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G W Zieve

Publications and source records attributed to G W Zieve.

At least 19 recordsLinked to original sources
PubMed1999

The cytoplasmic sites of the snRNP protein complexes are punctate structures that are responsive to changes in metabolism and intracellular architecture.

Five anti-Sm monoclonal antibodies, Y12, 7.13, KSm4, KSm6, and 128, stain similar discrete punctate structures distributed throughout the cytoplasm of hamster fibroblasts in addition to the expected intense nuclear staining. Several criteria suggest the cytoplasmic staining reflects the cytoplasmic pools of snRNP core proteins. The relative intensity of the cytoplasmic staining is similar to the 30% relative abundance of the cytoplasmic snRNP core proteins compared to the nuclear snRNP core proteins based on cell-fractionation studies. Moreover, the cytoplasmic staining is removed by the same extraction conditions that solubilize the pools of cytoplasmic snRNP core proteins. The cytoplasmic sites of staining are typically spherical but heterogeneous in diameter (0.2-0.5 microm). The larger particles greatly exceed the diameter of individual snRNP core particles and are likely to represent centers of many snRNP proteins or snRNP protein complexes. The staining, though punctate, is evenly dispersed throughout the cytoplasm with no evidence of major compartmentalization. The cytoplasmic staining pattern collapses into larger foci of intensely staining structures when cellular energy levels are depleted or when cells are exposed to hypertonic medium. Unlike the normal sites of snRNP protein cytoplasmic staining, these larger collapsed foci resist detergent extraction. These results suggest that the cytoplasmic staining identified with the anti-Sm monoclonal antibodies represents the large pools of snRNP core proteins in the cytoplasm.

Animals↗
PubMed1999

Thirteen anti-Sm monoclonal antibodies immunoprecipitate the three cytoplasmic snRNP core protein precursors in six distinct subsets.

The small nuclear ribonucleoprotein particle (snRNP) common core proteins are the lupus-associated Sm autoantigens. In mouse fibroblasts the seven snRNP core proteins form a particle with a suggested stoichiometry of B2[D1,D2(E,F,G)2] D3. Core particle assembly occurs in the cytoplasm where newly synthesized snRNAs assemble with core proteins stored in three RNA-free complexes of (1) a 6S complex of [D1,D2(E,F,G)2] (2) a 20S complex of (B,D3 and an unidentified 70 kDa protein) and (3) a 2S-6S complex that minimally contains the B protein. In this report a panel of 13 anti-Sm monoclonal antibodies is shown to immunoprecipitate six different subsets of the cytoplasmic snRNP proteins. Four epitopes are shared by the three aforementioned complexes and five other epitopes are shared by two of the complexes. In addition, the 6S or 20S complexes are apparently disrupted by five of the antibodies. Kinetic studies show that the three cytoplasmic snRNP protein complexes have independent half-lives. These studies provide another approach for characterizing the Sm epitopes. They also complement previous in vitro snRNP assembly studies and suggest that snRNP core assembly occurs by the initial binding of snRNA to the 6S particle followed by addition of the B and D3 proteins.

Animals↗
PubMed1997

Multiple protein: protein interactions between the snRNP common core proteins.

The snRNP core proteins (B, D3, D2, D1, E, F, and G) assemble with snRNA and form the snRNP core particle with a suggested stoichiometry of B2[D1, D2(E, F, G)2]D3. The newly synthesized snRNP core proteins are stored in the cytoplasm in three RNA-free complexes of (1) B at 2S-6S; (2) [D1, D2(E, F, G)2] at 6S; and (3) (B, D3, and 69 kDa) at 20S. The snRNP proteins assemble stepwise with snRNAs that appear transiently in the cytoplasm before returning to the nucleus as mature snRNP particles. In this report, two approaches are used to investigate the protein:protein interactions between the snRNP proteins. First, the 6S and 20S cytoplasmic complexes chromatographed as intact structures, supporting their identifications as discrete complexes. Second, the cDNAs for the proteins were used to test all pair-wise interactions between the seven major core proteins using the yeast two-hybrid system. The two-hybrid system identified four strong reciprocal interactions, one weak reciprocal interaction, five one-way interactions, and one homotypic interaction. The strongest interactions were between proteins within the 6S particle. Other interactions were between proteins in the 6S and 20S particles or within the 20S particle itself. These interactions are likely to occur within the cytoplasmic snRNP core protein complexes and the mature snRNP particle.

Animals↗
PubMed1996

U6 snRNA maturation and stability.

