The regulation of intermediate filament reorganization in mitosis: p34(cdc2) Phosphorylates vimentin at a unique N-terminal site

The disassembly of vimentin-containing intermediate filament (IF) networks during mitosis in BHK-21 cells is accompanied by increased phosphorylation of vimentin (Chou, Y.-H., Rosevear, E., and Goldman, R. D. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 1885-1889). We have recently identified p34(cdc2) as the catalytic subunit of one of the two endogenous vimentin kinases in mitotic baby hamster kidney cells (Chou, Y.-H., Bischoff, J.R., Beach, D., and Goldman, R.D. (1990) Cell 62, 1063-1071). To begin to characterize the biochemical basis of the p34(cdc2)-mediated IF disassembly process, we have purified and sequenced the ³²P-labeled tryptic peptides derived from in vitro-phosphorylated vimentin. The results demonstrate that Ser-55, in the N-terminal non-α-helical domain of vimentin, is the most favored phosphorylation site. This finding supports the idea that the N-terminal domain of type III IF protein plays a crucial role in regulating IF structure and supramolecular organization.