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 secreted 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. Here we report that Frzb-1 blocks Wnt-1 induced cytosolic accumulation of beta-catenin, a key component of the Wnt signaling pathway, in human embryonic kidney cells. Structure/function analysis reveals that complete removal of the frizzled domain of Frzb-1 abolishes the Wnt-1/Frzb-1 protein interaction and the inhibition of Wnt-1 mediated axis duplication in Xenopus embryos. In contrast, removal of the C-terminal portion of the molecule preserves both Frzb-Wnt binding and functional inhibition of Wnt signaling. Partial deletions of the Frzb-1 cysteine-rich domain maintain Wnt-1 interaction, but functional inhibition is lost. Taken together, these findings support the conclusion that the frizzled domain is necessary and sufficient for both activities. Interestingly, Frzb-1 does not block Wnt-5A signaling in a Xenopus functional assay, even though Wnt-5A coimmunoprecipitates with Frzb-1, suggesting that coimmunoprecipitation does not necessarily imply inhibition of Wnt function.
Lin
K
K
Craniofacial and Skeletal Diseases Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892, USA.
Wang
S
S
Julius
M A
MA
Kitajewski
J
J
Moos
M
M
Luyten
F P
FP
D000818:Animals; D019556:COS Cells; D002417:Cattle; D003545:Cysteine; D003598:Cytoskeletal Proteins / Q000096:biosynthesis*; D004625:Embryo, Nonmammalian; D006023:Glycoproteins; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis; D015398:Signal Transduction; D015534:Trans-Activators; D014162:Transfection; D051153:Wnt Proteins; D051155:Wnt1 Protein; D014981:Xenopus; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; D000099274:Frizzled-Related Proteins
D003545:Cysteine; D003598:Cytoskeletal Proteins; D006023:Glycoproteins; D047908:Intracellular Signaling Peptides and Proteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D015534:Trans-Activators; D051153:Wnt Proteins; D051155:Wnt1 Protein; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; D000099274:Frizzled-Related Proteins; C495266:CTNNB1 protein, Xenopus; C495270:CTNNB1 protein, human; C495249:WNT1 protein, Xenopus; C495247:WNT1 protein, human; C495248:wnt8a protein, Xenopus