Phosphorylation of vimentin in the C-terminal domain after exposure to calyculin-A

Exposure of 3T3 fibroblasts to the phosphatase inhibitor, calyculin-A, induces marked morphological changes and the formation of an aggregate of actin and myosin connected to the nucleus by intermediate filaments (Hirano, K., L. Chartier, R. G. Taylor, R. E. Allen, N. Fusetani, H. Karaki, D. J. Hartshorne: J. Muscle Res. Cell Motil. 13, 341-353 (1992)). Vimentin was isolated from this complex and shown to be phosphorylated. At least 4 phosphorylation sites were indicated. These sites were distinct from those phosphorylated by the cAMP-dependent protein kinase. Limited proteolysis was used to define the domains in which phosphorylation occurred. Vimentin was isolated from 32P-labeled calyculin-A-treated cells and digested with thrombin and alpha-chymotrypsin. Proteolysis with thrombin limited the phosphorylation to either the central core or C-terminal domain. Proteolysis with alpha-chymotrypsin indicated that the multiple phosphorylation sites were restricted to the C-terminal domain of vimentin.