p34cdc2 acts as a lamin kinase in fission yeast.

Abstract

Abstract. The nuclear lamina is an intermediate filament network that underlies the nuclear membrane in higher eukaryotic cells. During mitosis in higher eukaryotes, nuclear lamins are phosphorylated by a mitosis-specific kinase and this induces disassembly of the lamina structure. Recently, p34 ~dc2 protein kinase purified from starfish has been shown to induce phosphorylation of lamin proteins and disassembly of the nuclear lamina when incubated with isolated chick nuclei suggesting that p34 ~¢~ is likely to be the mitotic lamin kinase (Peter, M., J. Nakagawa, M. Dor6e, J. C. Labbe, and E. A. Nigg. 1990b. Cell. 45:145-153). To confirm and extend these studies using genetic techniques, we have investigated the role of p34 ~d¢2 in lamin phosphorylation in the fission yeast. As fission yeast lamins have not been identified, we have introduced a cDNA encoding the chicken lamin B2 protein into fission yeast. We report here that the chicken lamin B2 protein expressed in fission yeast is assembled into a structure that associates with the nucleus during interphase and becomes dispersed throughout the cytoplasm when cells enter mitosis. Mitotic reorganization correlates with phosphorylation of the chicken lamin B2 protein by a mitosis-specific yeast lamin kinase with similarities to the mitotic lamin kinase of higher eukaryotes. We show that a lamin kinase activity can be detected in cell-free yeast extracts and in p34 ~da immunoprecipitates prepared from yeast cells arrested in mitosis. The fission yeast lamin kinase activity is temperature sensitive in extracts and immunoprecipitates prepared from strains bearing temperature-sensitive mutations in the cdc2 gene. These results in conjunction with the previously reported biochemical studies strongly suggest that disassembly of the nuclear lamina at mitosis in higher eukaryotic cells is a consequence of direct phosphorylation of nuclear lamins by p34 ~c2.T HE fission yeast, Schizosaccharomyces pombe, has been a useful experimental system for the study of the eukaryotic cell cycle as numerous mutants deficient in cell cycle control have been identified (for review see reference 22). A central controlling element is the protein kinase p34 '~o2, activation of which is required for initiation of mitosis. Recent work has shown that cdc2 has been highly conserved throughout evolution and is required for initiation of mitosis in all eukaryotic cells (for review see reference 36). Entry into mitosis is characterized by profound changes in cellular organization (for review see reference 25), and it is important to determine how cdc2 activation causes these mitotic events.It is likely that protein phosphorylation plays an important role in promoting mitotic changes in structure as entry into mitosis is accompanied by a general increase in protein phosphorylation (16, 21,23,28). One prominent mitotic substrate is the nuclear lamina, a structure composed of polymers of intermediate filament type proteins called lamins. During interphase the nuclear lamins are found in an insoluble...

Fission yeast sts1+ gene encodes a protein similar to the chicken lamin B receptor and is implicated in pleiotropic drug-sensitivity, divalent cation-sensitivity, and osmoregulation.

“…Curiously, the lamin B protein became dispersed throughout the cytoplasm when cells entered mitosis, which may correlate with the cell cyclespecific assembly/disassembly of nuclear lamina observed in mammalian cells (for reviews, see Gerace and Burke, 1988;Nigg, 1989). These observations might suggest that interphase fission yeast nuclei contain a nuclear lamina similar to that in higher eukaryotes (Enoch et al, 1991).…”

“…Although no laminalike structures were identified in yeasts, the amino acid sequence of the SIR4 protein has some similarity to mammalian lamin proteins (Diffley and Stillman, 1989) and a ring of 10-nm-diameter filaments is found inside the plasma membrane in the neck connecting the mother cell to its bud (Haarer and Pringle, 1987;Kim et al, 1991). In the fission yeast, recently, Enoch et al (1991) described the functional expression of the chicken lamin B gene. They observed that the lamin B protein expressed formed a "ring-like" structure associated with the nucleus and existed as an insoluble nuclear matrix-like material.…”

Interphase phosphorylation of lamin A

“…The first region spanned the entire N-terminus, with seven identified phosphorylation sites, two of which (Ser12 and Ser22) were predominant high-turnover sites. Ser22 has been previously identified as one of the major Cdk1 targets that is phosphorylated during mitosis (Dessev et al, 1991;Enoch et al, 1991;Heald and McKeon, 1990;Ward and Kirschner, 1990). The second region lies between the end of the a-helical central rod domain and the Ig-fold, and contained ten sites, including five high-turnover sites (Ser390, Ser392, Ser404, Ser407, Ser423).…”

Protein dephosphorylation and the intracellular control of the cell number

“…One crucial mitotic function of PP1 may be to bring about the necessary inhibition of Cdk1 by dephosphorylating Thr 161 in Cdk1 (148). Another important role of PP1 may be to dephosphorylate the nuclear Cdk1 substrate lamin (149), thereby permitting the repolymerization that is necessary to rebuild the nuclear envelope at the onset of cytokinesis (150). An important substrate for PP1, at least in mammals, is the RB tumor suppressor protein that is dephosphorylated by a type-1 phosphatase activity during mitosis (151).…”