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T J Mitchison

Publications and source records attributed to T J Mitchison.

At least 73 records · Page 4Linked to original sources
PubMed1995

The SMC family: from chromosome condensation to dosage compensation.

Recent genetic analyses in yeasts and biochemical studies in vertebrate cells have led to the discovery of a family of putative ATPases that play a fundamental role in chromosome condensation and segregation in mitosis. One of the members was also found to be involved in dosage compensation in Caenorhabditis elegans, providing a new link between global regulation of gene expression and chromosome structure. This unique family of proteins may control higher-order chromosome dynamics by regulated self-assembly or mechanochemical activity.

Adenosine Triphosphatases↗
PubMed1995

Identification and partial characterization of mitotic centromere-associated kinesin, a kinesin-related protein that associates with centromeres during mitosis.

Using antipeptide antibodies to conserved regions of the kinesin motor domain, we cloned a kinesin-related protein that associates with the centromere region of mitotic chromosomes. We call the protein MCAK, for mitotic centromere-associated kinesin. MCAK appears concentrated on centromeres at prophase and persists until telophase, after which time the localization disperses. It is found throughout the centromere region and between the kinetochore plates of isolated mitotic CHO chromosomes, in contrast to two other kinetochore-associated microtubule motors: cytoplasmic dynein and CENP-E (Yen et al., 1992), which are closer to the outer surface of the kinetochore plates. Sequence analysis shows MCAK to be a kinesin-related protein with the motor domain located in the center of the protein. It is 60-70% similar to kif2, a kinesin-related protein originally cloned from mouse brain with a centrally located motor domain (Aizawa et al., 1992). MCAK protein is present in interphase and mitotic CHO cells and is transcribed as a single 3.4-kb message.

Amino Acid Sequence↗
PubMed1995

Myosin is involved in postmitotic cell spreading.

We have investigated a role for myosin in postmitotic Potoroo tridactylis kidney (PtK2) cell spreading by inhibitor studies, time-lapse video microscopy, and immunofluorescence. We have also determined the spatial organization and polarity of actin filaments in postmitotic spreading cells. We show that butanedione monoxime (BDM), a known inhibitor of muscle myosin II, inhibits nonmuscle myosin II and myosin V adenosine triphosphatases. BDM reversibly inhibits PtK2 postmitotic cell spreading. Listeria motility is not affected by this drug. Electron microscopy studies show that some actin filaments in spreading edges are part of actin bundles that are also found in long, thin, structures that are connected to spreading edges and substrate (retraction fibers), and that 90% of this actin is oriented with barbed ends in the direction of spreading. The remaining actin in spreading edges has a more random orientation and spatial arrangement. Myosin II is associated with actin polymer in spreading cell edges, but not retraction fibers. Myosin II is excluded from lamellipodia that protrude from the cell edge at the end of spreading. We suggest that spreading involves myosin, possibly myosin II.

Actins↗
PubMed1995

The bulk of unpolymerized actin in Xenopus egg extracts is ATP-bound.

Non-muscle cells contain 15-500 microM actin, a large fraction of which is unpolymerized. Thus, the concentration of unpolymerized actin is well above the critical concentration for polymerization in vitro (0.2 microM). This fraction of actin could be prevented from polymerization by being ADP bound (therefore less favored to polymerize) or by being ATP bound and sequestered by a protein such as thymosin beta 4, or both. We isolated the unpolymerized actin from Xenopus egg extracts using immobilized DNase 1 and assayed the bound nucleotide. High-pressure liquid chromatography analysis showed that the bulk of soluble actin is ATP bound. Analysis of actin-bound nucleotide exchange rates suggested the existence of two pools of unpolymerized actin, one of which exchanges nucleotide relatively rapidly and another that apparently does not exchange. Native gel electrophoresis of Xenopus egg extracts demonstrated that most of the soluble actin exists in complexes with other proteins, one of which might be thymosin beta 4. These results are consistent with actin polymerization being controlled by the sequestration and release of ATP-bound actin, and argue against nucleotide exchange playing a major role in regulating actin polymerization.

Actins↗
PubMed1994

A heterodimeric coiled-coil protein required for mitotic chromosome condensation in vitro.

