PhyB	Phytochrome B (photoreceptor of Arabidopsis thaliana), here only the first 651 amino acids are used, since this part is sufficient to bind PIF under 660 nm, but not under 740 nm light.
PIF	PhyB-interacting factor, here the first 100 aa of A. thaliana PIF6 are used, since they are sufficient to bind to PhyB.
PIFS	A mutant version of functional PIF developed in this study, that can move through the secretory pathway to the cell surface and remain in a functional conformation, i.e., it is still capable of binding PhyB.
PhyBt	Fluorescently labeled streptavidin-based tetramers of PhyB.
PhyBt(660)	PhyBt pre-illuminated with 660 nm light, so that equilibrium is reached, in which 80% of the PhyB molecules are in their ON state
PhyBt(740)	PhyBt pre-illuminated with 740 nm light, so that all PhyB molecules are in the OFF state
PhyB ON	PhyB in the active conformation allowing binding to PIF with nanomolar affinity (Figure 1).
PhyB OFF	PhyB in the inactive conformation, in which they cannot bind to PIF (Figure 1).
τKPR	The KPR time or KPR duration. The half-life of the bivalent binding of the soluble ligand to the TCR needs to be at least this time to induce downstream, activatory signaling.
moxGFP	A GFP mutant (S30R, Y29N, C48S, F64L, S65T, C70S, Q80R, F99S, N105T, Y145F, M153T, V163A, I171V, A206K) that was optimized for the folding in the endoplasmic reticulum (ox) and that is expressed as a monomer (m).
TCR	T cell receptor. The receptor on T cells controlling T cell activation. It consists of the TCRα, TCRβ, CD3γ, CD3δ, CD3ε and ζ chains.
PIFS-TCR	A preliminary non-functional TCR chimera, in which PIFS is fused to the ectodomain of the TCRβ subunit.
pMHC	Peptide bound to Major Histocompatibility Complex constitutes the ligand for the TCR.
GFP-PIFS-TCR	Our engineered TCR, in which the moxGFP and PIFS are fused to the ectodomain of the TCRβ subunit.
GFP-PIFS-TCR cells	Jurkat E6.1 T cells expressing the GFP-PIFS-TCR.