Showing papers by "David Beach published in 1991"

p34cdc2-mediated phosphorylation at T124 inhibits nuclear import of SV-40 T antigen proteins.

Abstract: The nuclear import of transcription regulatory proteins appears to be used by the cell to trigger transitions in cell cycle, morphogenesis, and transformation. We have previously observed that the rate at which SV-40 T antigen fusion proteins containing a functional nuclear localization sequence (NLS; residues 126-132) are imported into the nucleus is enhanced in the presence of the casein kinase II (CK-II) site S111/112. In this study purified p34cdc2 kinase was used to phosphorylate T antigen proteins specifically at T124 and kinetic measurements at the single-cell level performed to assess its effect on nuclear protein import. T124 phosphorylation, which could be functionally simulated by a T-to-D124 substitution, was found to reduce the maximal extent of nuclear accumulation whilst negligibly affecting the import rate. The inhibition of nuclear import depended on the stoichiometry of phosphorylation. T124 and S111/112 could be phosphorylated independently of one another. Two alternative mechanisms were considered to explain the inhibition of nuclear import by T124 phosphorylation: inactivation of the NLS and cytoplasmic retention, respectively. Furthermore, we speculate that in vivo T124 phosphorylation may regulate the small but functionally significant amount of cytoplasmic SV-40 T antigen. A sequence comparison showed that many transcription regulatory proteins contain domains comprising potential CK-II-sites, cdc2-sites, and NLS. This raises the possibility that the three elements represent a functional unit regulating nuclear protein import.

A complex containing p34cdc2 and cyclin B phosphorylates the nuclear lamin and disassembles nuclei of clam oocytes in vitro.

Abstract: Cell-free extracts prepared from activated clam oocytes contain factors which induce phosphorylation of the single 67-kD lamin (L67), disassemble clam oocyte nuclei, and cause chromosome condensation in vitro (Dessev, G., R. Palazzo, L. Rebhun, and R. Goldman. 1989. Dev. Biol. 131:469-504). To identify these factors, we have fractionated the oocyte extracts. The nuclear lamina disassembly (NLD) activity, together with a protein kinase activity specific for L67, appear as a single peak throughout a number of purification steps. This peak also contains p34cdc2, cyclin B, and histone H1-kinase activity, which are components of the M-phase promoting factor (MPF). The NLD/L67-kinase activity is depleted by exposure of this purified material to Sepharose conjugated to p13suc1, and is restored upon addition of a p34cdc2/p62 complex from HeLa cells. The latter complex phosphorylates L67 and induces NLD in the absence of other clam oocyte proteins. Our results suggest that a single protein kinase activity (p34cdc2-H1 kinase, identical with MPF) phosphorylates the lamin and is involved in the meiotic breakdown of the nuclear envelope in clam oocytes.

Interaction between ran1+ protein kinase and cAMP dependent protein kinase as negative regulators of fission yeast meiosis.

Abstract: In fission yeast, meiosis is initiated by transcriptional activation of the mei3+ gene under the combined influence of the four mating type genes. The mei3+ gene product acts as a meiotic inducer by binding to and inhibiting the ran1+ protein kinase. Inactivation of ran1+ kinase is both necessary and sufficient to allow meiotic differentiation. We describe a class of mutants which are unable to undergo both normal meiosis and meiosis induced by inactivation of ran1+. In addition to these defects, the cells are sterile and unable to enter stationary phase. We have determined that the mutants define two complementation groups, designated cgs1+ and cgs2+ (continues to grow in stationary). The wild type allele of each gene has been isolated and sequence analysis of cgs1+ shows that it encodes a protein homologous to the regulatory subunit of cyclic AMP dependent protein kinase (cAPK). Biochemical studies demonstrate that in cgs1-1 containing cells, cAPK activity is unregulated by cyclic AMP (cAMP). Sequence analysis of cgs2+ shows that the predicted protein it encodes shares homology with a phosphodiesterase from Dictyostelium discoideum and biochemical studies demonstrate that cells containing a mutant allele of cgs2+ have elevated levels of cAMP. Thus, both genes encode proteins that regulate the activity of cAPK. We have previously shown that cells overproducing ran1+ kinase are meiotically defective. Here, we provide direct evidence that the meiotic defect caused by either unregulated cAPK activity or unregulated ran1+ kinase activity is due to inability to induce transcription of the mei2+ gene, which is required for meiotic initiation. We propose that the switch from vegetative growth to meiosis in fission yeast requires inactivation of ran1+ kinase and is prevented by unregulated levels of cAPK.

A role for the p34cdc2 kinase and phosphatases in the regulation of phosphorylation and disassembly of lamin B2 during the cell cycle.

