Real innovations in medicine and science are historic and singular; the stories behind each occurrence are precious. At
we have established the Anthony Cerami Award in Translational Medicine to ...document and preserve these histories. The monographs recount the seminal events as told in the voice of the original investigators who provided the crucial early insight. These essays capture the essence of discovery, chronicling the birth of ideas that created new fields of research; and launched trajectories that persisted and ultimately influenced how disease is prevented, diagnosed, and treated. In this volume, the Cerami Award Monograph is by Paul Nurse, Director, The Francis Crick Institute in London, UK. A visionary in the field of cell biology, this is the story of Dr. Nurse's scientific journey.
Schizosaccharomyces pombe
is a good model to study cell-size control. These cells integrate size information into cell cycle controls at both the G1 S and G2 M transitions, although the primary ...control operates at the entry into mitosis. At G2 M there is both a size threshold, demonstrated by the fact that cells divide when they reach 14 μm in length, and also correction around this threshold, evident from the narrow distribution of sizes within a population. This latter property is referred to as size homeostasis. It has been argued that a population of cells accumulating mass in a linear fashion will have size homeostasis in the absence of size control, if cycle time is controlled by a fixed timer. Because fission yeast cells do not grow in a simple linear fashion, they require a size-sensing mechanism. However, current models do not fully describe all aspects of this control, especially the coordination of cell size with ploidy.
Nuclear size correlates with cell size, but the mechanism by which this scaling is achieved is not known. Here we screen fission yeast gene deletion mutants to identify essential factors involved in ...this process. Our screen has identified 25 essential factors that alter nuclear size, and our analysis has implicated RNA processing and LINC complexes in nuclear size control. This study has revealed lower and more extreme higher nuclear size phenotypes and has identified global cellular processes and specific structural nuclear components important for nuclear size control.
S phase and mitotic onset are brought about by the action of multiple different cyclin-CDK complexes. However, it has been suggested that changes in the total level of CDK kinase activity, rather ...than substrate specificity, drive the temporal ordering of S phase and mitosis. Here, we present a phosphoproteomics-based systems analysis of CDK substrates in fission yeast and demonstrate that the phosphorylation of different CDK substrates can be temporally ordered during the cell cycle by a single cyclin-CDK. This is achieved by rising CDK activity and the differential sensitivity of substrates to CDK activity over a wide dynamic range. This is combined with rapid phosphorylation turnover to generate clearly resolved substrate-specific activity thresholds, which in turn ensures the appropriate ordering of downstream cell-cycle events. Comparative analysis with wild-type cells expressing multiple cyclin-CDK complexes reveals how cyclin-substrate specificity works alongside activity thresholds to fine-tune the patterns of substrate phosphorylation.
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•Global analysis of CDK substrates and their phosphorylation dynamics in fission yeast•CDK substrate phosphorylation can be temporally ordered by a single cyclin-CDK•Substrate-specific activity thresholds set phosphorylation timing and cell-cycle order•Cyclin-substrate specificity provides a secondary, more minor, level of regulation
A systems analysis of CDK substrates in yeast shows that the phosphorylation of different CDK substrates can be temporally ordered during the cell cycle by a single cyclin-CDK via rising CDK activity and the differential sensitivity of substrates to CDK activity.
The size of the membrane-bound nucleus scales with cell size in a wide range of cell types but the mechanisms determining overall nuclear size remain largely unknown. Here we investigate the role of ...fission yeast inner nuclear membrane proteins in determining nuclear size, and propose that the Lap2-Emerin-Man1 domain protein Lem2 acts as a barrier to membrane flow between the nucleus and other parts of the cellular membrane system. Lem2 deletion increases membrane flow into and out of the nuclear envelope in response to changes in membrane synthesis and nucleocytoplasmic transport, altering nuclear size. The endoplasmic reticulum protein Lnp1 acts as a secondary barrier to membrane flow, functionally compensating for lack of Lem2. We propose that this is part of the mechanism that maintains nuclear size proportional to cellular membrane content and thus to cell size. Similar regulatory principles may apply to other organelles in the eukaryotic subcellular membrane network.
