Phosphorylation of non-histone proteins associated with mitosis in HeLa cells

Mitotic transcription repression in vivo in the absence of nucleosomal chromatin condensation

All nuclear RNA synthesis is repressed during the mitotic phase of the cell cycle. In addition, RNA polymerase II (RNAP II), nascent RNA and many transcription factors disengage from DNA during mitosis. It has been proposed that mitotic transcription repression and disengagement of factors are due to either mitotic chromatin condensation or biochemical modifications to the transcription machinery. In this study, we investigate the requirement for chromatin condensation in establishing mitotic transcription repression and factor loss, by analyzing transcription and RNAP II localization in mitotic cells infected with herpes simplex virus type 1. We find that virus-infected cells enter mitosis and that mitotic viral DNA is maintained in a nucleosome-free and noncondensed state. Our data show that RNAP II transcription is repressed on cellular genes that are condensed into mitotic chromosomes and on viral genes that remain nucleosome free and noncondensed. Although RNAP II may interact indirectly with viral DNA during mitosis, it remains transcriptionally unengaged. This study demonstrates that mitotic repression of transcription and loss of transcription factors from mitotic DNA can occur independently of nucleosomal chromatin condensation.

Mitotic repression of RNA polymerase II transcription is accompanied by release of transcription elongation complexes

Nuclear RNA synthesis is repressed during the mitotic phase of each cell cycle. Although total RNA synthesis remains low throughout mitosis, the degree of RNA polymerase II transcription repression on specific genes has not been examined. In addition, it is not known whether mitotic repression of RNA polymerase II transcription is due to polymerase pausing or ejection of transcription elongation complexes from mitotic chromosomes. In this study, we show that RNA polymerase II transcription is repressed in mammalian cells on a number of specific gene regions during mitosis. We also show that the majority of RNA polymerase II transcription elongation complexes are physically excluded from mitotic chromosomes between late prophase and late telophase. Despite generalized transcription repression and stripping of RNA polymerase II complexes from DNA, arrested RNA polymerase II ternary complexes appear to remain on some gene regions during mitosis. The cyclic repression of transcription and ejection of RNA polymerase II transcription elongation complexes may help regulate the transcriptional events that control cell cycle progression and differentiation.

Mitotic regulation of TFIID: inhibition of activator-dependent transcription and changes in subcellular localization

Mitosis in higher eukaryotes is accompanied by a general inhibition of transcription. To begin to understand the mechanisms underlying this inhibition we have examined the behavior of the general transcription factor TFIID during mitosis. Immunocytochemistry and subcellular fractionation studies indicate that the majority of TFIID is displaced from the disassembling prophase nucleus to the mitotic cytoplasm around the time of nuclear envelope breakdown. However, a subpopulation of TFIID remains associated tightly with the condensed mitotic chromosomes. Metabolic labeling of mitotic cells revealed that several subunits of TFIID undergo mitosis-specific phosphorylation, but in spite of these changes, the TFIID complex remains intact. Functional analysis of purified TFIID from mitotic cells shows that phosphorylated forms are unable to direct activator-dependent transcription, but that this activity is restored upon dephosphorylation. These results demonstrate that TFIID regulation by phosphorylation is likely to have an important role in mitotic inhibition of RNA polymerase II transcription. In addition, they suggest a mechanism for regulating gene expression through the selective disruption of polymerase II promoter structures during mitosis.

Mitotic regulation of a TATA-binding-protein-containing complex

The mitotic state is associated with a generalized repression of transcription. We show that mitotic repression of RNA polymerase III transcription can be reproduced by using extracts of synchronized HeLa cells. We have used this system to investigate the molecular basis of transcriptional repression during mitosis. We find a specific decrease in the activity of the TATA-binding-protein (TBP)-containing complex TFIIIB. TBP itself is hyperphosphorylated at mitosis, but this does not appear to account for the loss of TFIIIB activity. Instead, one or more TBP-associated components appear to be regulated. The data suggest that changes in the activity of TBP-associated components contribute to the coordinate repression of gene expression that occurs at mitosis.

