The PKD (Polycystic Kidney Disease) domain was first identified in the Polycystic Kidney Disease protein, polycystin-1 (PDK1 gene), and contains an Ig-like fold consisting of a β-sandwich of seven strands in two sheets with a Greek key topology, although some members have additional strands []. Polycystin-1 is a large cell-surface glycoprotein involved in adhesive protein-protein and protein-carbohydrate interactions; however it is not clear if the PKD domain mediates any of these interactions. PKD domains are also found in other proteins, usually in the extracellular parts of proteins involved in interactions with other proteins. For example, domains with a PKD-type fold are found in archaeal surface layer proteins that protect the cell from extreme environments [], and in the human VPS10 domain-containing receptor SorCS2 [].
