PURIFICATION AND MOLECULAR-CLONING OF A SECRETED, FRIZZLED-RELATED ANTAGONIST OF WNT ACTION

Frizzled polypeptides are integral membrane proteins that recently wer e shown to function as receptors for Wnt signaling molecules, Were, me report the identification of a novel, secreted 36-kDa protein that co ntains a region homologous to a putative Wnt-binding domain of Frizzle ds. This protein, called Frizzled-related, protein (FRP), was first id entified as a heparin-binding polypeptide that copurified with hepatoc yte growth factor/scatter factor in conditioned medium from a human em bryonic lung fibroblast line, Degenerate oligonucleotides, based on th e NH2-terminal sequence of the purified protein, were used to isolate corresponding cDNA clones. These encoded a 313-amino acid polypeptide, containing a cysteine-rich domain of approximate to 110 residues that was 30-40% identical to the putative ligand-binding domain of Frizzle d proteins, A 4.4-kb transcript of the FRP gene is present in many org ans, both in the adult and during embryogenesis, and homologs of the g ene are detectable in DNA from several vertebrate species, In biosynth etic studies, FRP was secreted but, like Wnts, tended to remain associ ated with cells, When coexpressed with several Wnt family members in e arly Xenopus embryos, FRP antagonized Wnt-dependent duplication of the embryonic dorsal asis. These results indicate that FRP may function a s an inhibitor of Wnt action during development and in the adult.