Description of species included in model and steady-state values in the absence of ligand.

<p><i>*</i>Values correspond to steady-state conditions in single-expressing cells where the total number of cell surface and intracellular receptors is 6×10⁵ and 1.5×10⁶ receptors/cell for CXCR4 and CXCR7, respectively. The total number of β-arrestin 2 molecules is 5.0×10⁵ and 7.5×10⁵ molecules/cell for endogenous β-arrestin 2 and β-arrestin 2-CBC, respectively. Receptor numbers are based on reasonable agreement with the data in Table C in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098328#pone.0098328.s001" target="_blank">File S1</a>, the assumption that a large portion of the receptors are intracellular in the absence of ligand, and ability to fit internalization data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098328#pone-0098328-g003" target="_blank">Fig. 3</a> E,F). β-arrestin 2 numbers are based on our data suggesting that the ratio of probe-labeled/endogenous β-arrestin 2 is ∼ 1.5 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098328#pone-0098328-g002" target="_blank">Fig. 2A</a>) and literature data (12).</p><p><i>**</i>Values correspond to steady-state conditions in co-expressing cells where the total number of cell surface and intracellular receptors is 6×10⁵ and 1.5×10⁶ receptors/cell for CXCR4 and CXCR7, respectively. The total number of β-arrestin 2 molecules is 5.0×10⁵ and 7.5×10⁵ molecules/cell for endogenous β-arrestin 2 and β-arrestin 2-CBC, respectively.