THE CYSTEINE-RICH FRIZZLED DOMAIN OF FRZB-1 IS REQUIRED AND SUFFICIENT FOR MODULATION OF WNT SIGNALING

Convincing evidence has accumulated to identify the Frizzled proteins as receptors for the Wnt growth factors, In parallel, a number of secr eted frizzled-like proteins with a conserved N-terminal frizzled motif have been identified, One of these proteins, Frzb-1, binds Wnt-1 and Xwnt-8 proteins and antagonizes Xwnt-8 signaling in Xenopus embryos, H ere we report that Frzb-1 blocks Wnt-1 induced cytosolic accumulation of beta-catenin, a key component of the Wnt signaling pathway, in huma n embryonic kidney cells, Structure/function analysis reveals that com plete removal of the frizzled domain of Frzb-1 abolishes the Wnt-1/Frz b-1 protein interaction and the inhibition of Wnt-1 mediated axis dupl ication in Xenopus embryos, In contrast, removal of the C-terminal por tion of the molecule preserves both Frzb-Wnt binding and functional in hibition of Wnt signaling, Partial deletions of the Frzb-1 cysteine-ri ch domain maintain Wnt-1 interaction, but functional inhibition is los t. Taken together, these findings support the conclusion that the friz zled domain is necessary and sufficient for both activities, Interesti ngly, Frzb-1 does not block Wnt-5A signaling in a Xenopus functional a ssay, even though Wnt-5A coimmunoprecipitates with Frzb-1, suggesting that coimmunoprecipitation does not necessarily imply inhibition of Wn t function.