Gating function of isoleucine-116 in TM-3 (position III: 16/3.40) for the activity state of the CC-chemokine receptor 5 (CCR5)

Background and PurposeA conserved amino acid within a protein family indicates a significance of the residue. In the centre of transmembrane helix (TM)-5, position V:13/5.47, an aromatic amino acid is conserved among class A 7TM receptors. However, in 37% of chemokine receptors - a subgroup of 7TM receptors - it is a leucine indicating an altered function. Here, we describe the significance of this position and its possible interaction with TM-3 for CCR5 activity. Experimental ApproachThe effects of [L203F]-CCR5 in TM-5 (position V:13/5.47), [I116A]-CCR5 in TM-3 (III:16/3.40) and [L203F;G286F]-CCR5 (V:13/5.47;VII:09/7.42) were determined in G-protein- and -arrestin-coupled signalling. Computational modelling monitored changes in amino acid conformation. Key Results[L203F]-CCR5 increased the basal level of G-protein coupling (20-70% of E-max) and -arrestin recruitment (50% of E-max) with a threefold increase in agonist potency. In silico, [I116A]-CCR5 switched 1-angle in [L203F]-CCR5. Furthermore, [I116A]-CCR5 was constitutively active to a similar degree as [L203F]-CCR5. Tyr(244) in TM-6 (VI:09/6.44) moved towards TM-5 in silico, consistent with its previously shown function for CCR5 activation. On [L203F;G286F]-CCR5 the antagonist aplaviroc was converted to a superagonist. Conclusions and ImplicationsThe results imply that an aromatic amino acid in the centre of TM-5 controls the level of receptor activity. Furthermore, Ile(116) acts as a gate for the movement of Tyr(244) towards TM-5 in the active state, a mechanism proposed previously for the (2)-adrenoceptor. The results provide an understanding of chemokine receptor function and thereby information for the development of biased and non-biased antagonists and inverse agonists.