Ted by immunoblot after LYP immunoprecipitation. CSK blots in panels B, C, D, E and F were scanned and the values inhibitor obtained were expressed as arbitrary units under the blot. doi:10.1371/journal.pone.0054569.gFigure 3. CSK SH3 and SH2 domains are involved in the association with LYP. A, Schematic representation of the NMR structure of the Pro rich motif P1 25033180 of Pep (orange) bound to the SH3 domain of CSK (blue) (PDB code 1JEG). P1 resid ues are numbered according to the LYP sequence (613IPPPLPVRTPESFIVVEE630). Arg620 is involved in intermolecular polar contacts with Asp27 and Gln26 of CSK; hydrogen bonds are shown by dashed lines. In addition, Arg620 could establish an intramolecular hydrogen bond with Ser624. B, Jurkat cells were electroporated with HA-CSK wild type and several mutants of CSK SH3 domain along myc-LYPR as indicated. Interaction was detected by IB after LYP IP. C, Several HA-CSK mutants in the SH3 and SH2 domains were tested for interaction with myc-LYP-RDA. Jurkat cells were left untreated or treated with PV and LYP was immunoprecipitated from cell lysates. CSK interaction was detected by IB with specific anti-HA antibody. D, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells cotransfected with different CSK plasmids, as indicated. The insert shows the IB of the CSK proteins expressed. doi:10.1371/journal.pone.0054569.gRegulation of TCR Signaling by LYP/CSK ComplexFigure 4. LYP/CSK interaction is not required to regulate TCR signaling. A, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells co-transfected with different myc-LYP plasmids, as indicated. The insert shows the expression of the LYP proteins as detected by IB. B, Erk activation was assayed in 
Jurkat cells transfected with different versions of LYP, as indicated, and stimulated with anti-CD3 AbRegulation of TCR Signaling by LYP/CSK Complexfor 5 min. Erk was immunoprecipitated from lysates of these cells and its phosphorylation was detected by IB. Expression was verified in total lysates (TL) by IB. Phospho-ERK (P-Erk) blot was inhibitor measured by densitometry and the data were expressed as arbitrary units under the blot.C, As in B, p38 activity was evaluated in Jurkat T cells stimulated with anti-CD3 and anti-CD28 Ab for 30 min by IP of HA-p38 and IB with a specific antibody for dually phosphorylated p38. Phospho-p38 (P-p38) blot was measured by densitometry scanning and the data were expressed as arbitrary units under the blot. D, Activation of a luciferase reporter gene driven by the NF-AT/AP1 site of IL-2 promoter in Jurkat cells co-transfected with LYPR, LYPTW, and CSK-W47A plasmids, as indicated. Expression of LYP and CSK proteins as detected by IB is shown in the insert. E, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells transfected with LYPR, LYPW, and CSK-W47A plasmids. The insert shows the IB of LYP and CSK proteins. R, 16574785 LYPR; W, LYPW. F, Expression of CD25 in Jurkat cells transfected with the plasmids indicated was measured by flow cytometry upon stimulation with anti-CD3 plus anti-CD28 antibodies for 24 hours. Expression of LYP and CSK proteins as detected by IB is shown in the insert. doi:10.1371/journal.pone.0054569.gfor T cells (Figure 5B). Co-expression of LCK, Fyn, and CSK lead to LYP Tyr phosphorylation, being the highest phosphorylation produced by LCK. To confirm that LCK was the main kinase involved in LYP phosphorylation, we us.Ted by immunoblot after LYP immunoprecipitation. CSK blots in panels B, C, D, E and F were scanned and the values obtained were expressed as arbitrary units under the blot. doi:10.1371/journal.pone.0054569.gFigure 3. CSK SH3 and SH2 domains are involved in the association with LYP. A, Schematic representation of the NMR structure of the Pro rich motif P1 25033180 of Pep (orange) bound to the SH3 domain of CSK (blue) (PDB code 1JEG). P1 resid ues are numbered according to the LYP sequence (613IPPPLPVRTPESFIVVEE630). Arg620 is involved in intermolecular polar contacts with Asp27 and Gln26 of CSK; hydrogen bonds are shown by dashed lines. In addition, Arg620 could establish an intramolecular hydrogen bond with Ser624. B, Jurkat cells were electroporated with HA-CSK wild type and several mutants of CSK SH3 domain along myc-LYPR as indicated. Interaction was detected by IB after LYP IP. C, Several HA-CSK mutants in the SH3 and SH2 domains were tested for interaction with myc-LYP-RDA. Jurkat cells were left untreated or treated with PV and LYP was immunoprecipitated from cell lysates. CSK interaction was detected by IB with specific anti-HA antibody. D, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells cotransfected with different CSK plasmids, as indicated. The insert shows the IB of the CSK proteins expressed. doi:10.1371/journal.pone.0054569.gRegulation of TCR Signaling by LYP/CSK ComplexFigure 4. LYP/CSK interaction is not required to regulate TCR signaling. A, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells co-transfected with different myc-LYP plasmids, as indicated. The insert shows the expression of the LYP proteins as detected by IB. B, Erk activation was assayed in Jurkat cells transfected with different versions of LYP, as indicated, and stimulated with anti-CD3 AbRegulation of TCR Signaling by LYP/CSK Complexfor 5 min. Erk was immunoprecipitated from lysates of these cells and its phosphorylation was detected by IB. Expression was verified in total lysates (TL) by IB. Phospho-ERK (P-Erk) blot was measured by densitometry and the data were expressed as arbitrary units under the blot.C, As in B, p38 activity was evaluated in Jurkat T cells stimulated with anti-CD3 and anti-CD28 Ab for 30 min by IP of HA-p38 and IB with a specific antibody for dually phosphorylated p38. Phospho-p38 (P-p38) blot was measured by densitometry scanning and the data were expressed as arbitrary units under the blot. D, Activation of a luciferase reporter gene driven by the NF-AT/AP1 site of IL-2 promoter in Jurkat cells co-transfected with LYPR, LYPTW, and CSK-W47A plasmids, as indicated. Expression of LYP and CSK proteins as detected by IB is shown in the insert. E, Activation of a luciferase reporter gene driven by the IL-2 minimal promoter in Jurkat cells transfected with LYPR, LYPW, and CSK-W47A plasmids. The insert shows the IB of LYP and CSK proteins. R, 16574785 LYPR; W, LYPW. F, Expression of CD25 in Jurkat cells transfected with the plasmids indicated was measured by flow cytometry upon stimulation with anti-CD3 plus anti-CD28 antibodies for 24 hours. Expression of LYP and CSK proteins as detected by IB is shown in the insert. doi:10.1371/journal.pone.0054569.gfor T cells (Figure 5B). Co-expression of LCK, Fyn, and CSK lead to LYP Tyr phosphorylation, being the highest phosphorylation produced by LCK. To confirm that LCK was the main kinase involved in 
LYP phosphorylation, we us.