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G-protein-coupled receptors (GPCRs) remain the primary conduit by which cells detect environmental stimuli and communicate with each other. Upon activation by extracellular agonists, these seven-transmembrane-domain-containing receptors interact with heterotrimeric G proteins to regulate downstream second messenger and/or protein kinase cascades. Crystallographic evidence from a prototypic GPCR, the β2-adrenergic receptor (β2AR), in complex with its cognate G protein, Gs, has provided a model for how agonist binding promotes conformational changes that propagate through the GPCR and into the nucleotide-binding pocket of the G protein α-subunit to catalyse GDP release, the key step required for GTP binding and activation of G proteins. The structure also offers hints about how G-protein binding may, in turn, allosterically influence ligand binding. Here we provide functional evidence that G-protein coupling to the β2AR stabilizes a ‘closed’ receptor conformation characterized by restricted access to and egress from the hormone-binding site. Surprisingly, the effects of G protein on the hormone-binding site can be observed in the absence of a bound agonist, where G-protein coupling driven by basal receptor activity impedes the association of agonists, partial agonists, antagonists and inverse agonists. The ability of bound ligands to dissociate from the receptor is also hindered, providing a structural explanation for the G-protein-mediated enhancement of agonist affinity, which has been observed for many GPCR–G-protein pairs. Our data also indicate that, in contrast to agonist binding alone, coupling of a G protein in the absence of an agonist stabilizes large structural changes in a GPCR. The effects of nucleotide-free G protein on ligand-binding kinetics are shared by other members of the superfamily of GPCRs, suggesting that a common mechanism may underlie G-protein-mediated enhancement of agonist affinity.

In 2015, as part of the Reproducibility Project: Cancer Biology, we published a Registered Report (Fung et al., 2015), that described how we intended to replicate selected experiments from the paper "Inhibition of BET recruitment to chromatin as an effective treatment for MLL-fusion leukaemia" (Dawson et al., 2011). Here, we report the results of those experiments. We found treatment of MLL-fusion leukaemia cells (MV4;11 cell line) with the BET bromodomain inhibitor I-BET151 resulted in selective growth inhibition, whereas treatment of leukaemia cells harboring a different oncogenic driver (K-562 cell line) did not result in selective growth inhibition; this is similar to the findings reported in the original study (Figure 2A and Supplementary Figure 11A,B; Dawson et al., 2011). Further, I-BET151 resulted in a statistically significant decrease in BCL2 expression in MV4;11 cells, but not in K-562 cells; again this is similar to the findings reported in the original study (Figure 3D; Dawson et al., 2011). We did not find a statistically significant difference in survival when testing I-BET151 efficacy in a disseminated xenograft MLL mouse model, whereas the original study reported increased survival in I-BET151 treated mice compared to vehicle control (Figure 4B,D; Dawson et al., 2011). Differences between the original study and this replication attempt, such as different conditioning regimens and I-BET151 doses, are factors that might have influenced the outcome. We also found I-BET151 treatment resulted in a lower median disease burden compared to vehicle control in all tissues analyzed, similar to the example reported in the original study (Supplementary Figure 16A; Dawson et al., 2011). Finally, we report meta-analyses for each result.