Effect of cell swelling on membrane and cytoplasmic distribution of pI(Cln)

pI(Cln) is found ubiquitously in mammalian cells and is postulated to play a critical role in cell volume regulation. Mutagenesis studies led to the proposal that pI(Cln) is a swelling-activated anion channel. However, recent studies in Madin-Darby canine kidney cells and endothelial cells have shown that the protein is localized primarily to the cytoplasm. It has therefore been postulated that activation involves reversible translocation of pI(Cln) from the cytoplasm and insertion into the plasma membrane. We tested this hypothesis using several different approaches. Fractionation of C₆ glioma cells into plasma membrane- and cytoplasm-containing fractions demonstrated that ~90% of the recovered pI(Cln) was confined to the cytosol. Swelling had no effect on the relative amount of protein present in the plasma membrane fraction. Immunofluorescence microscopy revealed that pI(Cln) is localized primarily, if not exclusively, to the cytoplasm of swollen and nonswollen cells. Similarly, transfection of cells with a green fluorescent protein-labeled pI(Cln) construct failed to reveal any membrane localization of the protein. These findings do not support the hypothesis that pI(Cln) is a volume regulatory anion channel activated by swelling-induced membrane insertion.