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1.
Cytokinesis must be initiated only after chromosomes have been segregated in anaphase and must be terminated once cleavage is completed. We show that the fission yeast protein Etd1 plays a central role in both of these processes. Etd1 activates the guanosine triphosphatase (GTPase) Spg1 to trigger signaling through the septum initiation network (SIN) pathway and onset of cytokinesis. Spg1 is activated in late anaphase when spindle elongation brings spindle pole body (SPB)–localized Spg1 into proximity with its activator Etd1 at cell tips, ensuring that cytokinesis is only initiated when the spindle is fully elongated. Spg1 is active at just one of the two SPBs during cytokinesis. When the actomyosin ring finishes constriction, the SIN triggers disappearance of Etd1 from the half of the cell with active Spg1, which then triggers Spg1 inactivation. Asymmetric activation of Spg1 is crucial for timely inactivation of the SIN. Together, these results suggest a mechanism whereby cell asymmetry is used to monitor cytoplasmic partitioning to turn off cytokinesis signaling. 相似文献
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The strategies of the sit-to-stand movement are investigated by describing the movement in terms of the topology of an associated
phase diagram. Kinematic constraints are applied to describe movement sequences, thus reducing the dimension of the phase
space. This dimensional reduction allows us to apply theorems of topological dynamics for two-dimensional systems to arrive
at a classification of six possible movement strategies, distinguished by the topology of their corresponding phase portrait.
Since movement is treated in terms of topological structure rather than specific trajectories, individual variations are automatically
included, and the approach is by nature model independent. Pathological movement is investigated, and this method clarifies
how subtle abnormalities in movement lead to difficulties in achieving a stable stance upon rising from a seated position.
This article was processed by the author using the LATEX style file pljour2 from Springer-Verlag. 相似文献
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Eight class I tRNA species have been purified to homogeneity and their proton nuclear magnetic resonance (NMR) spectra in the low-field region (-11 to -15 ppm) have been studied at 360 MHz. The low-field spectra contain only one low-field resonance from each base pair (the ring NH hydrogen bond) and hence directly monitor the number of long-lived secondary and tertiary base pairs in solution. The tRNA species were chosen on the basis of their sequence homology with yeast phenylalanine tRNA in the regions which form tertiary base pairs in the crystal structure of this tRNA. All of the spectra show 26 or 27 low-field resonances approximately 7 of which are derived from tertiary base pairs. These results are contrary to previous claims that the NMR spectra indicate the presence of resonances from secondary base pairs only, as well as more recent claims of only 1-3 tertiary resonances, but are in good agreement with the number of tertiary base pairs expected in solution based on the crystal structure. The tertiary base pair resonances are stable up to at least 46 degrees C. Removal of magnesium ions causes structural changes in the tRNA but does not result in the loss of any secondary or tertiary base pairs. 相似文献
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Scw1p antagonizes the septation initiation network to regulate septum formation and cell separation in the fission yeast Schizosaccharomyces pombe
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Cytokinesis in the fission yeast Schizosaccharomyces pombe is regulated by a signaling pathway termed the septation initiation network (SIN). The SIN is essential for initiation of actomyosin ring constriction and septum formation. In a screen to search for mutations that can rescue the sid2-250 SIN mutant, we obtained scw1-18. Both the scw1-18 mutant and the scw1 deletion mutant (scw1Δ mutant), have defects in cell separation. Both the scw1-18 and scw1Δ mutations rescue the growth defects of not just the sid2-250 mutant but also the other temperature-sensitive SIN mutants. Other cytokinesis mutants, such as those defective for actomyosin ring formation, are not rescued by scw1Δ. scw1Δ does not seem to rescue the SIN by restoring SIN signaling defects. However, scw1Δ may function downstream of the SIN to promote septum formation, since scw1Δ can rescue the septum formation defects of the cps1-191β-1,3-glucan synthase mutant, which is required for synthesis of the primary septum. 相似文献