FIG 4.

(a) If the TCR-endogenous pMHC off rate (k_{off self}) is greater than the rate at which Lck phosphorylates Tyr319 on Zap70 and binds to it (k_open), there will be a significant increase in the amount of active Zap70 upon stimulation with a small amount of agonist peptide. (b) Changing the rate k_open does not change the fraction of Zap70 that is bound at steady state in the absence of agonist peptide. (c) Negligible amounts of bound Zap70 are active in the regime for which k_{off self} is greater than k_open.