Dephosphorylation of nuclear non-histone proteins in submandibular glands of rats treated with isoproterenol

Protein phosphatase that removed [32P]phosphate from non-histone proteins, i.e., phenol-soluble acidic proteins, more rapidly and strongly than from histone proteins was present in nuclei of rat submandibular glands, but was not associated with chromatin. Cyclic AMP (10(-4)-10(-2) mM) stimulated the dephosphorylation of non-histone proteins, but not that of histone proteins. After a single injection of isoproterenol (IPR), the dephosphorylation of non-histone proteins in rat submandibular gland nuclei increased within 15 min, reached a maximum in 30 min and returned to normal control levels within 4 h. The stimulation of dephosphorylation of non-histone proteins induced by IPR was not observed after prior treatment of the animals with dichloroisoproterenol. The dephosphorylation of histone proteins was not affected by the injection of IPR. Stimulation of beta-adrenoceptors with IPR in rat submandibular glands resulted in increase in cyclic AMP and decrease in RNA synthesis in the tissues in the first few hours after the injection. This decrease in RNA synthesis was temporary and was preceded by the increase in cyclic AMP level and in the dephosphorylation of non-histone acidic proteins in the tissues. These results suggest that protein phosphatase in nuclei plays an important part in the events controlling RNA synthesis by regulating the state of phosphorylation of non-histone acidic proteins. In addition, the phosphatase may be regulated by a function of the cytoplasmic membranes.

Mitotic phosphorylation of the Oct-1 homeodomain and regulation of Oct-1 DNA binding activity

Oct-1 is a transcription factor involved in the cell cycle regulation of histone H2B gene transcription and in the transcription of other cellular housekeeping genes. Oct-1 is hyperphosphorylated as cells enter mitosis, and mitosis-specific phosphorylation is reversed as cells exit mitosis. A mitosis-specific phosphorylation site in the homeodomain of Oct-1 was phosphorylated in vitro by protein kinase A. Phosphorylation of this site correlated with inhibition of Oct-1 DNA binding activity in vivo and in vitro. The inhibition of Oct-1 DNA binding during mitosis suggests a mechanism by which the general inhibition of transcription during mitosis might occur.

Identification and partial purification of a chromatin bound calmodulin activated histone 3 kinase from calf thymus

A calcium-calmodulin (Ca2(+)-CaM) stimulated histone H3 phosphorylating activity was identified as a component of a nuclear protein complex purified from a 150 mM NaCl extract of calf thymus chromatin. This activity bound to a CaM-Sepharose affinity column in a Ca2+ dependent manner and was eluted off the column in the presence of EGTA. Equilibrium centrifugation of the EGTA eluate on a sucrose density gradient revealed that the activity is a component of a larger complex identified at 25% sucrose. This complex consisted of two major proteins, having Mr of 65 and 75 kDa. Using [125I] CaM and the gel overlay technique it was shown that the 75 kDa protein is the major CaM binding protein in the complex.

Microinjection of p34cdc2 kinase induces marked changes in cell shape, cytoskeletal organization, and chromatin structure in mammalian fibroblasts

We have examined the effects of elevating the intracellular levels of p34cdc2 kinase by microinjection into living mammalian cells. These studies reveal rapid and dramatic changes in cell shape with cells becoming round and losing the bulk of their cell-substratum contact. Such effects were induced at all times in the cell cycle except at S phase and were fully reversible at S phase or mitosis. Similar results were obtained with the homogeneous catalytic subunit of p34cdc2 kinase or p34cdc2 kinase associated with cyclin B. These alterations were accompanied by a marked reduction in interphase microtubules without the spindle formation, actin microfilament redistribution, and premature chromatin condensation. Although these changes closely mimic the events occurring during early phases of mitosis, p34cdc2 kinase-injected cells were not induced to pass further into division. These data provide detailed evidence that p34cdc2 kinase plays a major prerequisite role in the rearrangement of cellular structures associated with mammalian cell mitosis.

