Affinities of peptides#2–4 for BRS-3*, extracellular chimeric BRS-3*s, and wild-type GRPR expressed in CHOP cells (loss-of-affinity chimeras). The diagrams of the chimeric receptors formed are shown at the top. The chimeric BRS-3*s ([R127Q]BRS-3) were formed by replacing each of the extracellular domains of BRS-3 one at a time by the comparable GRPR extracellular domain. Results are expressed as the percentage of the saturable binding seen with no peptide#2-4 present that occurred when the indicated concentration of peptide#2-4 was present. Each point is the mean from at least 5 separate experiments, and in each experiment each point was measured in duplicate. EC2-, e3-, and e4-GRPR refer to the substitution of this extracellular domain of the GRPR for the comparable extracellular domain in BRS-3*. The arrows indicate large changes in affinity from BRS-3*.