PMID:16263759

Citation	Li, Y, Chen, J, Lu, W, McCormick, LM, Wang, J and Bu, G (2005) Mesd binds to mature LDL-receptor-related protein-6 and antagonizes ligand binding. J. Cell. Sci. 118:5305-14
Abstract	Wnt co-receptors LRP5 and LRP6 are two members of the low-density lipoprotein receptor family. Receptor-associated protein is not only a specialized chaperone but also a universal antagonist for members of the low-density lipoprotein receptor family. Here we test whether Mesd, a newly identified chaperone for members of the low-density lipoprotein receptor family, also binds to mature receptors at the cell surface and antagonizes ligand binding. We found that Mesd binds to cell surface LRP5 and LRP6, but not to other members of the low-density lipoprotein receptor family. Scatchard analysis revealed that Mesd binds cell surface LRP6 with high affinity (K(d) approximately 3.3 nM). Interestingly, the C-terminal region of Mesd, which is absent in sequences from invertebrates, is necessary and sufficient for binding to mature LRP6, and is required for LRP6 folding. We also found that LRP6 is not a constitutively active endocytosis receptor and binding of the receptor-associated protein to LRP6 partially competes for Mesd binding. Finally, we demonstrated that Mesd antagonizes ligand binding to LRP6 at the cell surface. Together our results show that in addition to serving as a folding chaperone, Mesd can function as a receptor antagonist by inhibiting ligand binding to mature LRP6.
Links	PubMed Online version:10.1242/jcs.02651
Keywords	Animals; Binding, Competitive; Cell Line, Tumor; Cell Membrane/drug effects; Cell Membrane/metabolism; Cytosol/metabolism; Endocytosis; Fibroblasts/metabolism; Humans; Intercellular Signaling Peptides and Proteins/pharmacology; LDL-Receptor Related Protein-Associated Protein/antagonists & inhibitors; LDL-Receptor Related Protein-Associated Protein/pharmacology; LDL-Receptor Related Proteins/antagonists & inhibitors; LDL-Receptor Related Proteins/chemistry; LDL-Receptor Related Proteins/deficiency; LDL-Receptor Related Proteins/metabolism; Ligands; Low Density Lipoprotein Receptor-Related Protein-6; Mice; Molecular Chaperones/chemistry; Molecular Chaperones/metabolism; Myocytes, Smooth Muscle/metabolism; Protein Binding; Protein Processing, Post-Translational; beta Catenin/metabolism
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