Identification of Protein Phosphatase 1 as a Mitotic Lamin Phosphatase*
- From the Sealy Center for Oncology and Hematology, University of Texas Medical Branch, Galveston, Texas 77555 and the‡Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium
Abstract
At the onset of mitosis, the nuclear lamins are hyperphosphorylated leading to nuclear lamina disassembly, a process required for nuclear envelope breakdown and entry into mitosis. Multiple lamin kinases have been identified, including protein kinase C, that mediate mitotic lamin phosphorylation and mitotic nuclear lamina disassembly. Conversely, lamin dephosphorylation is required for nuclear lamina reassembly at the completion of mitosis. However, the protein phosphatase(s) responsible for the removal of mitotic phosphates from the lamins is unknown. In this study, we use human lamin B phosphorylated at mitosis-specific sites as a substrate to identify and characterize a lamin phosphatase activity from mitotic human cells. Several lines of evidence demonstrate that the mitotic lamin phosphatase corresponds to type 1 protein phosphatase (PP1). First, mitotic lamin phosphatase activity is inhibited by high nanomolar concentrations of okadaic acid and the specific PP1 peptide inhibitor, inhibitor-2. Second, mitotic lamin phosphatase activity cofractionates with PP1 after ion exchange chromatography. Third, microcystin-agarose depletes mitotic extracts of both PP1 and lamin phosphatase activity. Our results demonstrate that PP1 is the major mitotic lamin phosphatase responsible for removal of mitotic phosphates from lamin B, a process required for nuclear lamina reassembly.
Footnotes
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↵* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵§ Supported by Fund for Medical Scientific Research-Flanders Grant G.0179.97.
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↵¶ Leukemia Society of America Scholar. To whom correspondence should be addressed: Sealy Center for Oncology and Hematology, University of Texas Medical Branch, Medical Research Bldg., 9.104, 301 University Blvd., Galveston, TX 77555-1048. Tel.: 409-747-1940; Fax: 409-747-1938; E-mail: afields{at}marlin.utmb.edu.
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↵1 The abbreviations used are: PKC, protein kinase C; PP1, protein phosphatase type 1; PP2A, protein phosphatase type 2A.
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↵2 L. J. Thompson and A. P. Fields, unpublished results.
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- Received August 13, 1997.
- Revision received September 8, 1997.
