Burger, W. A. C.; Das, S.; Venugopal, H.; Quinn, A. S.; Williams-Noonan, B. J.; Wong, L. L. L.; Nguyen, T. T.; Prendergast, J. R.; Kirk, N. S.; Leis, A. P.; Dramicanin, M.; Josephs, T.; Call, M. E.; Schulte, G.; O'Mara, M. L.; Huynh, K.; Glukhova, A.

Frizzleds (FZDs) are class F G protein-coupled receptors (GPCRs) that primarily signal through Dishevelled (DVL) to initiate Wnt signalling, with evidence for context-specific coupling to heterotrimeric G proteins. Every FZD-G protein structure to date has been determined without an agonist, implying that agonist binding is not required for stable complex formation in vitro and directly contradicting the difficulty of demonstrating G protein coupling in cells. All these structures contain unassigned density between the receptor and the heterotrimeric G protein. From structures generated by single-particle cryo-electron microscopy (cryo-EM) of FZD7-Gs and FZD5-Gq, we identify this density as the detergent lauryl maltose neopentyl glycol (LMNG), which bridges the receptor and G protein to stabilise a weak, shallow interface. Absence of LMNG - in biochemical preparations and molecular dynamics simulations - destabilises the complex, showing that constitutive FZD-G protein engagement is intrinsically weak. The weak G protein coupling is governed by a conserved intracellular interface that when mutated differentially influences DVL and G protein signalling. Our findings show that LMNG has masked the instability of purified FZD-G protein complexes and provide the structural basis of FZDs as inherently weak G protein activators that are predisposed to DVL signalling.