Statin, a protein specifically present in nonproliferating cells, is a phosphoprotein and forms a complex with a 45-kilodalton serine/threonine kinase.

The protein statin is found in nuclei of nonproliferating cells. Here we report that statin is a phosphoprotein, phosphorylated at serine residues in cultured cells. During immunoprecipitation with anti-statin (S44) antibody, a 45-kDa protein co-precipitates with the 57-kDa statin. In vitro kinase assays demonstrate that the S44 immunoprecipitates can phosphorylate, besides statin, immunoglobulins, enolase, and casein, at either serine or serine/threonine residues. Kinase assays with immunoprecipitated proteins performed on casein- or enolase-impregnated gels show that these substrates are phosphorylated by the 45-kDa (p45) protein. When the S44 immunoprecipitates from human cultured fibroblasts with different in vitro life-spans were compared, the p45 kinase activity was present only in young nongrowing and senescent cells, but not in young growing ones. In other cell cultures, the kinase is detected only in protein complexes precipitated from quiescent 3T3 cells, but not from cycling 3T3 cells or from transformed human glioma (U251-4) cells. Cell fractionation studies, indicating that the phosphorylating activity of S44 immunoprecipitates correlates both qualitatively and quantitatively with the amount of statin present, provide strong evidence that in vivo statin is specifically associated with the p45 kinase. These results suggest that the nonproliferation-specific nature of statin is indeed related to the phosphorylated property of this protein and maybe contributed by the associated kinase.