Two-dimensinal gel electrophoresis of rat liver nuclear washes, nuclear matrix, and hnRNA proteins

The growth inhibition in Friend erythroleukemia cells induces cycling of preexisting nonhistone proteins

The present investigation performed on Friend erythroleukemia cells provides evidence that the cellular reprogramming associated with transition of the cells from the growing to the resting state is accompanied by an intranuclear cycling of preexisting proteins migrating from the cytoplasm. The model system developed for this study affords an opportunity to follow these events in a conveniently short period. The flow microfluorometric analysis revealed that, with respect to DNA content, the quiescent nuclei exhibit distribution in G1, S and G2 phases, as do the nuclei of the control, exponentially growing cells. It was found further that the induction of quiescence arouses two waves of migration of preexisting cytoplasmic proteins toward the nucleus: an early one, during the transition to quiescence clearly showing density dependence, and a late one, in the established resting state. While the early-cycled proteins are undetectable in mass and once they have reached the nucleus have a short life time and, thus, could be considered as regulatory molecules, the late-cycled proteins accumulate within the nucleus and are associated most probably with structural reorganization of the resting nuclei; moreover, the late-cycled nonhistone proteins were found localized, at least partly, in the nuclear protein matrix structure. Control experiments confirmed that the early and late-cycled proteins have been synthesized in the cytoplasm in the proliferative state but are cycled intranuclearly only if the resting state has been induced. When finally the resting cells were stimulated to proliferate, the accumulated presynthesized and newly synthesized nonhistone proteins undergo a rapid degradation which suggests that the new cellular programme may require a complete elimination of the 'resting' nonhistone proteins.

Nuclear nonhistone proteins in mouse teratocarcinomas : I. Cell lineage specificity

The nonhistone protein pattern of four murine teratocarcinomas with different capacities for differentiation were compared: a multidifferentiated teratocarcinoma OTT2289, a nondifferentiated teratocarcinoma OTT2158, a teratocarcinoma-derived rhabdomyosarcoma TDR114, and a teratocarcinoma-derived neuroblastoma TDN2151. Their nonhistone proteins (NHP) were separated by differential salt extraction and hydroxyapatite chromatography into three fractions, NHP-I, NHP-II and NHP-III. Comparison of the NHP fractions by twodimensional gel electrophoresis in combination with a sensitive silver staining method reveals that there are several tumour line specific proteins in each NHP fraction. We suggest that specific NHP, which can be used as biochemical markers for each of the four investigated tumour lines, may be involved in cell lineage specific control of gene expression.

A nucleolar skeleton of protein filaments demonstrated in amplified nucleoli of Xenopus laevis

The amplified, extrachromosomal nucleoli of Xenopus oocytes contain a meshwork of approximately 4-nm-thick filaments, which are densely coiled into higher-order fibrils of diameter 30-40 nm and are resistant to treatment with high- and low-salt concentrations, nucleases (DNase I, pancreatic RNase, micrococcal nuclease), sulfhydryl agents, and various nonionic detergents. This filamentous "skeleton" has been prepared from manually isolated nuclear contents and nucleoli as well as from nucleoli isolated by fluorescence-activated particle sorting. The nucleolar skeletons are observed in light and electron microscopy and are characterized by ravels of filaments that are especially densely packed in the nucleolar cortex. DNA as well as RNA are not constituents of this structure, and precursors to ribosomal RNAs are completely removed from the extraction-resistant filaments by treatment with high-salt buffer or RNase. Fractions of isolated nucleolar skeletons show specific enrichment of an acidic major protein of 145,000 mol wt and an apparent pI value of approximately 6.15, accompanied in some preparations by various amounts of minor proteins. The demonstration of this skeletal structure in "free" extrachromosomal nucleoli excludes the problem of contaminations by nonnucleolar material such as perinucleolar heterochromatin normally encountered in studies of nucleoli from somatic cells. It is suggested that this insoluble protein filament complex forms a skeleton specific to the nucleolus proper that is different from other extraction-resistant components of the nucleus such as matrix and lamina and is involved in the spatial organization of the nucleolar chromatin and its transcriptional products.

Two-dimensional gel analysis of rat liver nuclear proteins after thyroidectomy and thyroid hormone treatment

The composition of nucleoproteins in normal rat liver, in livers from thyroidectomized animals, and in thyroidectomized animals treated with 3,3',5-triiodo-L-thyronine was examined by two-dimensional gel electrophoresis. A computer program was designed to measure the relative concentrations of each protein. Approximately 500 protein subunits could be distinguished. Of these, 102 either disappeared or were markedly decreased after thyroidectomy. Shortly after administration of the thyronine (6 hr), 13 proteins reappeared and, by 24 hr, 67 additional proteins had reappeared. These changes in protein concentrations are both the greatest in number and have the largest reported as an effect of thyroid hormone. Twenty-four hr after triiodothyronine administration, some 22 proteins seen in normal liver could not be found. Two new proteins appeared in thyroidectomized rat liver nuclei. We conclude that a major and rapid effect of thyroid hormone is exerted on hepatic nucleoproteins.

Association of newly replicated DNA with the nuclear matrix of Physarum polycephalum

We have studied the role of the nuclear matrix in DNA replication in a naturally synchronized eucaryote, Physarum polycephalum. When P. polycephalum. When P. polycephalum macroplasmodia were pulse labeled with 3H-thymidine, the DNA remaining tightly associated with the matrix was highly enriched in newly synthesized DNA. This enrichment was found both in nuclei that had just initiated DNA replication as well as in nuclei isolated later during S phase. Pulse chase experiments showed that the association of newly replicated DNA with the matrix is transient, since most of the newly replicated DNA could be chased from the matrix by incubating pulse labeled macroplasmodia in media containing unlabeled thymidine. Studies measuring the size distribution of the matrix DNA supported the hypothesis that replication forks are attached to the nuclear matrix. Reconstitution controls indicated that these results were unlikely to be due to preferential, nonspecific binding of nascent DNA to the matrix during the extraction procedures. These results with P. polycephalum in combination with previous studies in non-synchronized rodent cells, suggest that the association of newly replicated DNA with the nuclear matrix may be a general feature of eucaryotic DNA replication.

Arrangement of chromatin in the nucleus

The factors responsible for producing some degree of order to the arrangement of chromatin in the nucleus are reviewed. They are following: 1. Chromosomes are attached to the nuclear membrane, nucleolus and intranuclear matrix. As a result they have a relatively fixed position in the nucleus. 2. In some species somatic pairing results in alignment of homologs. This is rare in mammals. 3. The association of ribosomal DNA and 5S DNA with the nucleolous results in the close approximation of the chromosomes carrying these DNA sequences. In man and other animals the most obvious consequence is satellite association. 4. Heterochromatin is condensed onto the inner nuclear membrane and periphery of the nucleolous while genetically active chromatin occupies the more central portion of the nucleus. The results is a peripheral location of late replicating DNA and a central location of early relicating DNA. 5. The DNA replication points tend to be associated with the nuclear matrix. Autoradiography of briefly labelled cells shows a high frequency of grains associated with nuclear matrix material. 6. Heterochromatin association results in chromocenters and ectopic pairing. 7. In addition to all these is the Rabl orientation or alignment of centromeres with centromeres and telomeres with telomeres. This polarization of the chromosomes results from the traction on the centromeres by the spindle fibers. There is no firm evidence for any higher degrees of order that might bring specific functioning genes into close proximity.