Dephosphorylation of Cofilin Accompanies Heat Shock-induced Nuclear Accumulation of Cofilin

Cofilin is a widely distributed 21-kDa actin-modulating protein that forms intranuclear actin/cofilin rods in cultured fibroblastic cells exposed to heat shock or 10% dimethyl sulfoxide. In this study, cofilin was shown to be phosphorylated on a serine residue in cultured rat fibroblastic 3Y1 cells. Two-dimensional gel electrophoresis revealed that about 50% of the cofilin was phosphorylated in 3Y1 cells at 37 degrees C. Exposure of the cells to heat shock at 43 degrees C induced dephosphorylation of cofilin. The dephosphorylation of cofilin was detected about 30 min after the temperature shift and was completed within 120 min. Moreover, treatment of cells with 10% dimethyl sulfoxide also caused the dephosphorylation of cofilin. However, incubation of the cells with an isotonic NaCl solution, which induced cytoplasmic actin/cofilin rods, did not induce dephosphorylation of cofilin. Other cellular stress agents such as 6% ethanol or 50 microM sodium arsenite, which caused some heat shock responses in cells, did not induce dephosphorylation of cofilin. Thus, cofilin dephosphorylation was closely correlated with its nuclear accumulation. Incubation of the enucleated 3Y1 cells at 43 degrees C still induced dephosphorylation of cofilin, suggesting that the dephosphorylation occurred mostly in the cytoplasm in intact cells. It is likely that cofilin is dephosphorylated in the cytoplasm prior to its nuclear accumulation.