We report here a chromosomal protein that plays an essential role in mitotic chromosome condensation in Xenopus egg extracts. Two polypeptides, designated XCAP-C and XCAP-E, were found to associate with each other in the extracts, presumably forming a heterodimer. During chromosome assembly in mitotic extracts, XCAP-C/E was recruited to the chromatin and formed a discrete internal structure within assembled chromosomes. Antibody blocking experiments showed that XCAP-C function is required for both assembly and structural maintenance of mitotic chromosomes in vitro. Deduced amino acid sequences revealed that the two polypeptides share common structural motifs, consisting of an N-terminal NTP-binding domain, two central coiled-coil regions, and a C-terminal conserved domain. These motifs are highly conserved in a protein family, members of which have been identified recently in both prokaryotes and eukaryotes.

Amino Acid Sequence↗
PubMed1994

Involvement of profilin in the actin-based motility of L. monocytogenes in cells and in cell-free extracts.

Within hours of Listeria monocytogenes infection, host cell actin filaments form a dense cloud around the intracytoplasmic bacteria and then rearrange to form a polarized comet tail that is associated with moving bacteria. We have devised a cell-free extract system capable of faithfully reconstituting L. monocytogenes motility, and we have used this system to demonstrate that profilin, a host actin monomer-binding protein, is necessary for bacterial actin-based motility. We find that extracts from which profilin has been depleted do not support comet tail formation or bacterial motility. In extracts and host cells, profilin is localized to the back half of the surface of motile L. monocytogenes, the site of actin filament assembly in the tail. This association is not observed with L. monocytogenes mutants that do not express the ActA protein, a bacterial gene product necessary for motility and virulence. Profilin also fails to bind L. monocytogenes grown outside of host cytoplasm, suggesting that at least one other host cell factor is required for this association.

Actins↗
PubMed1994

Mitosis. Kinetochores pass the IQ test.

Kinetochores oscillate to and fro on the mitotic spindle. The oscillations seem to be biased by the forces acting on the kinetochore, explaining the variety of chromosome movements seen at different stages of mitosis.

Animals↗
PubMed1994

Actin-dependent motile forces and cell motility.

Force arising from actin polymerization and myosin activity drives a number of different actin-based cell movements. Several new reports support previous data suggesting that actin polymerization drives lamellipodial protrusion and bacterial propulsion, and one report describes a more indirect role for actin assembly in axonal elongation. The major new findings of the past year concerning possible motility roles for myosin describe myosin-driven protrusion of cell margins.

Actins↗
PubMed1994

Towards a pharmacological genetics.

In the past, pharmacological approaches have been instrumental in identifying proteins involved in biological processes. Combinatorial chemistry has the potential to make this type of approach even more powerful.

Animals↗
PubMed1994

Microtubule flux in mitosis is independent of chromosomes, centrosomes, and antiparallel microtubules.

We investigated the mechanism of poleward microtubule flux in the mitotic spindle by generating spindle subassemblies in Xenopus egg extracts in vitro and assaying their ability to flux by photoactivation of fluorescence and low-light multichannel fluorescence video-microscopy. We find that monopolar intermediates of in vitro spindle assembly (half-spindles) exhibit normal poleward flux, as do astral microtubule arrays induced by the addition of dimethyl sulfoxide to egg extracts in the absence of both chromosomes and conventional centrosomes. Immunodepletion of the kinesin-related microtubule motor protein Eg5, a candidate flux motor, suggests that Eg5 is not required for flux. These results suggest that poleward flux is a basic element of microtubule behavior exhibited by even simple self-organized microtubule arrays and presumably underlies the most elementary levels of spindle morphogenesis.

Animals↗
PubMed1993

Localization of an exchangeable GTP binding site at the plus end of microtubules.

Microtubule polarity arises from the head-to-tail orientation of alpha-beta tubulin heterodimers in the microtubule lattice. The identity of the polypeptide at each end of the microtubule is unknown, but structural models predict that the beta-tubulin end contains an exchangeable guanosine triphosphate (GTP) binding site. When GTP-coated fluorescent beads were incubated with microtubules, they bound specifically to plus ends, suggesting that tubulin is oriented in microtubules with beta-tubulin toward the plus end.

Adenosine Triphosphate↗
PubMed1993

Topoisomerase II does not play a scaffolding role in the organization of mitotic chromosomes assembled in Xenopus egg extracts.

