Fig. 8. RAP binds to LRP6 and partially competes for Mesd binding. (A) RAP binds to mature LRP6 at the cell surface. Binding of ¹²⁵I-RAP (5 nM) to LRP6-transduced HT1080 cells and the control cells was carried out for 4 hours at 4°C in the absence (total) or presence of 500 nM RAP, or 500 nM Mesd. (B) Binding of ¹²⁵I-Mesd (5 nM) to LRP6-transduced HT1080 cells was carried out for 2 hours at 4°C in the absence (total) or presence of various concentrations of RAP or 500 nM Mesd. (C) LRP6-mediated ¹²⁵I-Mesd (5 nM) degradation was carried out for 4 hours at 37°C in the absence or presence of 500 nM Mesd or 500 nM RAP. Values are the means of triple determinations with the s.d. indicated by error bars.