Abstract
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.
Highlights
Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) are essential coreceptors for morphogens Wnt/Wingless and cooperate with Frizzled (Fz) to activate Wnt signaling (Pinson et al, 2000; Tamai et al, 2000)
To establish a consensus view on LDL-receptor-related protein 6 (LRP6) dynamics and function, it is important to determine the conformational states of the LRP5/6 ectodomain before and after ligand binding on the cell surface
We found that the LRP6 ectodomain can flex at a hinge located at the P2-E2 boundary, and both the extent and the direction of interdomain mobility are modulated by highly conserved N-linked glycan chains located near the hinge
Summary
Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) are essential coreceptors for morphogens Wnt/Wingless and cooperate with Frizzled (Fz) to activate Wnt signaling (Pinson et al, 2000; Tamai et al, 2000). Several crystal structures are available for partial fragments of LRP6 with two submodules (PE12 or PE34), revealing a tightly packed arrangement between the P and the following E domains, as well as between the consecutive PE submodules (Ahn et al, 2011; Chen et al, 2011; Cheng et al, 2011; Holdsworth et al, 2012). To establish a consensus view on LRP6 dynamics and function, it is important to determine the conformational states of the LRP5/6 ectodomain before and after ligand binding on the cell surface. This will elucidate how such conformational changes would impact signaling events
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