Protein kinase C (PKC) inhibitors blocked CD3-stimulated Erk activation in P116 cells, while parental Jurkat cells were refractory to PKC inhibition. The physiologic relevance of these signaling events is further supported by the finding of PLCgamma1 tyrosine phosphorylation, Erk activation, and CD69 upregulation in P116 cells on stimulation with superantigen and antigen-presenting cells. These results demonstrate the existence of two pathways leading to TCR-stimulated Erk activation in Jurkat T cells: a ZAP-70-independent pathway requiring PKC and a ZAP-70-dependent pathway that is PKC independent.

Genetic evidence for differential coupling of Syk family kinases to the T-cell receptor: reconstitution studies in a ZAP-70-deficient Jurkat T-cell line.

T-cell antigen receptor (TCR) engagement activates multiple protein tyrosine kinases (PTKs), including the Src family member, Lck, and the Syk-related PTK, ZAP-70. Studies in ZAP-70-deficient humans have demonstrated that ZAP-70 plays crucial roles in T-cell activation and development. However, progress toward a detailed understanding of the regulation and function of ZAP-70 during TCR signaling has been hampered by the lack of a suitable T-cell model for biochemical and genetic analyses. In this report, we describe the isolation and phenotypic characterization of a Syk- and ZAP-70-negative somatic mutant derived from the Jurkat T-cell line. The P116 cell line displays severe defects in TCR-induced signaling functions, including protein tyrosine phosphorylation, intracellular Ca2+ mobilization, and interleukin-2 promoter-driven transcription. These signaling defects were fully reversed by reintroduction of catalytically active versions of either Syk or ZAP-70 into the P116 cells. However, in contrast to ZAP-70 expression, Syk expression triggered a significant degree of cellular activation in the absence of TCR ligation. Transfection experiments with ZAP-70-Syk chimeric proteins indicated that both the amino-terminal regulatory regions and the carboxy-terminal catalytic domains of Syk and ZAP-70 contribute to the distinctive functional properties of these PTKs. These studies underscore the crucial role of ZAP-70 in TCR signaling and offer a powerful genetic model for further analyses of ZAP-70 regulation and function in T cells.

Membrane raft-dependent regulation of phospholipase Cgamma-1 activation in T lymphocytes.

Numerous signaling molecules associate with lipid rafts, either constitutively or after engagement of surface receptors. One such molecule, phospholipase Cgamma-1 (PLCgamma1), translocates from the cytosol to lipid rafts during T-cell receptor (TCR) signaling. To investigate the role played by lipid rafts in the activation of this molecule in T cells, an influenza virus hemagglutinin A (HA)-tagged PLCgamma1 was ectopically expressed in Jurkat T cells and targeted to these microdomains by the addition of a dual-acylation signal. Raft-targeted PLCgamma1 was constitutively tyrosine phosphorylated and induced constitutive NF-AT-dependent transcription and interleukin-2 secretion in Jurkat cells. Tyrosine phosphorylation of raft-targeted PLCgamma1 did not require Zap-70 or the interaction with the adapters Lat and Slp-76, molecules that are necessary for TCR signaling. In contrast, the Src family kinase Lck was required. Coexpression in HEK 293T cells of PLCgamma1-HA with Lck or the Tec family kinase Rlk resulted in preferential phosphorylation of raft-targeted PLCgamma1 over wild-type PLCgamma1. These data show that localization of PLCgamma1 in lipid rafts is sufficient for its activation and demonstrate a role for lipid rafts as microdomains that dynamically segregate and integrate PLCgamma1 with other signaling components.

LAT, the linker for activation of T cells: a bridge between T cell-specific and general signaling pathways.

A key event in the regulation of the adaptive immune response is the binding of major histocompatibility complex-bound foreign peptides to T cell antigen receptors (TCRs) that are present on the cell surface of T lymphocytes. Recognition of the presence of cognate antigen in the host animal induces a series of biochemical changes within the T cell; these changes, in the context of additional signals from other surface receptors, ultimately result in massive proliferation of receptor-engaged T cells and the acquisition of effector and memory functions. Early studies established the importance of the activation of the enzymes phospholipase C-gamma1 (PLC-gamma1) and phosphatidylinositol 3-kinase (PI3K), as well as the small molecular weight heterotrimeric guanine nucleotide binding protein (G protein) Ras, in this process. These biochemical events are dependent on the activity of several protein tyrosine kinases that become activated immediately upon TCR engagement. An unresolved question in the field has been which molecules and what sequence of events tie together the early tyrosine phosphorylation events with the activation of these downstream signaling molecules. A likely candidate for linking the proximal and distal portions of the TCR signaling pathway is the recently described protein, LAT. LAT is a 36-kD transmembrane protein that becomes rapidly tyrosine-phosphorylated after TCR engagement. Phosphorylation of LAT creates binding sites for the Src homology 2 (SH2) domains of other proteins, including PLC-gamma1, Grb2, Gads, Grap, 3BP2, and Shb, and indirectly binds SOS, c-Cbl, Vav, SLP-76, and Itk. LAT is localized to the glycolipid-enriched membrane (GEM) subdomains of the plasma membrane by virtue of palmitoylation of two cysteine residues positioned near the endofacial side of the plasma membrane. Notably, in the absence of LAT, TCR engagement does not lead to activation of distal signaling events. This review examines the circumstances surrounding the discovery of LAT and our current understanding of its properties, and discusses current models for how LAT may be functioning to support the transduction of TCR-initiated, T cell-specific signaling events to the distal, general signaling machinery.

