Cell cycle-dependent phosphorylation of mammalian protein phosphatase 1 by cdc2 kinase.
Protein phosphatase 1 ( PP-1) is known to be a critical component of eukaryotic cell cycle progression. In vitro, our previous studies showed that cdc2 kinase phosphorylates Thr-320 (T320) in PP-1, and that this leads to inhibition of enzyme activity. To examine directly the phosphorylation of PP-1 in intact mammalian cells, an antibody has been prepared that specifically recognizes PP-1C alpha phosphorylated at T320. Cell synchronization studies revealed in a variety of cell types that T320 of PP-1 was phosphorylated to high levels only during early to mid-mitosis. The phosphorylation of T320 of PP-1 was reduced by the cyclin-dependent protein kinase inhibitor, olomoucine, and increased by the PP-1/PP-2A inhibitor, calyculin A. Immunofluorescence microscopy using phospho-T320 antibody indicated that in NIH 3T3 cells the phosphorylation of PP-1 began to increase from basal levels in prophase and to peak at metaphase. Immunostaining indicated that phospho- PP-1 was localized exclusively to nonchromosomal regions. Furthermore, in cell fractionation studies of mitotic cells, phospho- PP-1 was detectable only in the soluble fraction. These observations suggest that phosphorylation by cdc2 kinase in early to mid-mitosis and inhibition of PP-1 activity is likely to contribute to the increased state of phosphorylation of proteins that is critical to the initiation of normal cell division.[1]References
- Cell cycle-dependent phosphorylation of mammalian protein phosphatase 1 by cdc2 kinase. Kwon, Y.G., Lee, S.Y., Choi, Y., Greengard, P., Nairn, A.C. Proc. Natl. Acad. Sci. U.S.A. (1997) [Pubmed]
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