Amy Grunbeck developed a useful assay for measuring CCR5 (chemokine C-C type receptor 5) activity in a cell-free environment. The first step in the CCR5 signaling pathway is to activate a cytoplasmic heterotrimeric G protein to cause release of GDP and uptake of GTP.  Here she shows how GTP uptake by the G protein is enhanced in the presence of CCR5 and its agonist ligand.  The assay is adapted for use with the NABB (nanoscale apolipoprotein-bound bilayer) technology platform.