Engagement of the TCR can result in different functional outcomes such as activation, proliferation, survival, or apoptosis. However, it is not yet completely understood how signal specificity is achieved when different signals are relayed by the common intracellular signal transduction pathways. The aim of this project is to elucidate how a single-specificity TCR in peripheral T cells distinguishes between ligands which trigger distinct biological responses. Thus, an experimental system was established in which either proliferation (by treatment with OT1-streptamers) or apoptosis (by treatment with anti-CD3/CD8 mAbs) can be induced in mature OT1 TCR transgenic T cells. The cellular outcomes correlate with distinct activation/phosphorylation kinetics of key molecules involved in T cell activation, which appeared to bifurcate at the level of LAT / PLC gamma1. The input signals leading to apoptosis induced a strong, but transient activation of PLC gamma 1, PKD1 and ERK1/2. Among these molecules, ERK1/2 phosphorylation occurs mainly at the plasma membrane and its transient nature of activation is partially due to the Ą°activation-induced degradationĄą of zeta chain, Lck, and ZAP-70. In support of this observation, the inhibition of zeta chain degradation (by NH4Cl) led to sustained ERK1/2 phosphorylation and T cell activation instead of apoptosis upon antibody stimulation. In contrast, stimuli promoting proliferation generate a low and sustained activation of PLCgamma1, PKD1 and ERK1/2. Phosphorylated ERK1/2 shuttles from the plasma membrane to the endosomes, which are also positive for the TCR, PLC gamma1, and Ras. These endosomes serve as the second platform for sustained signal activation. Moreover, OT1-streptamers induce the expression of the anti-apoptotic molecule Bcl-xL and the degradation of the pro-apoptotic molecule Bim, which are required to prevent T cell apoptosis. Collectively, these data provide signaling signatures that are associated with proliferation or apoptosis of T cells.