Science ( IF 41.845 ) Pub Date : 2020-07-31 , DOI: 10.1126/science.aba3373 Daniel Hilger, Kaavya Krishna Kumar, Hongli Hu, Mie Fabricius Pedersen, Evan S. O’Brien, Lise Giehm, Christine Jennings, Gözde Eskici, Asuka Inoue, Michael Lerch, Jesper Mosolff Mathiesen, Georgios Skiniotis, Brian K. Kobilka
Family B heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) play important roles in carbohydrate metabolism. Recent structures of family B GPCR-Gs protein complexes reveal a disruption in the α-helix of transmembrane segment 6 (TM6) not observed in family A GPCRs. To investigate the functional impact of this structural difference, we compared the structure and function of the glucagon receptor (GCGR; family B) with the β2 adrenergic receptor (β2AR; family A). We determined the structure of the GCGR-Gs complex by means of cryo–electron microscopy at 3.1-angstrom resolution. This structure shows the distinct break in TM6. Guanosine triphosphate (GTP) turnover, guanosine diphosphate release, GTP binding, and G protein dissociation studies revealed much slower rates for G protein activation by the GCGR compared with the β2AR. Fluorescence and double electron-electron resonance studies suggest that this difference is due to the inability of agonist alone to induce a detectable outward movement of the cytoplasmic end of TM6.