Cloning and sequence analysis of the Saccharomyces cerevisiae RAD9 gene and further evidence that its product is required for cell cycle arrest induced by DNA damage

Procaryotic and eucaryotic cells possess mechanisms for arresting cell division in response to DNA damage. Eucaryotic cells arrest division in the G2 stage of the cell cycle, and various observations suggest that this arrest is necessary to ensure the completion of repair of damaged DNA before the entry of cells into mitosis. Here, we provide evidence that the Saccharomyces cerevisiae RAD9 gene, mutations of which confer sensitivity to DNA-damaging agents, is necessary for the cell cycle arrest phenomenon. Our studies with the rad9 delta mutation show that RAD9 plays a role in the cell cycle arrest of methyl methanesulfonate-treated cells and is absolutely required for the cell cycle arrest in the temperature-sensitive cdc9 mutant, which is defective in DNA ligase. At the restrictive temperature, cell cycle progression of cdc9 cells is blocked sometime after the DNA chain elongation step, whereas cdc9 rad9 delta cells do not arrest at this point and undergo one or two additional divisions. Upon transfer from the restrictive to the permissive temperature, a larger proportion of the cdc9 cells than of the cdc9 rad9 delta cells forms viable colonies, indicating that RAD9-mediated cell cycle arrest allows for proper ligation of DNA breaks before the entry of cells into mitosis. The rad9 delta mutation does not affect the frequency of spontaneous or UV-induced mutation and recombination, suggesting that RAD9 is not directly involved in mutagenic or recombinational repair processes. The RAD9 gene encodes a transcript of approximately 4.2 kilobases and a protein of 1,309 amino acids of Mr 148,412. We suggest that RAD9 may be involved in regulating the expression of genes required for the transition from G2 to mitosis.

Cloning and sequence analysis of the Saccharomyces cerevisiae RAD9 gene and further evidence that its product is required for cell cycle arrest induced by DNA damage

Procaryotic and eucaryotic cells possess mechanisms for arresting cell division in response to DNA damage. Eucaryotic cells arrest division in the G2 stage of the cell cycle, and various observations suggest that this arrest is necessary to ensure the completion of repair of damaged DNA before the entry of cells into mitosis. Here, we provide evidence that the Saccharomyces cerevisiae RAD9 gene, mutations of which confer sensitivity to DNA-damaging agents, is necessary for the cell cycle arrest phenomenon. Our studies with the rad9 delta mutation show that RAD9 plays a role in the cell cycle arrest of methyl methanesulfonate-treated cells and is absolutely required for the cell cycle arrest in the temperature-sensitive cdc9 mutant, which is defective in DNA ligase. At the restrictive temperature, cell cycle progression of cdc9 cells is blocked sometime after the DNA chain elongation step, whereas cdc9 rad9 delta cells do not arrest at this point and undergo one or two additional divisions. Upon transfer from the restrictive to the permissive temperature, a larger proportion of the cdc9 cells than of the cdc9 rad9 delta cells forms viable colonies, indicating that RAD9-mediated cell cycle arrest allows for proper ligation of DNA breaks before the entry of cells into mitosis. The rad9 delta mutation does not affect the frequency of spontaneous or UV-induced mutation and recombination, suggesting that RAD9 is not directly involved in mutagenic or recombinational repair processes. The RAD9 gene encodes a transcript of approximately 4.2 kilobases and a protein of 1,309 amino acids of Mr 148,412. We suggest that RAD9 may be involved in regulating the expression of genes required for the transition from G2 to mitosis.

Commitment to division in Paramecium: effect of nutrient level on the macronuclear DNA increment

In Paramecium, a fixed macronuclear DNA increment is associated with commitment to cell division. This study shows that this threshold DNA increment is about 70% of the final DNA increment in well-fed cells. The DNA increment is reduced when growth rate is decreased and decreases in parallel with growth rate to a minimum of 30% of the normal DNA increment. This minimum value is obtained when the growth rate is 20% of its normal level or lower. Further reduction in the growth rate produces no further reduction in the DNA increment. Following abrupt nutrient-level shifts, both the threshold DNA increment and the final DNA increment change progressively as the time of the shift is moved to later stages of the cell cycle. The threshold DNA increment is reset following nutrient-level shifts up to the point of commitment to division. These observations are consistent with the notion that the magnitude of the threshold DNA increment is strongly correlated with the rate of growth and is rapidly reset by factors which alter the growth rate. The implications of these observations for growth-driven regulation of the cell cycle are discussed.

