Calmodulin-binding proteins in the nuclei of quiescent and proliferatively activated rat liver cells

alpha-Spectrin, myosin light chain kinase (MLCK), and caldesmon have been detected in the nuclei of rat liver cells by 125I-calmodulin overlay, immunoblotting, and immunocytochemical methods. alpha-Spectrin is localized in the nuclear matrix, nuclear envelope, and nuclear pores. It has also been detected inside the nuclei in the form of small aggregates. MLCK is present in the nuclear matrix, envelope, nucleoli, and in a nuclease extract (S1 subfraction) but not in the nuclear pores. Caldesmon shows a diffuse distribution pattern inside the nuclei but it is not present in the nucleoli. Since all these proteins are components of the actin-myosin motility systems the presence of actin in the different nuclear subfractions has also been investigated: actin is present in the nuclear matrix, nuclear envelope, nucleoli, and nuclear pores. Proliferative activation of rat liver cells in vivo by partial hepatectomy induces the increase of alpha-spectrin, MLCK, and actin in different nuclear subfractions. This, together with the increase of nuclear calmodulin at the same time after hepatectomy (Pujol, M. J., Soriano, M., Aligue, R., Carafoli, E., and Bachs, O. (1989) J. Biol. Chem. 264, 18863-18865), indicates that nuclear calmodulin could activate a nuclear contractile system during proliferative activation. A 62-kDa protein (p62) which binds to calmodulin columns and shows immunological similarities to caldesmon is specifically located in the region surrounding the nuclear envelope and is associated with the heterochromatin.