The translocation of regucalcin to the nuclei of normal rat liver was investigated. The existence of endogenous regucalcin in isolated liver nuclei was confirmed by Western blotting using anti-regucalcin antibody. Nuclear translocation of regucalcin was estimated by sodium sulfate-polyacrylamide gel electrophoresis analysis. When isolated liver nuclei were incubated in the presence of exogenous regucalcin (50 microg/ml; 1.5 microM), potent band for regucalcin was found in the nuclei, indicating that the protein is translocated into the nucleus. This translocation was an early event. Nuclear regucalcin translocation was not appreciably changed in the presence of adenosine 5'-triphosphate (2 mM), guanosine 5'-triphosphate (2 mM), calcium chloride (0.1 mM), and the lectin wheat germ agglutinin (50 or 100 microg/ml), suggesting that its translocation is not mediated through nuclear localization signal. Moreover, Ca2+-dependent protein kinase and protein tyrosine phosphatase activities in isolated liver nuclei were significantly increased in the presence of anti-regucalcin monoclonal antibody (100 ng/ml) in the enzyme reaction mixture, and these increases were completely abolished by the addition of regucalcin (50 microg/ml). This study demonstrates that regucalcin is translocated into liver nucleus, and that it can regulate the nuclear function.

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