225-Kilodalton phosphoprotein associated with mitotic centrosomes in sea urchin eggs

Protein phosphorylation during development of sea urchin eggs from fertilization to first cleavage was examined by labeling cells with specific antiphosphoprotein antibodies. Indirect immunofluorescence staining with monoclonal antithiophosphoprotein antibody (Gerhart et al.: Cytobios 43:335-347, 1985) has revealed that nuclei as well as centrosomes, kinetochores, and midbodies were specifically thiophosphorylated in developing eggs incubated with adenosine 5'-O (3-thiotriphosphate) (ATP-gamma-S). The phosphorylation reaction required Mg2+ but was not dependent on cAMP or calmodulin in detergent-extracted models. Centrosomes were purified by fractionation of isolated mitotic spindles with 0.5 M KCl extraction. The thiophosphoproteins were retained in the purified centrosomes and the antibody recognized a major 225-Kd polypeptide on immunoblots. In an independent preparation, a monoclonal antiphosphoprotein antibody (CHO3) was found also to react with mitotic poles and stained a 225-Kd polypeptide, confirming the centrosome specificity of this protein. Immunoelectron microscopy showed that the 225-Kd thiophosphoprotein was found at mitotic poles associated with granules to which mitotic microtubules were directly attached. Unlike centrosomes in permeabilized eggs, those in isolated spindles could not be thiophosphorylated, possibly due to inactivation or loss of either phosphorylation enzymes or cofactors, or both, during isolation. The immunofluorescence labeling of thiophosphate could be inhibited by ATP and AMP.PNP in a concentration-dependent manner. Exogenous ATP could abolish thiophosphate-staining more effectively when added with phosphatase inhibitors, suggesting a dynamic state in which centrosomal proteins are being phosphorylated and dephosphorylated in rapid succession by the action of protein kinase(s) and phosphatase(s).

Phosphorylation of vimentin in mitotically selected cells. In vitro cyclic AMP-independent kinase and calcium-stimulated phosphatase activities

The phosphorylation of the intermediate filament protein vimentin was examined under in vitro conditions. Cell cytosol and Triton-insoluble cytoskeleton preparations from nonmitotic and mitotically selected mouse L-929 cells exhibited vimentin kinase activity that is apparently cAMP and Ca2+ independent. The level of vimentin kinase activity was greater in preparations from mitotically selected cells than nonmitotic cells. Addition of Ca2+ to mitotic cytosol decreased net vimentin phosphorylation. Dephosphorylation experiments indicated that there is phosphatase activity in these preparations which is stimulated by addition of Ca2+. Fractionation of cytosol from nonmitotic cells on DEAE-Sephacel and phosphocellulose revealed a single major vimentin kinase activity (peak I). Fractionation of cytosol from mitotically selected cells yielded a similar activity (peak I) and an additional vimentin kinase activity (peak II) that was not found in nonmitotic preparations. Based on substrate specificity and lack of inhibition to characteristic inhibitors, the semipurified peak I and II vimentin kinase activities appear to be cAMP-independent enzymes that are distinct from casein kinases I and II. Phosphopeptide mapping studies indicated that both peak I and peak II vimentin kinases phosphorylate tryptic peptides in the NH2-terminal region of vimentin that are phosphorylated in intact cells. Electron microscopic examination of reconstituted vimentin filaments phosphorylated with both semipurified kinases indicated that phosphorylation induced filament disassembly. These experiments indicate that the increased phosphorylation of vimentin during mitosis may be catalyzed by a discrete cAMP-independent protein kinase. In addition, preparations from mitotic cells exhibited a Ca2+-stimulated phosphatase activity, suggesting that Ca2+ may play a regulatory role in vimentin dephosphorylation during mitosis.

