Conformational Freedom of the LRP6 Ectodomain Is Regulated by N-glycosylation and the Binding of the Wnt Antagonist Dkk1.

LDL-receptor-related protein 6 (LRP6) is a single-pass membrane glycoprotein with a large modular ectodomain and forms a higher order signaling platform upon binding Wnt ligands on the cell surface. Although multiple crystal structures are available for fragments of the LRP6 ectodomain, we lack a consensus view on the overall molecular architecture of the full-length LRP6 and its dynamic aspects. Here, we used negative-stain electron microscopy to probe conformational states of the entire ectodomain of LRP6 in solution and found that the four-module ectodomain undergoes a large bending motion hinged at the junction between the second and the third modules. Importantly, the extent of inter-domain motion is modulated by evolutionarily conserved N-glycan chains proximal to the joint. We also found that the LRP6 ectodomain becomes highly compact upon complexation with the Wnt antagonist Dkk1, suggesting a potential role for the ectodomain conformational change in the regulation of receptor oligomerization and signaling.