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Figure 2


Fig. 2. The two-signal model of Fz-LRP signaling to ß-catenin showing the regulatory relationships between each component (A) and summarizing the probable physical interactions (B). In this model, Wnt stimulation promotes Fz-LRP oligomerization, which transduces two separate signals to the cytoplasm. The first signal is LRP phosphorylation, mediated by membrane-localized GSK3 and LRP-bound CKI{gamma}. We speculate that neither kinase is activated by Wnt. Rather, the LPR-Wnt-Fz interaction allows LRP to become phosphorylatable, which then promotes the recruitment of axin to the plasma membrane, leading to its inactivation and/or degradation. The second signal is Fz-dependent Dsh/Dvl phosphorylation, which we tentatively propose involves trimeric G proteins. Activated Dvl then inhibits the APC-axin-GSK3 complex by a poorly understood mechanism. The dashed line in panel A reflects the fact that Dsh/Dvl also participates in recruitment of axin to the plasma membrane. Although hyperactivation of either branch by overexpression can stabilize ß-catenin, both signals are required under physiological conditions.





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