The role of tyrosine kinases and phosphotyrosine-containing recognition motifs in regulation of the T cell-antigen receptor-mediated signal transduction pathway.

T cell-mediated immune responses are initiated by interaction of antigen bound to a glycoprotein encoded by the major histocompatibility complex with the T cell antigen receptor (TCR). These recognition and binding steps are followed by multiple intracellular biochemical events. The earliest event detected is an increase in intracellular protein tyrosine phosphorylation that involves a complex interaction of tyrosine kinases and phosphatases. Subsequently, one observes an increase in protein serine/threonine phosphorylation, phospholipid hydrolysis, and changes in intracellular Ca2+ levels. These and other biochemical changes lead to cell proliferation, differentiation, and acquisition of effector functions. While binding of extracellular growth factors to receptors containing cytoplasmic protein tyrosine kinase (PTK) domains induces direct activation of their kinase activity, the multichain TCR lacks an intrinsic kinase domain and therefore represents a distinct type of receptor. It transduces signals via the interaction with, and activation of, non-receptor PTKs. Recent efforts directed at defining the TCR-linked signaling pathways have provided insight into the regulatory role of three PTKs, and the functional importance of some unique protein motifs in both TCR subunits and PTKs, which mediate critical protein-protein interactions in this pathway.

Identification and characterization of a substrate specific for the T cell protein tyrosine kinase ZAP-70.

ZAP-70 is a protein tyrosine kinase (PTK) that plays a critical role in T cell activation. To study the role of ZAP-70 catalytic activity in this process, a substrate capable of distinguishing between the activities of ZAP-70 and other PTKs would be useful, especially since it has recently been shown that ZAP-70 interacts with another T cell PTK, Lck. We have thus identified a site of phosphorylation on the cytoplasmic fragment of the erythrocyte band 3 protein that is recognized by ZAP-70, but not Lck. A synthetic peptide based on this site has been demonstrated to be a good in vitro substrate for ZAP-70 and a poor substrate for the T cell PTKs Lck and Itk. This peptide molecule should thus prove useful to many investigators working in the field of T cell activation.

Itk/Emt/Tsk activation in response to CD3 cross-linking in Jurkat T cells requires ZAP-70 and Lat and is independent of membrane recruitment.

The Tec family tyrosine kinase, Itk has been implicated in T cell antigen receptor (TCR) signaling, yet little is known about Itk regulation. Here, we investigate the role of the tyrosine kinase ZAP-70 in regulating Itk. Whereas Itk was activated in Jurkat T cells in response to CD3 cross-linking, Itk activation was defective in the ZAP-70-deficient P116 Jurkat T cell line. Itk responsiveness to TCR engagement was restored in P116 cells stably transfected with ZAP-70 cDNA. ZAP-70 itself could not directly phosphorylate the Itk kinase domain, indicating an indirect regulation of Itk activity. No role was found for ZAP-70 in regulating Itk recruitment to the plasma membrane, an event that has been suggested to be rate-limiting for the activation of Tec family kinases. Indeed, Itk was found to be constitutively targeted to the membrane fraction in both Jurkat and P116 cells. Lat, a prominent in vivo substrate of ZAP-70 that mediates assembly of multimolecular signaling complexes at the plasma membrane of T cells was also found to be required for TCR-stimulated Itk activation. Itk could not be activated by CD3 cross-linking in a Lat-negative cell line, unless Lat expression was restored. Lat and Itk were observed to co-associate in response to CD3 cross-linking in Jurkat T cells, but not in P116 T cells. The Lat-Itk association correlated with Lat tyrosine phosphorylation, which was deficient in the P116 T cells. These data suggest that ZAP-70 and Lat play important, probably sequential, roles in regulating the activation of Itk following TCR engagement.

A tyrosine-phosphorylated 70-kDa protein binds a photoaffinity analogue of ATP and associates with both the zeta chain and CD3 components of the activated T cell antigen receptor.

