Extracellular Disulfide Bridges Serve Different Purposes in Two Homologous Chemokine Receptors, CCR1 and CCR5s

In addition to the 7 transmembrane receptor (7TM)-conserved disulfide bridge between transmembrane (TM) helix 3 and extracellular loop (ECL)-2, chemokine receptors (CCR) contain a disulfide bridge between the N terminus and what previously was believed to be ECL-3. Recent crystal and NMR structures of the CXC chemokine receptors (CXCR) CXCR4 and CXCR1, combined with structural analysis of all endogenous chemokine receptors indicate that this chemokine receptor–conserved bridge in fact connects the N terminus to the top of TM-7. By employing chemokine ligands that mainly target extracellular receptor regions and small-molecule ligands that predominantly interact with residues in the main binding crevice, we show that the 7TM-conserved bridge is essential for all types of ligandmediated activation, whereas the chemokine-conserved bridge is dispensable for small-molecule activation in CCR1. However, in striking contrast to previous studies in other chemokine receptors, high-affinity CCL3 chemokine binding was maintained in the absence of either bridge. In the highly related CCR5, a completely different dependency was observed as neither activation nor binding of the same chemokines was retained in the absence of either bridge. In contrast, both bridges were dispensable for activation by the same small molecules. This indicates that CCR5 activity is independent of extracellular regions, whereas in CCR1 the preserved folding of ECL-2 is necessary for activation. These results indicate that conserved structural features in a receptor subgroup do not necessarily provide specific traits for the whole subgroup but rather provide unique traits to the single receptors.