IDENTIFICATION OF PROTEIN PHOSPHATASE-1 AS A MITOTIC LAMIN PHOSPHATASE

At the onset of mitosis, the nuclear lamins are hyperphosphorylated le ading to nuclear lamina disassembly, a process required for nuclear en velope breakdown and entry into mitosis, Multiple Iamin kinases have b een identified, including protein kinase C, that mediate mitotic lamin phosphorylation and mitotic nuclear lamina disassembly. Conversely, l amin dephosphorylation is required for nuclear lamina reassembly at th e completion of mitosis, However, the protein phosphatase(s) responsib le for the removal of mitotic phosphates from the lamins is unknown, I n 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 demonstra te that the mitotic lamin phosphatase corresponds to type 1 protein ph osphatase (PP1), First, mitotic lamin phosphatase activity is inhibite d by high nanomolar concentrations of okadaic acid and the specific PP I peptide inhibitor, inhibitor-a, Second, mitotic Iamin phosphatase ac tivity cofractionates with PP1 after ion exchange chromatography, Thir d, microcystin-agarose depletes mitotic extracts of both PP1 and Iamin phosphatase activity. Our results demonstrate that PP1 is the major m itotic lamin phosphatase responsible for removal of mitotic phosphates from lamin B, a process required for nuclear lamina reassembly.