class F frizzled subfamilies 5, 8 and related proteins; member of 7-transmembrane G protein-coupled receptors
This group includes subfamilies 5 and 8 of the frizzled (FZD) family of seven transmembrane-spanning proteins, which constitute a novel and separate class of GPCRs, as well as their closely related proteins. This class F protein family consists of 10 isoforms (FZD1-10) in mammals. The FZDs are activated by the wingless/int-1 (WNT) family of secreted lipoglycoproteins and preferentially couple to stimulatory G proteins of the Gs family, which activate adenylate cyclase, but can also couple to G proteins of the Gi/Gq families. In the WNT/beta-catenin signaling pathway, the WNT ligand binds to FZD and a lipoprotein receptor-related protein (LRP) co-receptor. This leads to the stabilization and translocation of beta-catenin to the nucleus, where it induces the activation of TCF/LEF family transcription factors. The conserved cytoplasmic motif of FZD, Lys-Thr-X-X-X-Trp, is required for activation of the WNT/beta-catenin pathway, and for membrane localization and phosphorylation of Dsh (dishevelled) protein, a key component of the WNT pathway that relays the WNT signals from the activated receptor to downstream effector proteins. The WNT pathway plays a critical role in many developmental processes, such as cell-fate determination, cell proliferation, neural patterning, stem cell renewal, tissue homeostasis and repair, and tumorigenesis, among many others.