Saturation binding of Alexa-647-labeled RANTES analogs to CCR5-SNAP-488 by FCCS. (a) Cross-correlation traces and fits from one representative experiment of 5P12-647, 5P14-647, 6P4-647, and PSC-647 binding to CCR5-SNAP-488 at various ligand concentrations are shown. Cross-correlation traces were fitted to one translational component with 3D diffusion. (b) Saturation-binding isotherms for 5P12-647, 5P14-647, 6P4-647, and PSC-647 binding to CCR5-SNAP-488 are shown. Ligand-binding data were analyzed by global nonlinear regression of a model with a shared dissociation constant Kd and one active receptor fraction parameter for each of three to five independent experiments. The binding isotherms show the pooled receptor occupancies with respect to the active receptor fractions.