Peptide mapping of phosphorylated vimentin. Evidence for a site-specific alteration in mitotic cells.

Vimentin, the subunit protein of one type of intermediate filament, has been isolated from 32Pi-labeled nonmitotic and mitotic mouse L-929 cells. Analysis of tryptic phosphopeptides by two-dimensional maps indicates that vimentin is phosphorylated at multiple sites in mitotic cells. Comparison of nonmitotic and mitotic vimentin phosphotryptic peptides indicated that in addition to the 6-7 major phosphorylated tryptic peptides found in nonmitotic cells, vimentin isolated from mitotic cells contained an additional 2 distinct phosphorylated peptides following trypsin digestion. Partial acid hydrolysis and one-dimensional phosphoamino acid analysis indicates that phosphoserine is present in all 9 major phosphopeptides. Treatment of L-929 cells with 8-bromo-cAMP did not result in a qualitative change in the phosphopeptide map of vimentin isolated from a normal cell population. These results suggest that the reorganization of vimentin filaments during mitosis is accompanied by a site-specific change in phosphorylation.