The U6 snRNP is found as a monomer and as a heterodimer, complexed with the U4 snRNP (U4/U6). Northern blotting detects approximately equal amounts of U4/U6 heterodimer and U6 monomer in the nucleus but only U6 monomer in bona fide cytoplasm. In mammalian cells, newly synthesized U6 appears transiently in the cytoplasm before returning to the nucleus. Sedimentation analysis identifies cytoplasmic U6 in similarly sized structures as nuclear U4 and U6 and smaller structures than cytoplasmic U4. Inhibitor studies demonstrate that newly synthesized U6 can move from the cytoplasm into the nucleus in the absence of U4 synthesis. The nuclear half-life of U6 is significantly shorter than that of U4 and the other spliceosomal snRNAs. These data support a model in which U4 and U6 snRNAs undergo distinct cytoplasmic maturation pathways before returning to the nucleus, where the U4/U6 snRNP assembles.

Animals↗
PubMed1991

Assembly and intracellular transport of snRNP particles.

The assembly of the major small nuclear ribonucleoprotein (snRNP) particles begins in the cytoplasm where large pools of common core proteins are preassembled in several RNA-free intermediate particles. Newly synthesized snRNAs transiently enter the cytoplasm and complex with core particles to form pre-snRNP particles. Subsequently, the cap structure at the 5' end of the snRNA is hypermethylated. The resulting trimethylguanosine (TMG) cap is an integral part of the nuclear localization signal for snRNP particles and the pre-snRNP particles are rapidly transported into the nucleus. SnRNP particles mature when snRNA-specific proteins complex with the particles, in some cases, just before or during nuclear transport, but in most instances after the particles are in the nucleus. In addition, U6 snRNA hybridizes with U4 snRNA to form a U4/U6 snRNP in the nucleus. The transport signals are retained on the snRNP particles and proteins since existing particles and proteins enter the reformed nucleus after mitosis.

Amino Acid Sequence↗
PubMed1990

Cytoplasmic assembly of small nuclear ribonucleoprotein particles from 6 S and 20 S RNA-free intermediates in L929 mouse fibroblasts.

Newly transcribed small nuclear RNAs (snRNAs) appear transiently in the cytoplasm where they assemble with snRNP core proteins (B, D, E, F, and G) stored in large pools of snRNA-free intermediates before returning permanently to the nucleus. In this report, the cytoplasmic assembly of snRNP core particles in L929 mouse fibroblasts was investigated by kinetic analysis of assembly intermediates resolved on sucrose gradients. Immunoprecipitation of gradient fractions with anti-snRNP autoimmune antisera identify pools of 6 and 20 S snRNA-free snRNP protein intermediates. The snRNP B protein has a heterodisperse sedimentation from 4 to 20 S with peaks at 6 and 20 S, and the snRNP D protein is in a bimodal distribution at 6 and 20 S. At 6 S the D protein is assembled with the E, F, and G proteins into a RNA-free core particle with a stoichiometry of D4EFG. SnRNP assembly proceeds by snRNA assembling initially with the 6 S D4EFG particle and then two copies of the B protein to form an 11-15 S SnRNP particle. The 20 S forms of the D protein in the cytoplasm are less stable than the 6 S D4EFG particle. The U1-specific A and C proteins leak from isolated nuclei and appear in the cytoplasmic fractions where they sediment from 10 to 20 S and from 4 to 8 S, respectively.

Animals↗
PubMed1990

Nuclear exchange of the U1 and U2 snRNP-specific proteins.

The snRNP particles include a set of common core snRNP proteins and snRNP specific proteins. In rodent cells the common core proteins are the B, D, D', E, F and G proteins in a suggested stoichiometry of B2D'2D2EFG. The additional U1- and U2-specific proteins are the 70-kD, A and C proteins and the A' and B" proteins, respectively. Previous cell fractionation and kinetic analysis demonstrated the snRNP core proteins are stored in the cytoplasm in large partially assembled snRNA-free intermediates that assemble with newly synthesized snRNAs during their transient appearance in the cytoplasm (Sauterer, R. A., R. J. Feeney, and G. W. Zieve. 1988. Exp. Cell Res. 176:344-359). This report investigates the assembly and intracellular distribution of the U1 and U2 snRNP-specific proteins. Cell enucleation and aqueous cell fractionation are used to prepare nuclear and cytoplasmic fractions and the U1- and U2-specific proteins are identified by isotopic labeling and immunoprecipitation or by immunoblotting with specific autoimmune antisera. The A, C, and A' proteins are found both assembled into mature nuclear snRNP particles and in unassembled pools in the nucleus that exchange with the assembled snRNP particles. The unassembled proteins leak from isolated nuclei prepared by detergent extraction. The unassembled A' protein sediments at 4S-6S in structures that may be multimers. The 70-kD and B" proteins are fully assembled with snRNP particles which do not leak from isolated nuclei. The kinetic studies suggest that the B" protein assembles with the U2 particle in the cytoplasm before it enters the nucleus.

