NUCLEAR CALMODULIN 62-KDA CALMODULIN-BINDING PROTEIN COMPLEXES IN INTERPHASIC AND MITOTIC CELLS

We report here that a 62 kDa calmodulin-binding protein (p62), recentl y identified in the nucleus of rat hepatocytes, neurons and glial cell s, consists of four polypeptides showing pI values between 5.9 and 6.1 . By using a DNA-binding overlay assay we found that the two most basi c of the p62 polypeptides bind both single- and double-stranded DNA. T he intranuclear distribution of calmodulin and p62 was analysed in hep atocytes and astrocyte precursor cells, and in proliferating and diffe rentiated astrocytes in primary cultures by immunogold-labeling method s. In non-dividing cells nuclear calmodulin was mostly localized in he terochromatin although it was also present in euchromatin and nucleoli . A similar pattern was observed for p62, with the difference that it was not located in nucleoli, p62/calmodulin complexes, mainly located over heterochromatin domains were also observed in interphasic cells. These complexes remained associated with the nuclear matrix after in s itu sequential extraction with nucleases and high-salt containing buff ers. In dividing cells, both calmodulin and p62 were found distributed over all the mitotic chromosomes but the p62/calmodulin aggregates we re disrupted. These results suggest a role for calmodulin and p62 in t he condensation of the chromatin.