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CUTTING EDGE |


,
*
Division of Tumor Immunology, Department of Cancer Immunology and AIDS,
Dana Farber Cancer Institute and Departments of Medicine and
Pathology, Harvard Medical School, Boston, MA 02115
| Abstract |
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| Introduction |
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Among the substrates identified are the hemopoietic cell-specific adaptor proteins, SLP-76 (Src homology (SH) 2-domain-containing leukocyte protein of 76 kDa)2 and FYB/SLAP (Fyn T-binding protein/SLP-76-associated protein) (4, 5, 6). SLP-76 is an adaptor protein that is comprised of three domains allowing for protein-protein interactions: an amino-terminal acidic region containing tyrosine phosphorylation sites (7), a central proline-rich region that binds to the SH3 domain of Grb2 family members (8, 9), and a carboxyl-terminal SH2 domain that associates with FYB/SLAP (5, 6). The importance of SLP-76 in T cell signaling has been demonstrated in experiments in the Jurkat human T cell leukemia line and in SLP-76-deficient mice. SLP-76 overexpression in Jurkat cells augments TCR-mediated activation of the IL-2 promoter (9, 10). Furthermore, Jurkat T cells lacking SLP-76 show defects in TCR-mediated signals (11). Studies of SLP-76-/- mice also indicate that SLP-76 is required for pre-TCR signaling, as these mice exhibit arrest of thymocyte development at the double negative CD25+ CD44- stage (12, 13).
FYB/SLAP was independently cloned on the basis of its ability to bind to the src kinase FYN-T and SLP-76 (5, 6). It also has the hallmarks of an adaptor protein with several proline-rich regions, two putative nuclear localization sequences, a carboxyl-terminal SH3 domain, and multiple tyrosine-containing motifs. Two forms of FYB at 120 and 130 kDa exits that differ due to an insertion of 46 aa toward the C terminus (14). FYB-120 corresponds to the version of SLAP termed SLAP-130 (5). FYB undergoes tyrosine phosphorylation in response to TCR ligation, an event diminished in FYN-T-deficient T cells (15). However, unlike with SLP-76, transfection studies with FYB/SLAP have yielded conflicting results on the role of FYB/SLAP in the regulation of IL-2 production. Recent studies from our laboratory demonstrated that FYB is selectively phosphorylated between residue 585 and 670 by FYN-T (16), providing a template for the recruitment of the SH2 domains of FYN-T and SLP-76. The interaction between FYN-T, FYB, and SLP-76 is unusual in its distinct cytoplasmic localization and its stable kinetics of phosphorylation. Furthermore, coexpression of all three components of the FYN-T-FYB-SLP-76 matrix can cooperatively up-regulate anti-CD3-mediated IL-2 transcription in Jurkat T cells (16). However, previous studies have not addressed whether direct association between these components is required for their synergy.
In this paper, we demonstrate that two YDDV sites on FYB are major sites of phosphorylation by FYN-T and serve as major sites for SLP-76 SH2 domain binding. We further demonstrate that the loss of SLP-76 binding by mutation of these sites markedly reduced the ability of FYN-T-FYB-SLP-76 to up-regulate IL-2 transcription. By contrast, mutation of the FYN-T SH2 domain binding site had no consistent effect. Overall, these results indicate a role for SLP-76/FYB complex formation in the regulation of TCR-driven IL-2 production.
| Materials and Methods |
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Monoclonal Ab to SLP-76 was kindly provided by Dr. Paul R. Findell (Syntex, Palo Alto, CA). Anti-phosphotyrosine mAb 4G10 was kindly provided by Dr. Tom Roberts (Dana-Farber Cancer Institute, Boston, MA). Anti-human CD3 (OKT3) was obtained from American Type Culture Collection (Manassas, VA).
DNA constructs
Wild-type FYB were cloned into the EcoRI and
SalI sites of pSR
mammalian expression vector containing
a sequence encoding the influenza hemaaglutinin (HA) epitope tag at the
N terminal. The FYB deletion mutations have been described. The
tyrosine mutants were created with the Quickchange Mutagenesis kit
(Stratagene, La Jolla, CA) using oligonucleotides (Life Technologies,
Gaithersburg, MD) encoding the appropriate mutations plus silent
mutations to provide novel restriction sites for diagnosis. The NF-AT
luciferase reporter construct was provided by Dr. Burakoff (Dana-Farber
Cancer Institute).
