Helix 8 of the Leukotriene B4 Receptor Is Required for the Conformational Change to the Low Affinity State after G-protein Activation

Abstract

Recent studies have revealed that G-protein-coupled receptors contain a putative cytoplasmic helical domain, helix 8. Leukotriene B4 (LTB4) receptor 1 derivatives with truncated or mutated helix 8 showed much higher LTB4 binding than wild-type (WT) receptors. Similar to the WT receptor, LTB4 promoted guanosine 5′-3-O-(thio)triphosphate (GTPγS) binding in these mutants. Unlike the WT receptor, however, the addition of GTPγS did not inhibit LTB 4 binding to the mutant receptors. Scatchard analyses revealed that mutants maintained high affinity for LTB4, even in the presence of excess GTPγS. Consistently, mutant receptors showed a more prolonged Ca2+ mobilization and cellular metabolic activation than the WT receptor. From mutational studies and three-dimensional modeling based on the structure of bovine rhodopsin, we conclude that the helix 8 of LTB4 receptor 1 plays an important role in the conformational change of the receptor to the low affinity state after G-protein activation, possibly by sensing the status of coupling Gα subunits as GTP-bound.