Functional Role of the C-Terminal Amphipathic Helix 8 of Olfactory Receptors and Other G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) transduce various extracellular signals, such as neurotransmitters, hormones, light, and odorous chemicals, into intracellular signals via G protein activation during neurological, cardiovascular, sensory and reproductive signaling. Common and unique features of interactions between GPCRs and specific G proteins are important for structure-based design of drugs in order to treat GPCR-related diseases. Atomic resolution structures of GPCR complexes with G proteins have revealed shared and extensive interactions between the conserved DRY motif and other residues in transmembrane domains 3 (TM3), 5 and 6, and the target G protein C-terminal region. However, the initial interactions formed between GPCRs and their specific G proteins remain unclear. Alanine scanning mutagenesis of the murine olfactory receptor S6 (OR-S6) indicated that the N-terminal acidic residue of helix 8 of OR-S6 is responsible for initial transient and specific interactions with chimeric Gα, resulting in a response that is 2.2-fold more rapid and 1.7-fold more robust than the interaction with Gα. Our mutagenesis analysis indicates that the hydrophobic core buried between helix 8 and TM1-2 of OR-S6 is important for the activation of both Gα and Gα. This review focuses on the functional role of the C-terminal amphipathic helix 8 based on several recent GPCR studies.