Animals↗
PubMed1990

Identification and characterization of the small nuclear ribonucleoprotein particle D' core protein.

The addition of urea to sodium dodecyl sulfate (SDS)-polyacrylamide gels has allowed the identification and characterization of the small nuclear ribonucleoprotein particle (snRNP) D' protein and has also improved resolution of the E, F, and G snRNP core proteins. In standard SDS-polyacrylamide gels, the D' and D snRNP core proteins comigrate at approximately 16 kilodaltons. The addition of urea to the separating gel caused the D' protein to shift to a slower electrophoretic mobility that is distinct from that of the D protein. The shift to a slower electrophoretic mobility in the presence of urea suggests that the D' protein has extensive secondary structure that is not totally disrupted by SDS alone. Both N-terminal sequencing and partial peptide maps indicate that the D and D' proteins are distinct gene products, and the sequence data have identified the faster moving of the two proteins as the previously cloned D protein (L. A. Rokeach, J. A. Haselby, and S. O. Hoch, Proc. Natl. Acad. Sci. USA 85:4832-4836, 1988). In the cytoplasm, the D protein is found primarily in the small-nuclear-RNA-free 6S protein complexes, while the D' protein is found primarily in the 20S protein complexes. Like the D protein, the D' protein is an autoantigen in patients with systemic lupus erythematosus and is recognized by some of the Sm class of autoimmune antisera.

Animals↗
PubMed1989

Cytoplasmic assembly and nuclear accumulation of mature small nuclear ribonucleoprotein particles.

The assembly pathway of small nuclear ribonucleoprotein (snRNP) particles in the cytoplasm of L929 mouse fibroblasts was analyzed by observing the nuclear accumulation of snRNP proteins. Immunoprecipitations of nuclear and cytoplasmic fractions after a pulse label and chase indicate that the snRNP D, E, F, and G proteins assemble first, followed by the small nuclear RNA (snRNA), then the snRNP B protein and, in the case of the U1 snRNP, the A and C proteins. The snRNP B' protein is not detected in the L929 cells. The U1-specific A and C proteins can enter the nucleus in the absence of snRNP assembly, suggesting that these proteins exchange on the mature nuclear snRNP particles. Two-dimensional electrophoresis using nonequilibrium pH gradient electrophoresis identifies the A, B, B", C, D, E, F, and G proteins in a distribution similar to that reported previously by immunoprecipitation (Sauterer, R. A., and Zieve, G. W. (1989) J. Biol. Chem., submitted for publication). The D protein appears in multiple isoelectric variants in the cytoplasm and shifts toward more basic variants during maturation. Kinetic experiments analyzed by two-dimensional electrophoresis indicate a quantitative maturation of the cytoplasmic B protein into nuclear particles. Quantitative densitometry of immunoprecipitated stable nuclear snRNPs labeled with [35S] methionine corrected for the published methionine content of the A, B, C, D, and E proteins indicates that the mature nuclear U1 snRNP probably contains four copies of D, two copies each of B, C, and A, and one copy of E.

Animals↗
PubMed1988

Newly synthesized small nuclear RNAs appear transiently in the cytoplasm.

Newly synthesized small nuclear RNA (snRNA) species U1 and U2 are easily identified in cytoplasmic fractions prepared by standard aqueous cell fractionation. However, because the mature stable snRNA species leak from isolated nuclei during cell fractionation, the possibility exists that these newly synthesized species also leak from the nucleus. To overcome the problems of nuclear leakage, mouse L929 cells were fractionated by cell enucleation. Enucleation extrudes the nuclei from cytochalasin-treated cells and produces cytoplasts that, by several criteria, are a bona fide cytoplasmic fraction uncontaminated by nuclear material. All six of the major snRNAs are present in the cytoplasts (c-snRNAs) shortly after synthesis. The species are identified by immunoprecipitation with specific antisera against the ribonucleoproteins and by Northern blotting and hybrid selection using cloned probes. This confirms and extends similar studies that used non-aqueous cell fractionation and manual dissection to overcome nuclear leakage. Kinetic studies demonstrate that the c-snRNAs return to the interphase nucleus after approximately 20 minutes in the cytoplasm. The U2 precursor U2' is processed to mature-sized U2 in the cytoplast fractions before returning to the nucleus. The c-snRNAs occur in ribonucleoprotein particles with similar antigenicity to the mature nuclear particles within six minutes of transcription. This suggests that in mammalian cells, important steps in the assembly of these ribonucleoproteins occur in the cytoplasm.