An early event in T cell antigen receptor (TCR)-mediated signal transduction is the activation of a protein tyrosine kinase (PTK) pathway. An unidentified PTK activity and a kinase substrate termed ZAP-70 have previously been shown to associate with TCR zeta upon cross-linking of TCR beta. Here we report that TCR activation, by antibody cross-linking of either TCR beta or CD3 epsilon, results in the association of a PTK activity with both CD3 and TCR zeta. A number of in vitro PTK substrates are also associated with CD3 and TCR zeta, including CD3 epsilon, TCR zeta, p60fyn, p62yes, and a predominant 70-kDa protein (ZAP-70). The shared PTK activity and PTK substrates suggest that both CD3 and TCR zeta are involved in signal transduction via a shared pathway. We used [alpha-32P]gamma-azidoanilido ATP, a photoreactive analogue of ATP, to detect CD3-associated proteins that bound ATP upon TCR activation, reasoning that such proteins could represent PTKs. A 70-kDa protein bound [alpha-32P]gamma-azidoanilido ATP only upon TCR activation, and we propose that this protein and the 70-kDa PTK substrate are the same protein. Furthermore, we propose that this protein is responsible for the PTK activity observed to be associated with TCR zeta and CD3 upon TCR activation.

ZAP-70-dependent and -independent activation of Erk in Jurkat T cells. Differences in signaling induced by H2o2 and Cd3 cross-linking.

Oxidative stress in T cells induces signaling events similar to those initiated by T cell antigen receptor engagement, including tyrosine phosphorylation and activation of the critical protein-tyrosine kinase ZAP-70. Distal signaling events such as the activation of mitogen-activated protein kinases and downstream transcription factors are also initiated by oxidative stimuli. In this study P116, a ZAP-70-negative Jurkat T cell line, was used to investigate the role of ZAP-70 in mediating activation of Erk in response to H2O2. Consistent with the hypothesis that ZAP-70 is required for activation of Erk in response to an oxidative stimulus, Erk1 and Erk2 could be rapidly activated in Jurkat cells but not in P116 cells upon addition of H2O2. P116 cells became competent for H2O2-induced Erk activation upon stable transfection with wild-type ZAP-70. An in vivo ZAP-70 substrate, SLP-76, implicated in Erk activation, also became rapidly tyrosine-phosphorylated in Jurkat cells, but not in P116 cells, upon treatment with H2O2. Surprisingly, although ZAP-70 was required for H2O2-mediated Erk activation, Erk activation in response to T cell antigen receptor engagement did not require ZAP-70. In addition to demonstrating a requirement for ZAP-70 in H2O2-stimulated Erk activation, these results provide the first evidence for the existence of a ZAP-70-independent pathway for Erk activation in T cells.

The protein product of the c-cbl protooncogene is the 120-kDa tyrosine-phosphorylated protein in Jurkat cells activated via the T cell antigen receptor.

Tyrosine phosphorylation of multiple cellular proteins is a critical event in T cell receptor (TCR)-mediated activation. This pathway has also been implicated in cellular transformation in multiple systems. The viral oncogene v-cbl is the transforming gene of a murine retrovirus that induces pre-B cell lymphomas and myelogenous leukemias. The product of its cellular homolog, p120cbl, is a 120-kDa cytoplasmic protein that is non-transforming when overexpressed. Here we show that the 120-kDa protein tyrosine phosphorylated in Jurkat T cells upon TCR engagement is p120cbl. Following stimulation through the TCR, this tyrosine phosphorylation is rapid and reversible. Tyrosine-phosphorylated p120cbl binds to glutathione S-transferase fusion proteins generated from SH2 domains of the Fyn, Lck, and Blk protein tyrosine kinases, GTPase-activating protein and phospholipase C gamma. The p120cbl from unactivated and activated cells also binds to full-length glutathione S-transferase-Grb2 and the Grb2 N-terminal SH3 domain, but not to the Grb2 C-terminal SH3 domain. Additionally, p120cbl binds to SH3 domains of Fyn and Lck, but not Blk. These data expand our knowledge of protein tyrosine kinase signaling pathways in T cells by identifying a prominent tyrosine kinase substrate. This protein, the product of the cellular homolog of a transforming oncogene, can interact with several known signaling molecules.

Tandem SH2 domains of ZAP-70 bind to T cell antigen receptor zeta and CD3 epsilon from activated Jurkat T cells.

A proximal and critical biochemical event upon T cell antigen receptor (TCR) stimulation is the activation of a protein tyrosine kinase (PTK) pathway. ZAP-70, a PTK of the p72syk family, associates with phosphorylated TCR subunits upon TCR stimulation. Here we report that the tandem SH2 domains of ZAP-70, expressed as a fusion protein, bind to tyrosine-phosphorylated CD3 epsilon and TCR zeta from activated Jurkat T cell lysates. The single N- and C-terminal SH2 domains of ZAP-70, expressed separately, do not bind these TCR subunits. In comparison to fusion proteins containing SH2 domains from other proteins, the tandem SH2 domains of ZAP-70 demonstrate a remarkably restricted repertoire of protein binding, binding only TCR zeta and CD3 epsilon. ZAP-70 is also recovered in the binding assay, but this is likely to be a consequence of its interaction with multiple SH2 binding sites on the zeta-zeta and CD3 epsilon-containing dimers.

