Ca2+-dependent Conformational Changes in a C-terminal Cytosolic Domain of Polycystin-2

The PKD1 and PKD2 genes are the genes that are mutated in patients suffering from autosomal dominant polycystic kidney disease. The human PKD2 gene codes for a968-amino acid long membrane protein called polycystin-2 that represents a cation channel whose activity can be regulated by Ca2+ ions. By CD, fluorescence, and NMR spectroscopy, we have studied a 117-amino acid-long fragment of the cytoplasmic domain of polycystin-2, polycystin-2-(680-796) that was proposed to contain a Ca2+-binding site. NMR structure determination reveals the existence of two Ca2+-binding sites in polycystin-2-(680-796) arranged in a typical and an a typical EF-hand motif. In the absence of Ca2+ the protein forms a dimer that is dissociated by Ca2+ binding. This dissociation may be related to the Ca2+ inactivation observed earlier. The calcium affinity of the protein was determined by fluorescence and NMR spectroscopy. At 293 K, the K-D values for the high and low affinity sites are 55 mu M and 179 mu M, respectively.