Description and values of parameters.

?Fit to internalization and β-arrestin 2 binding data with CXCL12 and CXCR4 in CXCR4+ cells.??Fit to internalization and β-arrestin 2 binding data with CXCL12 and CXCR7 in CXCR7+ cells.*Reference gives maximum rate of β-arrestin 2 binding as kf[β]total = 0.136 s−1. Assuming a range of 105–5×106 β-arrestin 2 per cell gives kf≈10−8–10−6 (#/cell) −1s−1. Reference gives β-arrestin 2 dissociation rate constant as kr≈0.024 s−1, which gives a β-arrestin 2/receptor equilibrium dissociation constant KD≈104–106 (#/cell).**Rate constants (k) and equilibrium dissociation constants (K) are converted from #/cell to their effective value in nM using: andCell volume (Vcell) is assumed to be 8.4×10−12 L based on a spherical, 20 µm diameter cell.***These parameters do not affect model output of fold change of β-arrestin bound (see Fig. A in File S1) but are included for completeness.°Experimental conditions. Cells are assumed to grow to 2–3x above confluence at the time of plating.∧Value was converted from units reported to these units by using Avogadro's number and cell volume from original paper.