C-terminal putative kinase domain of FAM20 (family with sequence similarity 20), Drosophila Four-jointed (Fj), and related proteins
Drosophila Fj is a Golgi kinase that phosphorylates Ser or Thr residues within extracellular cadherin domains of a transmembrane receptor Fat and its ligand, Dachsous (Ds). The Fat signaling pathway regulates growth, gene expression, and planar cell polarity (PCP). Defects from mutation in the Drosophila fj gene include loss of the intermediate leg joint, and a PCP defect in the eye. Fjx1, the murine homologue of Fj, has been shown to be involved in both the Fat and Hippo signaling pathways, these two pathways intersect at multiple points. The Hippo pathway is important in organ size control and in cancer. FAM20B is a xylose kinase that may regulate the number of glycosaminoglycan chains by phosphorylating the xylose residue in the glycosaminoglycan-protein linkage region of proteoglycans. This domain has homology to a kinase-active site, mutation of three conserved Asp residues at the Drosophila Fj putative active site abolished its ability to phosphorylate Ft and Ds cadherin domains. FAM20A may participate in enamel development and gingival homeostasis, FAM20B in proteoglycan production, and FAM20C in bone development. FAM20C, also called Dentin Matrix Protein 4, is abundant in the dentin matrix, and may participate in the differentiation of mesenchymal precursor cells into functional odontoblast-like cells. Mutations in FAM20C are associated with lethal Osteosclerotic Bone Dysplasia (Raine Syndrome), and mutations in FAM20A with Amelogenesis imperfecta (AI) and Gingival Hyperplasia Syndrome. This model includes the FAM20_C domain family, previously known as DUF1193; FAM20_C appears to be homologous to the catalytic domain of the phosphoinositide 3-kinase (PI3K)-like family.
Jiyao W et al.(2023). "The conserved domain database in 2023.",