Helix 8 Leu in the CB₁ Cannabinoid Receptor Contributes to Selective Signal Transduction Mechanisms

The intracellular C-terminal helix 8 (H8) of the CB1 cannabinoid receptor deviates from the highly conserved NPXXY(X)5,6F G-protein-coupled receptor motif, possessing a Leu instead of a Phe. We compared the signal transduction capabilities of CB1 with those of an L7.60F mutation and an L7.60I mutation that mimics the CB2 sequence. The two mutant receptors differed from wild type (WT) in their ability to regulate G-proteins in the [35S]guanosine 5′-3-O-(thio)triphosphate binding assay. The L7.60F receptor exhibited attenuated stimulation by agonists WIN-55,212-2 and CP-55,940 but not HU-210, whereas the L7.60I receptor exhibited impaired stimulation by all agonists tested as well as by the inverse agonist rimonabant. The mutants internalized more rapidly than WT receptors but could equally sequester G-proteins from the somatostatin receptor. Both the time course and maximal N-type Ca2+ current inhibition by WIN-55,212-2 were reduced in the mutants. Reconstitution experiments with pertussis toxin-insensitive G-proteins revealed loss of coupling to Gαi3 but not Gα0A in the L7.60I mutant, whereas the reduction in the time course for the L7.60F mutant was governed by Gαi3. Furthermore, Gαi3 but not Gα0A enhanced basal facilitation ratio, suggesting that Gαi3 is responsible for CB1 tonic activity. Co-immunoprecipitation studies revealed that both mutant receptors were associated with Gαi1 or Gαi2 but not with Gαi3. Molecular dynamics simulations of WT CB1 receptor and each mutant in a 1-palmitoyl-2-oleoylphosphatidylcholine bilayer suggested that the packing of H8 is different in each. The hydrogen bonding patterns along the helix backbones of each H8 also are different, as are the geometries of the elbow region of H8 (R7.56(400)-K7.58(402)). This study demonstrates that the evolutionary modification to NPXXY(X)5,6L contributes to maximal activity of the CB1 receptor and provides a molecular basis for the differential coupling observed with chemically different agonists.