Amphibian oocyte maturation induced by extracts of Physarum polycephalum in mitosis

The orderly progression of eukaryotic cells from interphase to mitosis requires the close coordination of various nuclear and cytoplasmic events. Studies from our laboratory and others on animal cells indicate that two activities, one present mainly in mitotic cells and the other exclusively in G1-phase cells, play a pivotal role in the regulation of initiation and completion of mitosis, respectively. The purpose of this study was to investigate whether these activities are expressed in the slime mold Physarum polycephalum in which all the nuclei traverse the cell cycle in natural synchrony. Extracts were prepared from plasmodia in various phases of the cell cycle and tested for their ability to induce germinal vesicle breakdown and chromosome condensation after microinjection into Xenopus laevis oocytes. We found that extract of cells at 10-20 min before metaphase consistently induced germinal vesicle breakdown in oocytes. Preliminary characterization, including purification on a DNA-cellulose affinity column, indicated that the mitotic factors from Physarum were functionally very similar to HeLa mitotic factors. We also identified a number of mitosis-specific antigens in extracts from Physarum plasmodia, similar to those of HeLa cells, using the mitosis-specific monoclonal antibodies MPM-2 and MPM-7. Interestingly, we also observed an activity in Physarum at 45 min after metaphase (i.e., in early S phase since it has no G1) that is usually present in HeLa cells only during the G1 phase of the cell cycle. These are the first studies to show that maturation-promoting factor activity is present in Physarum during mitosis and is replaced by the G1 factor (or anti-maturation-promoting factor) activity in a postmitotic stage. A comparative study of these factors in this slime mold and in mammalian cells would be extremely valuable in further understanding their function in the regulation of eukaryotic cell cycle and their evolutionary relationship to one another.

Mitotic induction and maintenance by overexpression of a G2-specific gene that encodes a potential protein kinase

There may be a causal relationship between expression of the G2-specific gene nimA and mitotic regulation in Aspergillus. To test this relationship we have introduced extra inducible copies of nimA into Aspergillus and determined the effect of nimA overproduction on mitotic regulation. The results show that nimA overexpression causes mitotic induction in less than a cell cycle and maintains chromatin in a condensed state. These effects occur even if cells are first blocked in S phase. Sequence analysis shows that the nimA gene encodes a potential protein kinase. These data indicate that there is indeed a causal relationship between expression of nimA and the regulation of mitosis and further implicate protein phosphorylation in mitotic control.

Cell-cycle modulation of MPM-2-specific spindle pole body phosphorylation in Aspergillus nidulans

MPM-2 is a monoclonal antibody that interacts with mitosis-specific phosphorylated proteins in many different organisms. Immunocytochemistry of tissue culture cells has shown that MPM-2 stains centrosomes, chromosomes, kinetochores, and spindles. In this paper, we demonstrate that MPM-2 staining colocalizes with the spindle pole body (SPB) of Aspergillus nidulans and that SPB staining varies during the mitotic cycle. In an unsynchronized population, about one-fourth to one-third of the cells stain with MPM-2 at the spindle plaques or SPBs. Nuclei in mitosis have two SPBs localized at the ends of the spindle, both of which stain with MPM-2. To determine when MPM-2 staining appears, we have examined the effects of temperature-sensitive cell-cycle mutations that block nuclear division in S or G2. Only a very small fraction of cells blocked in S-phase stain with MPM-2. In contrast, a large fraction of cells blocked in G2 stain brightly at the SPB. These data suggest that MPM-2 reactivity of SPBs appears in G2. Moreover, the fact that cells blocked in G2 showed MPM-2 staining but no spindles suggests that reactivity of SPBs occurs prior to mitosis but is not sufficient to trigger spindle formation. When G2-blocked cells were downshifted to permissive temperature, they generated a mitotic spindle with an SPB at each end. Both SPBs stained with MPM-2 in all of the mitotic cells.