Transfections, immunoprecipitation, and immunoblotting
Transfections and immunoprecipitations were conducted as previously described (17, 18). Briefly, 20 x 106 SV40 large tumor Ag-transfected Jurkat cells were electroporated and allowed to sit for 20 h. Cells were harvested and lysed with 200 µl lysis buffer (20 mM Tris-HCl, pH 8.0, 150 mM NaCl, 1% (v/v) Triton X-100, 1 mM sodium vanadate, 1 mM PMSF, 1 mM leupeptin). Immunoprecipitation was conducted by incubation of the lysate with the Ab for 1 h at 4°C, followed by incubation with 50 µl of protein A-Sepharose beads (10% w/v) for 1 h at 4°C. Immunoprecipitates were washed three times with ice-cold lysis buffer and subjected to SDS-PAGE. For immunoblotting, the immunoprecipitates were separated by SDS-PAGE and transferred onto nitrocellulose filters (Schleicher and Schuell, Keene, NH). Filters were blocked with 5% (w/v) skim milk for 1 h in TBS, pH 8.0, and then probed with the indicated Ab. Bound Ab was revealed with HRP-conjugated rabbit anti-mouse or donkey anti-rabbit Abs using enhanced chemiluminescence (Amersham, Arlington Heights, IL).
Luciferase assays
A total of 5 x 105 transfected Jurkat cells were stimulated with 1 µg/ml OKT3 and 2 µg/ml rabbit anti-mouse Ab at 37°C for 6 h and subsequently assayed for luciferase activity following manufacturer suggested protocols (dual luciferase system kit; Promega, Madison, WI). Luciferase activity was determined using the luminometer (MicroLumat, EG&G Berthold Bad WildBad, Germany). Luciferase units of the experimental vector were normalized to the level of the control vector in each sample.
| Results and Discussion |
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Recent studies in our laboratory demonstrated that the region
between residues 585 and 670 on FYB was selectively
phosphorylated by FYN-T, but not LCK and ZAP-70, and
mediates binding to SH2 domains of FYN-T and SLP-76 (16).
Fig. 1
A shows the amino acid
sequence of a portion of this region highlighting the three potential
tyrosine phosphorylation motifs that would be suited
for SH2 domain binding (YDDV, residues 595598; YDGI, residues
625628; YDDV, residues 651654). The YDGI site has previously been
identified as the predominant FYN-T-binding site in vitro by peptide
binding assays and in vivo using mutant FYB in which the tyrosine of
the YDGI site was replaced by phenylalanine (16). The two
Y595DDV and Y651DDV motifs
are similar, each containing conserved EV residues preceding the YDDV
sequence (Fig. 1
A). By in vitro peptide binding assay, the
C-terminal YDDV motif was shown to be able to bind to SLP-76 SH2 domain
(16). However, in vivo studies are necessary to confirm
this result and to test whether the N-terminal YDDV motif or other
tyrosine-containing motifs in FYB can bind to SLP-76 as well.
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Both YDDV sites are necessary for optimal SLP-76 binding
To assess whether the two pYDDV sites are responsible for binding
to SLP-76, the three FYB mutants (Y595F, Y651F, and Y595/651F) were
coexpressed with FYN-T and HA-tagged SLP-76 in Jurkat T cells followed
by immunoprecipitation with anti-SLP-76 mAb and anti-HA
blotting. The expression levels of FYB and the mutants and SLP-76 were
similar as shown in the whole-cell lysates (Fig. 2
A, right upper and
lower panels, respectively). As expected, SLP-76
coprecipitated WT FYB (left panel, lane
1), whereas the double mutant Y595/651F fail to coprecipitate with
SLP-76. The single mutations resulted in significant reductions in
SLP-76 binding, with Y651F having a greater effect (55% reduction)
than Y595F (20% reduction) (left panel, lanes
2 and 3; also see Fig. 2
B: densiometric
readings in histogram).