We have investigated the role of topoisomerase II (topo II) in mitotic chromosome assembly and organization in vitro using Xenopus egg extracts. When sperm chromatin was incubated with mitotic extracts, the highly compact chromatin rapidly swelled and concomitantly underwent local condensation. Further incubation induced the formation of entangled thin chromatin fibers that eventually resolved into highly condensed individual chromosomes. This in vitro system made it possible to manipulate mitotic chromosomes in their assembly condition without any isolation or stabilization steps. Two complementary approaches, immunodepletion and antibody blocking, demonstrated that topo II activity is required for chromosome assembly and condensation. Once condensation was completed, however, blocking of topo II activity had little effect on the chromosome morphology. Immunofluorescent studies showed that topo II was uniformly distributed throughout the condensed chromosomes and was not restricted to the chromosomal axis. Surprisingly, all detectable topo II molecules were easily extracted from the chromosomes under mild conditions where the shape of chromosomes was well preserved. Our results show that topo II is essential for mitotic chromosome assembly, but does not play a scaffolding role in the structural maintenance of chromosomes assembled in vitro. We also present evidence that changes of DNA topology affect the distribution of topo II in mitotic chromosomes in our system.

Amino Acid Sequence↗
PubMed1993

Moving and stationary actin filaments are involved in spreading of postmitotic PtK2 cells.

We have investigated spreading of postmitotic PtK2 cells and the behavior of actin filaments in this system by time-lapse microscopy and photoactivation of fluorescence. During mitosis PtK2 cells round up and at cytokinesis the daughter cells spread back to regain their interphase morphology. Normal spreading edges are quite homogenous and are not comprised of two distinct areas (lamellae and lamellipodia) as found in moving edges of interphase motile cells. Spreading edges are connected to a network of long, thin, actin-rich fibers called retraction fibers. A role for retraction fibers in spreading was tested by mechanical disruption of fibers ahead of a spreading edge. Spreading is inhibited over the region of disruption, but not over neighboring intact fibers. Using photoactivation of fluorescence to mark actin filaments, we have determined that the majority of actin filaments move forward in spreading edges at the same rate as the edge. As far as we are aware, this is the first time that forward movement of a cell edge has been correlated with forward movement of actin filaments. In contrast, actin filaments in retraction fibers remain stationary with respect to the substrate. Thus there are at least two dynamic populations of actin polymer in spreading postmitotic cells. This is supported by the observation that actin filaments in some spreading edges not only move forward, but also separate into two fractions or broaden with time. A small fraction of postmitotic cells have a spreading edge with a distinct lamellipodium. In these edges, marked actin polymer fluxes backward with respect to substrate. We suggest that forward movement of actin filaments may participate in generating force for spreading in postmitotic cells and perhaps more generally for cell locomotion.

Actin Cytoskeleton↗
PubMed1992

Microtubule-motor activity of a yeast centromere-binding protein complex.

During cell division, sister chromosomes segregate from each other on a microtubule-based structure called the mitotic spindle. Proteins bind to the centromere, a region of chromosomal DNA, to form the kinetochore, which mediates chromosome attachment to the mitotic spindle microtubules. In the budding yeast Saccharomyces cerevisiae, genetic analysis has shown that the 28-basepair (bp) CDEIII region of the 125-bp centromere DNA sequence (CEN sequence) is the main region controlling chromosome segregation in vivo. Therefore it is likely that proteins binding to the CDEIII region link the centromeres to the microtubules during mitosis. A complex of proteins (CBF3) that binds specifically to the CDEIII DNA sequence has been isolated by affinity chromatography. Here we describe kinetochore function in vitro. The CBF3 complex can link DNA to microtubules, and the complex contains a minus-end-directed microtubule-based motor. We suggest that microtubule-based motors form the fundamental link between microtubules and chromosomes at mitosis.

Adenosine Triphosphate↗
PubMed1992

Mitotic spindle organization by a plus-end-directed microtubule motor.

Intracellular microtubule motor proteins may direct the motile properties and/or morphogenesis of the mitotic spindle (reviewed in ref. 3). The recent identification of kinesin-like proteins important for mitosis or meiosis indicates that kinesin-related proteins may play a universal role in eukaryotic cell division, but the precise function of such proteins in mitosis remains unknown. Here we use an in vitro assay for spindle assembly, derived from Xenopus egg extracts, to investigate the role of Eg5, a kinesin-like protein in Xenopus eggs. Eg5 is localized along spindle microtubules, and particularly enriched near spindle poles. Immunodepletion of Eg5 from egg extracts markedly reduces the extent of spindle formation in extracts, as does direct addition of anti-Eg5 antibodies. We also demonstrate that Eg5 is a plus-end-directed microtubule motor in vitro. Our results suggest a novel mechanism for the dynamic self-organization of spindle poles in mitosis.

Animals↗