Redistribution of the nuclear mitotic apparatus protein (NuMA) during mitosis and nuclear assembly. Properties of purified NuMA protein

Monoclonal antibodies and human autoimmune sera specific for the nuclear mitotic apparatus protein (NuMA protein) were applied to study the structure of this protein and its intracellular distribution. The NuMA protein was purified using immuno-affinity columns. Studies on this large (250 kD) nuclear protein indicated that it is a highly asymmetric phosphoprotein. It is present in all mammalian cells examined and in those of some non-mammals. Immunofluorescence studies on fixed cells demonstrated that its intracellular distribution is essentially the same in all species at all stages of the cell cycle. Immunoblot (western blot) analysis showed that the size of the NuMA protein varies slightly in different species. At the onset of mitosis the NuMA protein redistributes from the nucleus to two centrosomal structures that later will become part of the mitotic spindle pole. This occurs at the time of nuclear breakdown and eventually leads to an accumulation of the NuMA protein at the polar region of the mitotic spindle. After anaphase the protein redistributes from the spindle polar region into the reforming nucleus and concentrates initially at the site where nuclear lamins and perichomatin have been reported to assemble. Living cells microinjected with fluorescent anti-NuMA antibodies were studied to examine parameters that effect the redistribution of the NuMA protein in vivo. These experiments indicate that microtubule assembly is essential for the NuMA protein to accumulate in the polar region.

Possible target of Abelson virus phosphokinase in cell transformation

By fusing interphase cells to cells undergoing mitosis, the interphase chromosomes can be visualized. When analyzed in this way, chromosomes of normal mouse cells show characteristic undercondensed centromeric regions. We have found that the centromeric regions of chromosomes from Abelson virus-transformed cells are fully condensed. Abelson virus transforms mouse cells by introducing into them a virally encoded phosphokinase that is expressed constitutively. Thus, we propose that the condensation of centromeric chromatin is a result of overphosphorylation by the Abelson virus phosphokinase, and that the centromeric region is the relevant target of overphosphorylation in transformed cell growth.

The cell cycle control gene cdc2+ of fission yeast encodes a protein kinase potentially regulated by phosphorylation

The cdc2+ gene function has an important role in controlling the commitment of the fission yeast cell to the mitotic cycle and the timing of mitosis. We have raised antibodies against the cdc2+ protein using synthetic peptides and have demonstrated that it is a 34 kd phosphoprotein with protein kinase activity. The protein level and phosphorylation state remain unchanged during the mitotic cycle of rapidly growing cells. When cells cease to proliferate and arrest in G1 the protein becomes dephosphorylated and loses protein kinase activity. Exit from the mitotic cycle and entry into stationary phase may be controlled in part by modulation of the cdc2 protein kinase activity by changes in its phosphorylation state.

Differential localization and androgen sensitivity of prostatic nuclear protein kinases in euchromatin and heterochromatin fractions

We have examined the distribution and androgenic regulation of protein kinases and phosphoproteins in euchromatin and heterochromatin fractions of rat ventral prostate chromatin. Available procedures to prepare euchromatin and heterochromatin fractions were found to result in the loss of various chromatin-associated protein kinases even though there was no gross change in the gel electrophoretic profile of proteins in these fractions. This loss was prevented by the addition of 0.5 mM phenylmethylsulfonyl fluoride throughout the preparative procedures, which indicates that the protein kinases associated with the chromatin may be particularly susceptible to proteolytic degradation during further subfractionation. By utilizing an improved method for fractionation of chromatin, we have demonstrated a marked enrichment of protein kinase activity (towards phosvitin and endogenous chromosomal proteins) in the euchromatin fraction as compared with heterochromatin. Both of these fractions were also examined for the incorporation of 32P into two main classes of nonhistone proteins (namely, H2SO4-soluble and -insoluble nonhistones). The amount of 32P incorporated into heterochromatin-associated proteins of both classes was markedly less than that in the euchromatin-associated proteins. Protein kinase activities (especially those active towards phosvitin and nonhistone proteins) in the euchromatin fraction as compared with the heterochromatin were significantly reduced within 24 h after androgenic deprivation in the animal. The decreased phosphorylation of nonhistone proteins could be attributed to the loss of endogenous protein kinase activity. The results indicate that not only are chromatin-associated protein phosphokinases preferentially localized in euchromatin fractions but also that these euchromatin-associated protein kinases display the greatest sensitivity to androgenic status of the animal.

