One does not often look to analytic cubism for insights into the control of the cell cycle, but Pablo Picasso beautifully encapsulated the fundamentals when he said that "every act of creation is, ...first of all, an act of destruction". The rapid destruction of specific cell cycle regulators at just the right moment in the cell cycle ensures that daughter cells receive an equal and identical set of chromosomes from their mother and that DNA replication always follows mitosis. Remarkably, one protein complex is responsible for this surgical precision, the APC/C (anaphase-promoting complex, also known as the cyclosome). The APC/C is tightly regulated by its co-activators and by the spindle assembly checkpoint.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The spindle assembly checkpoint (SAC) maintains genomic stability by delaying chromosome segregation until the last chromosome has attached to the mitotic spindle. The SAC prevents the anaphase ...promoting complex/cyclosome (APC/C) ubiquitin ligase from recognizing cyclin B and securin by catalysing the incorporation of the APC/C co-activator, CDC20, into a complex called the mitotic checkpoint complex (MCC). The SAC works through unattached kinetochores generating a diffusible 'wait anaphase' signal that inhibits the APC/C in the cytoplasm, but the nature of this signal remains a key unsolved problem. Moreover, the SAC and the APC/C are highly responsive to each other: the APC/C quickly targets cyclin B and securin once all the chromosomes attach in metaphase, but is rapidly inhibited should kinetochore attachment be perturbed. How this is achieved is also unknown. Here, we show that the MCC can inhibit a second CDC20 that has already bound and activated the APC/C. We show how the MCC inhibits active APC/C and that this is essential for the SAC. Moreover, this mechanism can prevent anaphase in the absence of kinetochore signalling. Thus, we propose that the diffusible 'wait anaphase' signal could be the MCC itself, and explain how reactivating the SAC can rapidly inhibit active APC/C.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the ...proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.
The cyclin B-Cdk1 kinase triggers mitosis in most eukaryotes. In animal cells, cyclin B shuttles between the nucleus and cytoplasm in interphase before rapidly accumulating in the nucleus at ...prophase, which promotes disassembly of the nuclear lamina and nuclear envelope breakdown (NEBD). What triggers the nuclear accumulation of cyclin B1 is presently unclear, although the prevailing view is that the Plk1 kinase inhibits its nuclear export. In this study, we use a biosensor specific for cyclin B1-Cdk1 activity to show that activating cyclin B1-Cdk1 immediately triggers its rapid accumulation in the nucleus through a 40-fold increase in nuclear import that remains dependent on Cdk1 activity until NEBD. Nevertheless, a substantial proportion of cyclin B1-Cdk1 remains in the cytoplasm. The increase in nuclear import is driven by changes in the nuclear import machinery that require neither Plk1 nor inhibition of nuclear export. Thus, the intrinsic link between cyclin B1-Cdk1 activation and its rapid nuclear import inherently coordinates the reorganization of the nucleus and the cytoplasm at mitotic entry.
The CyclinB1-Cdk1 kinase is the catalytic activity at the heart of mitosis-promoting factor (MPF), yet fundamental questions concerning its role in mitosis remained unresolved. It is not known when ...and how rapidly CyclinB1-Cdk1 is activated in mammalian cells, nor how its activation coordinates the substantial changes in the cell at mitosis. Here, we have developed a FRET biosensor specific for CyclinB1-Cdk1 that enables us to assay its activity with very high temporal precision in living human cells. We show that CyclinB1-Cdk1 is inactive in G2 phase and activated at a set time before nuclear envelope breakdown, thereby initiating the events of prophase. CyclinB1-Cdk1 levels rise to their maximum extent over the course of approximately 30 min, and we demonstrate that different levels of CyclinB1-Cdk1 kinase activity trigger different mitotic events, thus revealing how the remarkable reorganization of the cell is coordinated at mitotic entry.
► We have developed a FRET probe that can assay Cyclin B1-Cdk1 activity in living cells ► Cyclin B1-Cdk1 is activated ∼30 min before nuclear envelope breakdown ► Different levels of CyclinB1-Cdk1 activity trigger different events in prophase ► Plk1 is required upstream of CyclinB1-Cdk1 activation, not as part of an amplification loop
The spindle assembly checkpoint (SAC) is essential in mammalian mitosis to ensure the equal segregation of sister chromatids. The SAC generates a mitotic checkpoint complex (MCC) to prevent the ...anaphase-promoting complex/cyclosome (APC/C) from targeting key mitotic regulators for destruction until all of the chromosomes have attached to the mitotic apparatus. A single unattached kinetochore can delay anaphase for several hours, but how it is able to block the APC/C throughout the cell is not understood. Present concepts of the SAC posit that either it exhibits an all-or-nothing response or there is a minimum threshold sufficient to block the APC/C (ref. 7). Here, we have used gene targeting to measure SAC activity, and find that it does not have an all-or-nothing response. Instead, the strength of the SAC depends on the amount of MAD2 recruited to kinetochores and on the amount of MCC formed. Furthermore, we show that different drugs activate the SAC to different extents, which may be relevant to their efficacy in chemotherapy.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
There are two major problems for the cell to solve in mitosis: how to ensure that each daughter cell receives an equal and identical complement of the genome, and how to prevent cell separation ...before chromosome segregation. Both these problems are solved by controlling when two specific proteins are destroyed: securin, an inhibitor of chromosome segregation, and cyclin B, which inhibits cell separation (cytokinesis). It has recently become clear that several other proteins are degraded at specific points in mitosis. This review (which is part of the
Chromosome Segregation and Aneuploidy series) focuses on how specific proteins are selected for proteolysis at defined points in mitosis and how this contributes to the proper coordination of chromosome segregation and cytokinesis.
PDF