Animals↗
PubMed1988

Cordycepin rapidly collapses the intermediate filament networks into juxtanuclear caps in fibroblasts and epidermal cells.

The nucleoside analog 3'-deoxyadenosine (cordycepin) rapidly collapses the intermediate filaments into juxtanuclear caps in interphase fibroblasts and keratinocytes. A minimum of 80 micrograms/ml cordycepin or 20 micrograms/ml cordycepin in combination with 2 micrograms/ml of the deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenosine (EHNA) to inhibit its degradation is required to see these effects. This is the same concentration required for cordycepin to arrest cells at the onset of mitosis and depolymerize the microtubules to small asters. Cordycepin enters the cells rapidly and is phosphorylated to 3'-dATP with a concomitant drop in ATP levels. However, the direct reduction of ATP levels does not mimic the same rapid effects of cordycepin on either the intermediate filaments or microtubules. In addition, similar effects are not produced by a variety of other adenosine analogs with alterations in the 2'- and 3'-ribose positions. Although other pharmacological reagents result in alterations of the fibroblastic intermediate filaments, cordycepin is unusual because of the rapidity with which the fibroblastic intermediate filaments collapse into the juxtanuclear caps. The juxtanuclear caps have a morphology different from that of the perinuclear bundles of intermediate filaments that arise after long-term depolymerization of the microtubules. The keratin fibers in the epidermal cells retract to a perinuclear ring when treated with cordycepin.

Adenine↗
PubMed1988

Cytoplasmic assembly of snRNP particles from stored proteins and newly transcribed snRNA's in L929 mouse fibroblasts.

Newly synthesized snRNAs appear transiently in the cytoplasm where they assemble into ribonucleoprotein particles, the snRNP particles, before returning permanently to the interphase nucleus. In this report, bona fide cytoplasmic fractions, prepared by cell enucleation, are used for a quantitative analysis of snRNP assembly in growing mouse fibroblasts. The half-lives and abundances of the snRNP precursors in the cytoplasm and the rates of snRNP assembly are calculated in L929 cells. With the exception of U6, the major snRNAs are stable RNA species; U1 is almost totally stable while U2 has a half-life of about two cell cycles. In contrast, the majority of newly synthesized U6 decays with a half-life of about 15 h. The relative abundances of the newly synthesized snRNA species U1, U2, U3, U4 and U6 in the cytoplasm are determined by Northern hybridization using cloned probes and are approximately 2% of their nuclear abundance. The half-lives of the two major snRNA precursors in the cytoplasm (U1 and U2) are approximately 20 min as determined by labeling to steady state. The relative abundance of the snRNP B protein in the cytoplasm is determined by Western blotting with the Sm class of autoantibodies and is approximately 25% of the nuclear abundance. Kinetic studies, using the Sm antiserum to immunoprecipitate the methionine-labeled snRNP proteins, suggest that the B protein has a half-life of 90 to 120 min in the cytoplasm. These data are discussed and suggest that there is a large pool of more stable snRNP proteins in the cytoplasm available for assembly with the less abundant but more rapidly turning-over snRNAs.

Animals↗
PubMed1987

Cordycepin disrupts the microtubule networks and arrests Nil 8 hamster fibroblasts at the onset of mitosis.