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FYB and SLP-76 interaction is required to up-regulate NF-AT activity in the FYN-T-FYB-SLP-76 pathway
To address the role of SLP-76 binding to FYB in T cell function,
Jurkat T cells were transiently transfected with FYB mutants or a
control SR
vector together with FYN-T and SLP-76 and a luciferase
reporter construct driven by an IL-2 NF-AT/AP-1 promoter. The mutants
used include the YDDV mutants, a C-terminal deletion mutant D585
(deletion from amino acid 585 to C terminus), and a FYB mutant with the
Tyr625 in the YDGI motif substituted by
phenylalanine (Y625F). This latter site has previously been shown to
bind to FYN-T, but had not been examined for an effect on IL-2
transcription (16). As reported, the combined expression
of FYN-T/FYB/SLP-76 potentiated IL-2 transcription by about 100-fold
beyond vector-transfected control (Fig. 3
, A and B).
Deletion mutant D585, which lacks the region of FYN-T and SLP-76
binding, effectively eliminated the potentiating effect (Fig. 3
A, left panel). By contrast, the Y625F mutant
had little effect on FYB synergy with FYN and SLP-76 (left
panel). Several experiments showed no efect, while a slight degree
of inhibition. This result can be explained by two scenarios. One
possibility is that FYN-T complex formation with FYB and SLP-76 is not
necessary for the FYB-FYN-SLP-76 pathway. FYN-T may only be required to
phosphorylate FYB, which can then be accessed by a third
anchor protein such as SLP-76. Alternatively, the Y625F mutant retains
some 20% WT FYB binding to FYN-T, indicating a site other than YDGI
can mediate the FYB-FYN-T interaction, albeit much less efficiently.
This limited interaction may be sufficient to mediate the up-regulation
of TCR-stimulated NF-AT/AP-1 activity
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Recent studies have demonstrated that the importance of the SLP-76 (and
LAT) adaptors in T cell signaling (9, 10, 11, 17, 20, 21, 22).
Further, the combined expression of SLP-76 with the FYB adaptor and
FYN-T has demonstrated potent increases in TCR-driven IL-2 production
(14, 16). Whether this cooperativity was related to
separate effects of each component on different pathways or was
genuinely related to binding between the components had been unclear.
Our mapping of the SLP-76 binding sites and our finding that binding is
needed for the potentiation of IL-2 transcription demonstrates that
FYBs role as a scaffold for SLP-76 binding contributes to optimal
activation. The functional significance of the FYB-SLP-76 interaction
in SLP-76 function is also consistent with the report that the SLP-76
SH2 domain is needed for its ability to augment IL-2 transcription
(9). FYB/SLAP is the principal protein that has been found
to bind to the SLP-76 SH2 domain (5, 15). The residual
stimulatory effects of the Y595/651F double mutant further argue that
other signaling components (albeit less potent than SLP-76) can also
use other sites on the FYB scaffold in the mediation of signals leading
to IL-2 transcription (Fig. 3
B). We are now in the process
of determining the identity of the second messenger pathways downstream
of the interaction.
| Footnotes |
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2 Abbreviations used in this paper: SLP-76, SH2-domain-containing leukocyte protein of 76 kDa; SH, Src homology; FYB, Fyn T-binding protein; SLAP, SLP-76-associated protein; HA, hemagglutinin. ![]()
Received for publication September 13, 1999. Accepted for publication September 29, 1999.
| References |
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/CD3 and CD28 signaling in T cells: SH2/SH3 domains, protein-tyrosine and lipid kinases. Immunol. Today 15:225.[Medline]
1 in a SLP-76-deficient T cell. Science 218:413.
/CD3-mediated tyrosine phosphorylation pathway that up-regulates interleukin 2 transcription in T-cells. J. Biol. Chem. 274:21170.
/CD3 induction of interleukin 2. Immunity 6:1.[Medline]
chain. Biochem. Biophys. Res. Commun. 222:50.[Medline]
1 and the Ras pathway. Immunity 9:617.[Medline]
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