(A) The superposition of the 20 best-fit NMR structures of mouse MESD. The superposition is based on the most conserved and rigid secondary structure region. The β-strands are colored in green, the loops are in grey and αhelices are in other colors. (B) A ribbon representation of the average structure of mouse MESD using Pymol. The colors are coded the same as (A). (C) The same ribbon representation as (B). The core domain is colored in green and the flexible C-terminal helical domain is in yellow. The sidechain heavy atoms of the key residues in both domains are highlighted using sticks and labeled in red for the core domain and in light green for the flexible helical domain. (D) Surface representation of the average NMR structure of mouse MESD, with negatively charged surface in red and positively charged surface in blue. The electrostatic potential is calculated using APBS and the surface is displayed using Pymol. The same orientation and position of MESD are shown as (C). The key residues in both domains are also labeled as (C). See also Figure S2.

(A) An overview of the chaperone complex. The chaperone domain is in green and the C-terminal flexible helical domain of MESD is in yellow. The BP domain of LRP6 is shown in pink. The sidechain atoms of the interacting residues in BP domain are shown in either blue or yellow sticks, whereas the interacting residues in MESD are shown in either brown or purple sticks. (B) Binding interface between Q31, W32, D35 in MESD and R9 and R13 in the BP domain. (C) An aromatic bridge between H46 of MESD and H207 of the BP domain. (D) Binding interface between K13, K15 of MESD and E32, D33, D50, E53 of the BP domain. (E) Binding interface between K68, K71 and K72 in MESD and D94/E96/N98/W115/E117 and Q120/D137/W138/G139/E140 in the BP domain. In (B–E), the salt bridges are indicated with red dashed lines and H-bonds are shown with blue dashed lines. See also Figure S4.

(A) An overview of the escort complex. The chaperone domain of MESD is shown in green and the escort domain of MESD is shown in yellow. The first BP domain of LRP6 is shown in pink. The sidechain heavy atoms of the interacting residues in the BP domain are shown in blue sticks, whereas the interacting residues in MESD are shown in brown sticks. (B) Binding interfaces between K162, K164 in MESD and D119/Q120/D137/W138/G139/E140 in the BP domain, and between K166, K169 in MESD and D79/D94/E96/N98/E117 in the BP domain. (C) Binding interface between K173, K183, R187 in MESD and E32/D33/D50/E53/E54 in the BP domain. In (B) and (C), the salt bridges are indicated with red dashed lines and H-bonds are shown with blue dashed lines. See also Figure S4.

The rigid chaperone domain (Ch) of MESD binds to the newly synthesized BP domain of LRP6, serving as a folding template. After BP domain properly folded, MESD switches the binding from the rigid chaperone domain to the flexible escort domain (Es), safely guarding the mature receptor traveling from the ER to the Golgi, preventing premature ligand (Lig) binding. The acidic environment of the Golgi activates the histidine switch in the BP domain that leads to the dissociation of MESD from the receptor. MESD will be retrieved back to the ER by the KDEL-receptor. The properly folded receptor will be properly post-translationally modified and further reach the cell membrane for activation of canonical WNT pathway. RAP goes through a similar cycle for promoting the ligand-binding domain folding and trafficking as described before (Herz and Marschang, 2003).

(A) The chaperone function of the wild type MESD is demonstrated as more mature BP domain of LRP6 is secreted into medium compared with pcDNA transfection (Left). As a control, an equal amount of BP domain is observed in the cell lysate as compared with pcDNA transfection (Right). Another control for equal loading is the actin in the cell lysate. (B) Impaired chaperone function of a typical mutant, MESD_W32R. (C) Normal chaperone function of MESD_R111A. (D) Quantification of the Western blot results of different mutants, showing the relative secretion of BP domain of LRP6 after subtraction of the values from pcDNA-transfected cells. The secretion of the BP domain promoted by the wild type MESD was set at 100%. (E) Cell surface binding analyses of ¹²⁵I-MESD (5 nM) to LRP6-transduced HT1080 cells. MESD mutants that have the impaired chaperone function were selected for the binding competition with wildtype MESD (50 nM). In both (D) and (E), values are the means of triplicates with the standard deviation indicated by error bars. See also Figure S1.

(A) Ribbon representation of the C-terminal flexible helical domain of MESD with the conserved lysine and arginine sidechain heavy atoms highlighted by sticks. (B) Competition binding results of different K or R mutations in the C-terminal flexible domain of MESD as compared with the wild-type protein, showing the binding ability of the MESD mutants to the mature, cell surface located, intact LRP6. The details of all the mutations are list underneath. (C) Competition binding results of different concentrations of the noK mutant of MESD as compared with the wild-type protein. (D) A comparison of relative secretion of the BP domain of LRP6 into the cell culture medium promoted by the noK mutant and wild-type MESD, as monitored by a Western blot of both the cell culture medium and cell lystate. (E) Quantification of the Western blot results shown in (D). In (B), (C) and (E), values shown are the means of triplet results with the standard deviation indicated by error bars. See also Figure S1.

(A) The chaperone complex. For MESD, the chaperone domain is in green ribbon and the escort domain is in yellow ribbon. The BP domain is in pink with the YWTD motifs highlighted in blue, whereas the EGF repeat is in light blue. The hydrophobic interface between the β-propeller and the EGF is labeled as “Hydrophobic Face”. The hydrophobic faces of the β-propeller are in brown and the hydrophobic faces of the EGF are in lemon. In this panel, five major interaction sits are labeled and highlighted with the stick models of the side chains of the interacting residues. (B) The escort complex. The colors are the same as A, except the hydrophobic interface between the β-propeller and the EGF is highlighted in red. In addition, three YWTD-motifs in the binding interface between the escort domain of MESD and the BP domain of LRP6 are colored in green, whereas the other three YWTD-motifs with the histidine switch are colored in brown. The major interaction sites are labeled and highlighted with the stick models of the side chains of the interacting residues. The interacting lysine residues in MESD are colored in brown and the interacting residues in the BP domain are colored in blue. A histidine switch is also displayed with the highlight of three histidine residues of the BP domain, H207, H226 and H243, in a space-filling model. The surrounding lysine residues in MESD are displayed as pink sticks and labeled. See also Figure S4.