The nucleoside analogue 3'-deoxyadenosine (cordycepin) arrests dividing cells at the onset of mitosis in prometaphase. The microtubules in the arrested prometaphase cells depolymerize to two small asters. A minimum of 80 micrograms/ml cordycepin or 20 micrograms/ml cordycepin in combination with 2 micrograms/ml of the deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl) adenosine (EHNA) to inhibit its degradation is required to see these effects. Analysis of cell extracts by high-pressure liquid chromatography indicates that cordycepin enters the cells rapidly and is phosphorylated to 3'-dATP. The intracellular concentration rises almost linearly from 0.7 mM after 15 min to 7 mM by 210 min. Concomitantly the ATP concentration shows a rapid drop from the 4 mM present in controls. However, the direct reduction of ATP levels does not mimic the same rapid effects of cordycepin on the microtubules. In addition, similar effects are not produced by a variety of other adenosine analogues with alterations in the 2' and 3' ribose positions. Although other pharmacological reagents arrest cells at the onset of mitosis, cordycepin is unusual because of the collapse of the microtubule networks to two small asters that radiate from the microtubule-organizing center. 3'-dATP can replace the requirement for ATP or GTP in the vitro polymerization of microtubules from microtubule protein: however, at limiting concentrations of nucleotide it requires approximately two times the concentration of 3'-dATP as ATP to support an equivalent level of microtubule polymerization. This suggests that the effects of cordycepin in vivo may be the result of the depletion of cellular ATP pools and the altered ability of 3'dATP to substitute for ATP-dependent reactions. Current experiments are testing this hypothesis.

Adenine↗
PubMed1987

Cytoplasmic maturation of the snRNAs.

The snRNAs are abundant and stable components of the interphase nucleus. Aqueous and non-aqueous cell fractionation demonstrate that the snRNAs appear transiently in the cytoplasm shortly after transcription, before returning permanently to the interphase nucleus. In pulse label and chase experiments, the newly synthesized snRNA species appear in the cytoplasm after 1 min of labeling and then return to the interphase nucleus after approximately 15 min in the cytoplasm. In order to study the maturation and intracellular transport of these particles, a battery of metabolic inhibitors and alterations in cell culture conditions were investigated for their ability to interfere with the return of the newly synthesized snRNAs to the nucleus. A wide range of inhibitors of the cytoskeleton did not interfere with this process. Only the inhibition of protein synthesis and exposure of cells to medium of at least twice the normal tonicity block the return of the snRNAs to the nucleus. Immunofluorescent staining of cells exposed to hypertonic medium identifies discrete foci in the cytoplasm that stain with the Sm antiserum, directed against proteins associated with the snRNAs. Using a detergent extraction procedure that preserves the cytoskeleton, the newly synthesized snRNAs in the cytoplasm fractionate as soluble complexes. These data are consistent with the hypothesis that the snRNAs partition into the interphase nucleus because of a preferential solubility and the existence of specific binding sites.

Autoantigens↗
PubMed1986

Removal of cellular water prevents the reformation of the interphase nucleus.

The mature snRNP (small nuclear ribonucleoprotein) particles are localized quantitatively in the interphase nucleus. Like many nuclear antigens, they distribute throughout the cytoplasm after the nuclear envelope breaks down during mitosis and then return to the newly formed daughter nuclei in early G1. Their abundance and stability and the availability of monoclonal antibodies that recognize them, make the snRNP particles a useful model system for studying the reformation of the nucleus at the completion of mitosis. A wide variety of metabolic inhibitors and alterations in normal culture conditions were investigated for their ability to interfere with the return of the snRNP particles to daughter nuclei after mitosis. None of the well-characterized cytoskeletal inhibitors, biosynthetic inhibitors, calcium antagonists, nor ionophores were effective in interfering with this return. However, the removal of cellular water by exposure of cells to hypertonic medium during mitosis blocked the reformation of the nucleus and trapped the snRNP particles in the cytoplasm. In medium of twice the normal tonicity, the function of the mitotic spindle and the cleavage furrow are inhibited, however, the cells reattach to the substratum as if returning to interphase. The chromatin stays condensed and does not form a normal interphase nucleus and the snRNP particles stay dispersed throughout the cytoplasm. This condition is reversible and after return to normal medium the nucleus reforms and the snRNP particles collect in the new nuclei. After gentle extraction of metaphase cells, about 30% of the snRNP particles are soluble, however, the remainder are associated with an insoluble remnant. These data are consistent with the notion that the snRNP particles accumulate in the nucleus due to both preferential solubility and specific binding sites in the interphase nucleus.

Biological Transport, Active↗
PubMed1984

Nocodazole and cytochalasin D induce tetraploidy in mammalian cells.

Nocodazole, a rapidly reversible inhibitor of microtubule assembly is useful for preparing mammalian cells synchronized at all stages of mitosis. When synchronized cells are allowed to progress through mitosis in the presence of cytochalasin D, the cleavage furrow is inhibited and dikaryon cells are formed. These cells become homogeneous populations of stable mononuclear tetraploid cells after the following cell division. This procedure is applicable to a wide range of mammalian cells in culture.

Animals↗