Synthesis Question: Co-Stimulatory and Inhibitory Receptors Modulate Antigen Receptor Signaling

Synthesis question: Co-stimulatory and inhibitory receptors modulate antigen receptor signaling in T and B lymphocytes. A new receptor is discovered, expressed on the surface of T cells, and called 'X'. An antibody to X is generated, and used in T cell stimulation experiments. In these experiments, antibodies to the TCR complex (anti-CD3) and to CD28 (anti-CD28) are known to stimulate signaling through those receptors, as does the antibody to X. The data from an experiment measuring IL-2 secretion by the T cells stimulated with different combinations of antibodies are shown in Figure.
a) Does stimulation of receptor X alone induce IL-2 production by T cells? Does it enhance or inhibit TCR signaling? Indicate the evidence supporting your answers.
b) If you examined the amino acid sequence of the receptor X cytoplasmic tail, what motif would you expect to find?
Biochemical studies show that when receptor X is stimulated, a tyrosine residue in the cytoplasmic tail becomes phosphorylated.
c) From these data, what are the two most likely signaling proteins that might be recruited to receptor X when it is stimulated? Does the T cell stimulation data shown in the graph rule in or out either of your candidate proteins? Why or why not?
d) Describe a biochemical experiment (analysis of proteins) that would indicate which enzyme is recruited to receptor X when it is stimulated.

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a) Stimulation of X does not induce IL-2...

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