Specific CD3 epsilon association of a phosphodiesterase 4B isoform determines its selective tyrosine phosphorylation after CD3 ligation.

cAMP-specific phosphodiesterases (PDE) comprise an extensive family of enzymes that control intracellular levels of cAMP and thus regulate T cell responses. It is not known how the function of these enzymes is altered by TCR engagement. We have examined this issue by studying one of the PDE isozymes (PDE4B). PDE4B RNA and protein were detected in resting PBLs, and the levels of PDE4B protein increased with cell cycling. In peripheral blood T cells, two previously reported PDE4B isoforms could be detected: one was 75-80 kDa (PDE4B1) and the other was 65-67 kDa (PDE4B2). These two isoforms differed in their N-terminal sequence, with the presence of four potential myristylation sites in the PDE4B2 that are absent in PDE4B1. Consequently, only PDE4B2 was found in association with the CD3var epsilon chain of the TCR. In addition, although both isoforms were phosphorylated in tyrosines in pervanadate-stimulated T cells, only the TCR-associated PDE4B2 was tyrosine-phosphorylated following CD3 ligation. The kinetics of phosphorylation of TCR-associated PDE4B2 correlated with changes in cAMP levels, suggesting that tyrosine phosphorylation of the TCR-associated PDE4B isoform upon engagement of this receptor may be an important regulatory step in PDE4B function. Our results reveal that selectivity of PDE4B activation can be achieved by differential receptor association and phosphorylation of the alternatively spliced forms of this PDE.

Purification and characterization of human ZAP-70 protein-tyrosine kinase from a baculovirus expression system.

The ZAP-70 protein tyrosine kinase is essential for T cell antigen receptor (TCR)-mediated signaling. The absence of ZAP-70 results in impaired differentiation of T cells and a lack of responsiveness to antigenic stimulation. In order to study the characteristics of ZAP-70 in vitro, we overexpressed an epitopically tagged human ZAP-70 in a recombinant baculovirus expression system and purified it by column chromatography. The kinase activity of purified, recombinant ZAP-70 required cation and exhibited a strong preference for Mn2+ over Mg2+. The apparent Km of ZAP-70 for ATP was approximately 3.0 microM. The activity of the recombinant ZAP-70, unlike that of the homologous protein tyrosine kinase, Syk, was not affected by binding of TCR-derived tyrosine phosphorylated immunoreceptor tyrosine-based activation motif peptides. Several proteins were tested as potential in vitro substrates of ZAP-70. Only alpha-tubulin and the cytoplasmic fragment of human erythrocyte band 3 (cfb3), which have a region of sequence identity at the phosphorylation site, proved to be good substrates, exhibiting Kmvalues of approximately 3.3 and approximately 2.5 microM, respectively ([ATP] = 50 microM). alpha- and beta-Casein were poor substrates for ZAP-70, and no activity toward enolase, myelin basic protein, calmodulin, histone proteins, or angiotensin could be detected. In contrast to the T cell protein tyrosine kinase, Lck, ZAP-70 did not phosphorylate the cytoplasmic portion of the TCRzeta chain or short peptides corresponding to the CD3epsilon or the TCRzeta immunoreceptor tyrosine-based activation motifs. Our studies suggest that ZAP-70 exhibits a high degree of substrate specificity.

Photoaffinity labeling of two rat liver plasma membrane proteins with [32P]gamma-azidoanilido GTP in response to vasopressin. Immunologic identification as alpha subunits of the Gq class of G proteins.

Two proteins have been identified in rat liver plasma membranes that bind a photoreactive GTP analogue, [32P]gamma-azidoanilido GTP, in response to incubation with the Ca(2+)-mobilizing agonist, vasopressin. The labeled proteins possess apparent molecular masses of 42 and 43 kDa. Their labeling requires Mg2+ and can be inhibited by GTP, its analogues, and GDP but not by other nucleotides. Vasopressin-stimulated labeling is attenuated by a V1 receptor-selective antagonist. The concentration of vasopressin required to stimulate labeling is in the same range (EC50 = 4 nM) as that required for activation of GTPase and phosphoinositide-specific phospholipase C activities in liver plasma membranes. Immunodetection and immunoprecipitation of the [32P]gamma-azidoanilido GTP-labeled 42- and 43-kDa proteins with antisera raised against peptide sequences in alpha q indicate that these proteins are members of the recently described Gq class of G proteins.