The cysteine-rich frizzled domain of Frzb-1 is required and sufficient for modulation of Wnt signaling

The cysteine-rich frizzled domain of Frzb-1 is required and sufficient for modulation of Wnt signaling

^*Craniofacial and Skeletal Diseases Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892; ^†Laboratory of Developmental Biology, Center for Biologics and Evaluation, Food and Drug Administration, Bethesda, MD 20892; and ^‡Department of Pathology and Center for Reproductive Sciences, Columbia University, College of Physicians and Surgeons, 630 West 168th Street, New York, NY 10032

Edited by Arnold L. Levine, Princeton University, Princeton, NJ, and approved August 13, 1997 (received for review May 21, 1997)

Abstract

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 β-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.

Footnotes

↵ § To whom reprint requests should be addressed at: Craniofacial and Skeletal Diseases Branch, National Institute of Dental Research, National Institutes of Health, Building 10, Room 1N108, Bethesda, MD 20892-1188. e-mail: luyten{at}yoda.nidr.nih.gov.
This paper was submitted directly (Track II) to the Proceedings Office.
Abbreviations: CRD, cysteine-rich domain; HA, hemagglutinin.