class F frizzled/smoothened family, member of the 7-transmembrane G protein-coupled receptor superfamily
The class F G protein-coupled receptors includes the frizzled (FZD) family of seven-transmembrane proteins consisting 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. Also included in the class F family is the closely related smoothened (SMO), which is a transmembrane G protein-coupled receptor that acts as the transducer of the hedgehog (HH) signaling pathway. SMO is activated by the hedgehog (HH) family of proteins acting on the 12-transmembrane domain receptor patched (PTCH), which constitutively inhibits SMO. Thus, in the absence of HH proteins, PTCH inhibits SMO signaling. On the other hand, binding of HH to the PTCH receptor activates its internalization and degradation, thereby releasing the PTCH inhibition of SMO. This allows SMO to trigger intracellular signaling and the subsequent activation of the Gli family of zinc finger transcriptional factors and induction of HH target gene expression (PTCH, Gli1, cyclin, Bcl-2, etc). The WNT and HH signaling pathways play critical roles 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.