Partial purification and characterization of mitotic factors from HeLa cells

Extracts from mitotic HeLa cells, when injected into Xenopus laevis oocytes, exhibit maturation-promoting activity (MPA) as evidenced by the breakdown of the germinal vesicle and the condensation of chromosomes. In this study we have attempted to purify and characterize these mitotic factors. When 0.2 M NaCl-soluble extracts of mitotic HeLa cells were concentrated by ultrafiltration and subjected to affinity chromatography on hydroxylapatite followed by DNA-cellulose, the proteins with MPA eluted as a single peak and their specific activity was increased approx. 200-fold compared with crude extracts. The molecular weight of the mitotic factors was estimated to be 100 kD as determined by chromatography on Sephacryl S-200. SDS-PAGE of the partially-purified mitotic factors indicated the presence of several polypeptides ranging from 40-150 kD with a major band of about 50 kD. The majority of these polypeptides were found to be phosphoproteins as revealed by 32P-labeling and autoradiography. Very little or no phosphorylation was observed at the 50 kD band. Several of these polypeptides were reactive with mitosis-specific monoclonal antibodies, MPM-1 or MPM-2, as shown by immunoblots of these proteins but the major polypeptide band at 50 kD was not. Removal of the immunoreactive polypeptides by precipitation with these antibodies did not destroy the MPA. The MPA of the crude or the partially-purified mitotic factors was destroyed by injection of (but not pretreatment with) alkaline phosphatase within 45 min after injection of mitotic factors. These results are discussed in terms of a possible role of phosphorylation-dephosphorylation of non-histone proteins in the regulation of mitosis and meiosis.

Nucleocytoplasmic RNA transport

A number of closely related post-transcriptional facets of RNA metabolism show nuclear compartmentation, including capping, methylation, splicing reactions, and packaging in ribonucleoprotein particles (RNP). These nuclear 'processing' events are followed by the translocation of the finished product across the nuclear envelope. Due to the inherent complexity of these interrelated events, in vitro systems have been designed to examine the processes separately, particularly so with regard to translocation. A few studies have utilized nuclear transplantation/microinjection techniques and specialized systems to show that RNA transport occurs as a regulated phenomenon. While isolated nuclei swell in aqueous media and dramatic loss of nuclear protein is associated with this swelling, loss of RNA is not substantial, and most studies on RNA translocation have employed isolated nuclei. The quantity of RNA transported from isolated nuclei is related to hydrolysis of high-energy phosphate bonds in nucleotide additives. The RNA is released predominantly in RNP: messenger-like RNA is released in RNP which have buoyant density and polypeptide composition similar to cytoplasmic messenger RNP, but which have distinctly different composition from those in heterogeneous nuclear RNP. Mature 18 and 28S ribosomal RNA is released in 40 and 60S RNP which represent mature ribosomal subunits. RNA transport proceeds with characteristics of an energy-requiring process, and proceeds independently of the presence or state of fluidity of nuclear membranes. The energy for transport appears to be utilized by a nucleoside triphosphatase (NTPase) which is distributed mainly within heterochromatin at the peripheral lamina. Photoaffinity labeling has identified the pertinent NTPase as a 46 kD polypeptide which is associated with nuclear envelope and matrix preparations. The NTPase does not appear to be modulated via direct phosphorylation or to reflect kinase-phosphatase activities. A large number of additives (including RNA and insulin) produce parallel effects upon RNA transport and nuclear envelope NTPase, strengthening the correlative relationship between these activities. Of particular interest has been the finding that carcinogens induce specific, long-lasting increases in nuclear envelope (and matrix) NTPase; this derangement may underlie the alterations in RNA transport associated with cancer and carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)