Abstract: While the p34cdc2 kinase is considered to be a critical regulator of mitosis, its function has not yet been directly linked to one of the key events during the onset of mitosis: nuclear envelope breakdown. Here we show that a major structural protein of the nuclear envelope, lamin B2, is phosphorylated by p34cdc2. Results from two-dimensional phosphopeptide mapping experiments demonstrate that the p34cdc2-specific phosphopeptides represent both mitotic and interphase specific phosphorylations of lamin B2 and include the major interphase phosphorylation site. In mitotic cells we detected two distinct forms of lamin B2 which differ in electrophoretic mobility and in degree of phosphorylation. The phosphorylation pattern of lamin B2 generated in vitro by p34cdc2 was more closely related to the less phosphorylated mitotic lamin B2, suggesting that another kinase(s) in addition to p34cdc2 is involved in generating the mitotic phosphorylation pattern. In addition, we show that treatment of interphase cells with okadaic acid, a potent phosphatase inhibitor, leads to the acquisition of mitosis-specific phosphopeptides and can reversibly increase the detergent-solubility of lamin B2. However, the M-phase-like phosphorylation of lamin B2 in itself is not sufficient to induce its disassembly from the nuclear lamina suggesting that an additional event(s) besides phosphorylation is required.

Cell cycle regulation of histone H1 kinase activity associated with the adenoviral protein E1A

Abstract: Several cellular proteins form stable complexes with the proteins encoded by the adenovirus early region 1A (E1A) gene in extracts derived from adenovirus infected or transformed cells. Two of the cellular proteins that bind to E1A have been identified; one, a 105-kilodalton protein (pRb), is the product of the retinoblastoma gene, and the other, a 60-kilodalton protein, is a human cyclin A. Two other proteins that bind E1A have now been shown to be related to p34cdc2. This E1A complex displayed histone H1-specific kinase activity; the kinase activity was modulated during the cell division cycle, and association of pRb with E1A apparently was not required for this activity.

A cdc2-like kinase phosphorylates histone H1 in the amitotic macronucleus of Tetrahymena

Abstract: Genetic and biochemical studies have shown that cdc2 protein kinase plays a pivotal role in a highly conserved mechanism controlling the entry of cells into mitosis. It is generally believed that one function of cdc2 kinase is to phosphorylate histone H1 which in turn promotes mitotic chromosome condensation. However, direct evidence linking H1 phosphorylation to mitotic chromatin condensation is limited and the exact cellular function(s) of H1 phosphorylation remains unclear. In this study, we show that mammalian cdc2 kinase phosphorylates H1 from the amitotic macronucleus of Tetrahymena with remarkable fidelity. Furthermore, we demonstrate that macronuclei from Tetrahymena contain a growth-associated H1 kinase activity which closely resembles cdc2 kinase from other eukaryotes. Using polyclonal antibodies raised against yeast p34cdc2, we have detected a 36 kd immunoactive polypeptide in macronuclei which binds to Suc1 (p13)-coated beads and closely follows H1 kinase activity. Since macronuclei divide without mitotic chromosome condensation, these data demonstrate that H1 phosphorylation by cdc2 kinase may be necessary, but is not sufficient to promote mitotic chromatin condensation. The fact that an activity which strongly resembles mammalian cdc2 kinase is active during cell growth in a nucleus which does not undergo mitosis and chromosome condensation suggests that other factors are needed for a true mitotic division to occur. These data also reinforce the notion that H1 phosphorylation has important functions outside mitosis both in Tetrahymena and in mammalian cells.

The spil GTPase interacts with RCCl in cell cycle dependency.

Abstract: Eukaryotic cell cycles can be broadly classified into two types, those that display dependency and those that do not. In a dependent cycle, sequential events follow, each progressively due to the existence of molecular checkpoints. Failure to complete one event, such as DNA replication, activates a checkpoint mechanism that prevents the onset of subsequent events, such as mitosis. All somatic cells display a variety of such checkpoints that monitor cell growth, DNA replication, DNA repair, and so on (for review, see Hartwell and Weinert 1989). However, in embryos, particularly those that accumulate a substantial cell mass during oogenesis, such checkpoints are cryptic, and the division cycle is usually very rapid and may occur without appreciable cell growth (Kimelman et al. 1987).

Spi1 GTPase interacts with RCC1 to maintain interdependency of cell cycle events.

Abstract: A mutant which can enter mitosis at any cell cycle stage has been isolated and characterized in fission yeast. The pim1 (premature initiation of mitosis) mutant prearrested at G1/S can develop a mitotic spindle and has tightly condensed chromosomes upon shift to the restrictive temperature. pim1-induced mitosis requires maturation promoting factor (MPF) activity, but not the essential mitotic inducer, cdc25. The pim1+ gene encodes a homolog of regulator of chromosome condensation 1 (RCC1), a regulator of onset of mitosis in mammalian cells. A multicopy suppressor of pim1, spi1, was isolated, and found to encode a 25 kDa GTPase. The primary sequence of the spi1 GTPase shows extensive identity (80%) to human TC4, whose function is unknown. The spi1/TC4 GTPase defines a novel class in the "ras-like" GTPase family, which is distinct from ras, rho, or ypt. Disruption of the spi1+ gene causes genomic instability in a heterozygous diploid. These genetic data suggest that pim1+ and spi1+ interact to coordinate correct entry into mitosis. Immunological experiments demonstrate that the pim1+ and spi1+ products are physically associated. Mutation in the pim1 gene results in lowered affinity of the protein for the spi1 protein in vitro, which may explain why high dosages of the spi1 protein can rescue the pim1 mutant in vivo. The pim1/spi1 complex dissociates in the presence of Mg2+ and GTP. The current data suggests that pim1+ acts as a GTP exchanger for the spi1 GTPase.