Structure and function of a near fully-activated intermediate GPCR-Gαβγ complex

Unraveling the signaling roles of intermediate complexes is pivotal for G protein-coupled receptor (GPCR) drug development. Despite hundreds of GPCR-G alpha beta gamma structures, these snapshots primarily capture the fully activated complex. Consequently, the functions of intermediate GPCR-G protein complexes remain elusive. Guided by a conformational landscape visualized via 19F quantitative NMR and molecular dynamics (MD) simulations, we determined the structure of an intermediate GPCR-mini-G alpha s beta gamma complex at 2.6 & Aring; using cryo-EM, by blocking its transition to the fully activated complex. Furthermore, we present direct evidence that the complex at this intermediate state initiates a rate-limited nucleotide exchange before transitioning to the fully activated complex. In this state, BODIPY-GDP/GTP based nucleotide exchange assays further indicated the alpha-helical domain of the G alpha is partially open, allowing it to grasp a nucleotide at a non-canonical binding site, distinct from the canonical nucleotide-binding site. These advances bridge a significant gap in our understanding of the complexity of GPCR signaling.