Molecular characterization of CCR6: Involvement of multiple domains in ligand binding and receptor signaling

The CC chemokine receptor 6 (CCR6) is selectively expressed on memory T cells, B cells, and dendritic cells and appears to be involved in the initiation of a memory immune response. The only chemokine ligand for CCR6 is CCL20/MIP-3. In the present study, we attempted to define the extracellular domains (ECDs) of CCR6 responsible for CCL20/MIP-3 binding using a domain-swapping approach in which the ECDs of CCR6 were substituted with the corresponding CCR5 domains to generate various CCR6/CCR5 chimeras. These chimeras were tested for receptor expression, ligand binding, and functional activity as evaluated by calcium flux and chemotaxis. All chimeras showed respectable surface expression; however only one, substituted with extracellular loop 1 from CCR5, showed reduced functional activity. The general failure of functionality of the CCR6/CCR5 chimeras may imply that characteristics of each ECD are critical for coordination among all the ECDs of CCR6. Additionally, of interest, a chimera containing all of the ECDs from CCR5 in the context of CCR6 neither responded to CCR5 ligands nor served as a coreceptor for macrophage-tropic HIV-1. These results suggest that not only ECDs but also transmembrane and intracellular domains of CCR5 are involved in both ligand binding and coreceptor activity.