Calcium Dependence of Polycystin-2 Channel Activity Is Modulated by Phosphorylation at Ser⁸¹²

Polycystin-2 (PC-2) is a non-selective cation channel that, when mutated, results in autosomal dominant polycystic kidney disease. In an effort to understand the regulation of this channel, we investigated the role of protein phosphorylation in PC-2 function. We demonstrated the direct incorporation of phosphate into PC-2 in cells and tissues and found that this constitutive phosphorylation occurs at Ser⁸¹², a putative casein kinase II (CK2) substrate domain. Ser⁸¹² can be phosphorylated by CK2 in vitro and substitution S812A results in failure to incorporate phosphate in cultured epithelial cells. Non-phosphorylated forms of PC-2 traffic normally in the endoplasmic reticulum and cilial compartments and retain homo- and hetero-multimerization interactions with PC-2 and polycystin-1, respectively. Single-channel studies of PC-2, S812A, and a substitution mutant, T721A, not related to phosphorylation show that PC-2 and S812A function as divalent cation channels with similar current amplitudes across a range of holding potentials; the T721A channel is not functional. Channel open probabilities for PC-2 and S812A show a bell-shaped dependence on cytoplasmic Ca²⁺ but there is a shift in this Ca²⁺ dependence such that S812A is 10-fold less sensitive to Ca²⁺ activation/inactivation than the wild type PC-2 channel. In vivo analysis of PC-2-dependent enhanced intracellular Ca ²⁺ transients found that S812A resulted in enhanced transient duration and relative amplitude intermediate between control cells and those overexpressing wild type PC-2. Phosphorylation at Ser⁸¹² modulates PC-2 channel activity and factors regulating this phosphorylation are likely to play a role in the pathogenesis of polycystic kidney disease.