Involvement of colchicine-sensitive cytoplasmic element in premitotic nuclear positioning ofAdiantum protonemata

Summary When the red-light grown protonema ofAdiantum capillus-veneris was transferred to the dark, the nucleus ceased its migration ca. 5 hours before cell plate formation (Mineyuki andFuruya 1980). To see whether the nucleus was held by some cytoplasmic structure during nuclear positioning, protonemata were treated with various centrifugal forces at different stages of the cell cycle. Nuclei of G₁ phase were easily displaced by centrifugation at 360×g for 15 minutes, but those of G₂ or M phase were not displaced by it, suggesting that the nuclei were held by some cytoplasmic elements in G₂ or M phase. This nuclear anchoring was not detectable in protonemata that were treated with 5mM colchicine. With this treatment, the nucleus did not stop its migration at late G₂ and moved even in prophase. And the retardation of organelle movement which was observed in cytoplasm on the lateral side of the nucleus after the cessation of premitotic nuclear migration (Mineyuki andFuruya 1984) was not observed in the presence of colchicine. Thus the nuclei appear to be held by colchicine-sensitive structure in cytoplasm between the lateral surface of the nucleus and cell wall during the premitotic nuclear positioning. Electron micrographs showing cytoplasmic microtubules were consistent with the idea.Abbreviations PPN Premitotic positioning of the nucleus - L region Cytoplasm between the lateral surface of the nucleus and cell wall (seeMineyuki et al. 1984)     

Reconstitution mechanism of nucleosome core particles mediated by poly(l-glutamic acid)

Poly(l-glutamic acid) has been reported to mediate in vitro nucleosome assembly (Stein, A., Whitlock, J.P., Jr. and Bina, M. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 5000–5004). To study the reaction mechanism, we have reconstituted nucleosome core particles from chicken erythrocyte core DNA and core histones in the presence of poly(l-glutamic acid) and analyzed the assembly products by polyacrylamide gel electrophoresis. Poly(l-glutamic acid), which binds and forms a large complex with core histones, is replaced with core DNA in the reconstitution process. When histone-poly(l-glutamic acid) complex and core DNA are mixed with a histone:DNA ratio of 1.0, the yield of core particles increases by prolonged reconstitution time. Two phases with a distinct time range appear in the process. In the fast phase within 30 min, 60% of the DNA is involved in products containing histones: reconstituted core particles, a larger nucleoprotein complex and aggregation. In the second phase, the remaining DNA and the DNA in the aggregation decrease, and the core particles increase slowly. The yield of core particles is approx. 60% after 24 h. The slow phase is not observed by reconstitution with a histone:DNA ratio of 2.0 in the initial mixture. The reaction scheme of the assembly process derived from these data is given. Based on the in vitro reaction scheme, the possible role of in vivo ‘nucleosome assembly factors’ is also discussed.     

Germ cell nucleosomes contain remodeled core protein complex

Electrophoretic analysis of the nucleosomal histones from MN1 and MN2 subpopulations of the seminiferous tubules in gels containing either 6.25 or 2.5 M urea revealed the presence of testis specific histone H2S, H1 and protein ‘A’ in addition to the somatic histones in the core protein complex. Size analysis indicated the presence of a 150–160 bp DNA segment in the MNI subpopulation, whereas, an approx 180 bp DNA fragment was present in the MN2 subpopulation of both liver and tubule nucleosomes. These data suggest an extensive remodeling of the nucleosomal core protein complex during mammalian spermatogenesis.     

The presence of extended phragmosomes containing cytoskeletal elements in fusiform cambial cells ofFraxinus excelsior L.

Summary Fusiform cambial cells of the ash (Fraxinus excelsior L.), which are strongly elongated and vacuolated, contain a phragmosome which traverses the whole length of the cells during preprophase and karyokinesis and which remains present during cytokinesis until it is integrated in cell plate with adjacent cytoplasm.The phragmosome consists of a thin perforated cytoplasmic layer located in the plane of the future cell plate. Otherwise oriented transvacuolar cytoplasmic layers or strands are not present in these cells.The phragmosome contains cytoskeletal elements, namely microtubules and also microfilament bundles both of which are oriented mainly in longitudinal direction.The phragmosomal microtubules are a new category of microtubules associated with cell division; presumably they guide the centrifugally growing cell plate to the parental cell wall site previously marked by the preprophase band of microtubules.     

Spectrum structures and biological functions of 8-mers in the human genome

The spectra of k-mer frequencies can reveal the structures and evolution of genome sequences. We confirmed that the trimodal spectrum of 8-mers in human genome sequences is distinguished only by CG2, CG1 and CG0 8-mer sets, containing 2,1 or 0 CpG, respectively. This phenomenon is called independent selection law. The three types of CG 8-mers were considered as different functional elements. We conjectured that (1) nucleosome binding motifs are mainly characterized by CG1 8-mers and (2) the core structural units of CpG island sequences are predominantly characterized by CG2 8-mers. To validate our conjectures, nucleosome occupied sequences and CGI sequences were extracted, then the sequence parameters were constructed through the information of the three CG 8-mer sets respectively. ROC analysis showed that CG1 8-mers are more preference in nucleosome occupied segments (AUC > 0.7) and CG2 8-mers are more preference in CGI sequences (AUC > 0.99). This validates our conjecture in principle.     

The coordination of spindle‐positioning forces during the asymmetric division of the Caenorhabditis elegans zygote

In Caenorhabditis elegans zygote, astral microtubules generate forces essential to position the mitotic spindle, by pushing against and pulling from the cortex. Measuring microtubule dynamics there, we revealed the presence of two populations, corresponding to pulling and pushing events. It offers a unique opportunity to study, under physiological conditions, the variations of both spindle‐positioning forces along space and time. We propose a threefold control of pulling force, by polarity, spindle position and mitotic progression. We showed that the sole anteroposterior asymmetry in dynein on‐rate, encoding pulling force imbalance, is sufficient to cause posterior spindle displacement. The positional regulation, reflecting the number of microtubule contacts in the posterior‐most region, reinforces this imbalance only in late anaphase. Furthermore, we exhibited the first direct proof that dynein processivity increases along mitosis. It reflects the temporal control of pulling forces, which strengthens at anaphase onset following mitotic progression and independently from chromatid separation. In contrast, the pushing force remains constant and symmetric and contributes to maintaining the spindle at the cell centre during metaphase.     

The impact of microgravity-based proteomics research

Proteomics is performed in microgravity research in order to determine protein alterations occurring qualitatively and quantitatively, when single cells or whole organisms are exposed to real or simulated microgravity. To this purpose, antibody-dependent (Western blotting, flow cytometry, Luminex^® technology) and antibody-independent (mass spectrometry, gene array) techniques are applied. The anticipated findings will help to understand microgravity-specific behavior, which has been observed in bacteria, as well as in plant, animal and human cells. To date, the analyses revealed that cell cultures are more sensitive to microgravity than cells embedded in organisms and that proteins changing under microgravity are highly interactive. Furthermore, one has to distinguish between primary gravity-induced and subsequent interaction-dependent changes of proteins, as well as between direct microgravity-related effects and indirect stress responses. Progress in this field will impact on tissue engineering and medicine and will uncover possibilities of counteracting alterations of protein expression at lowered gravity.