How cells correct deviations from a mean cell size at mitosis remains uncertain. Classical cell-size homeostasis models are the sizer, timer, and adder 1. Sizers postulate that cells divide at some ...threshold size; timers, that cells grow for a set time; and adders, that cells add a constant volume before division. Here, we show that a size-based probabilistic model of cell-size control at the G2/M transition (P(Div)) can generate realistic cell-size homeostasis in silico. In fission yeast cells, Cyclin BCdc13 scales with size, and we propose that this increases the likelihood of mitotic entry, while molecular noise in its expression adds a probabilistic component to the model. Varying Cdc13 expression levels exogenously using a newly developed tetracycline inducible promoter shows that both the level and variability of its expression influence cell size at division. Our results demonstrate that as cells grow larger, their probability of dividing increases, and this is sufficient to generate cell-size homeostasis. Size-correlated Cdc13 expression forms part of the molecular circuitry of this system.
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•A size-correlated division probability can generate cell-size homeostasis•Cyclin B concentration scales noisily with size in fission yeast•Cells with stochastically suprathreshold cyclin B are the ones that divide•A new tetracycline inducible promoter with linear dose response is developed
Patterson et al. show that a probabilistic model of cell-size control can generate cell-size homeostasis in fission yeast. Cyclin B levels scale noisily with cell size, providing a molecular correlate for the phenomenological model. Using a new inducible promoter, they show that cyclin B and cell-size variability scale proportionally.
Cells sense their size and use this information to coordinate cell division with cell growth to maintain a constant cell size within a given population. A model has been proposed for cell size ...control in the rod-shaped cells of the fission yeast, Schizosaccharomyces pombe. This involves a protein localized to the cell ends, which inhibits mitotic activators in the middle of the cell in a cell size-dependent manner. This protein, Pom1, along with another tip-localized protein, Nif1, have been implicated as direct sensors of cell size controlling the onset of mitosis. Here we have investigated cell size variability and size homeostasis at the G
2
/M transition, focusing on the role of pom1 and nif1. Cells deleted for either of these 2 genes show wild-type size homeostasis both in size variability analyses and size homeostasis experiments. This indicates that these genes do not have a critical role as direct cell size sensors in the control mechanism. Cell size homeostasis also seems to be independent of Cdc2-Tyr15 phosphorylation, suggesting that the size sensing mechanism in fission yeast may act through an unidentified pathway regulating CDK activity by an unknown mechanism.
Only those exacting editors at Cell could seriously ask you to review the past century of cell cycle research and to predict the course of research for the next century, and to do it all in 10 pages! ...It is unrealistic to try to be comprehensive in such a review, and so I will focus on what I consider to be the important principles underlying the cell cycle, with less emphasis on detailed descriptions of molecular mechanisms which would degenerate into lists of genes and proteins. Referencing will be minimal, and will be restricted to reviews, a few key primary publications, and to books written in English for summaries of the earlier literature. Let us begin at the end of the century by summarizing what is now known about the cell cycle. We know that the cell cycle is the universal process by which cells reproduce, and that it underlies the growth and development of all living organisms. The most important events of the cell cycle are those concerned with the copying and partitioning of the hereditary material, that is replicating the chromosomal DNA during S phase and separating the replicated chromosomes during mitosis. Controls operate that regulate onset of these events and compensate for errors in their execution. The molecular basis of these controls is highly conserved from simple unicellular eukaryotes such as yeast to complex metazoans such as ourselves. The precision with which cell cycle events are executed ensures the survival of living organisms, while loss of this precision increases genomic instability, an important factor in the formation of cancer. The mitotic cell cycle is modified to a meiotic cycle during gamete formation, leading to a reduction in chromosome number that is essential for sexual reproduction, and to an increase in genetic variation that is a driving force for evolution. Thus, the cell cycle plays a central role in the operation and development of all life, and in ensuring the continuity of life across generations. These discoveries were made over a period that extends into the previous century, and so the scope of this review will be similarly extended, hence "A Long Twentieth Century of the Cell Cycle."
A long-standing biological question is how a eukaryotic cell controls the size of its nucleus. We report here that in fission yeast, nuclear size is proportional to cell size over a 35-fold range, ...and use mutants to show that a 16-fold change in nuclear DNA content does not influence the relative size of the nucleus. Multi-nucleated cells with unevenly distributed nuclei reveal that nuclei surrounded by a greater volume of cytoplasm grow more rapidly. During interphase of the cell cycle nuclear growth is proportional to cell growth, and during mitosis there is a rapid expansion of the nuclear envelope. When the nuclear/cell (N/C) volume ratio is increased by centrifugation or genetic manipulation, nuclear growth is arrested while the cell continues to grow; in contrast, low N/C ratios are rapidly corrected by nuclear growth. We propose that there is a general cellular control linking nuclear growth to cell size.
The Rho GTPase family proteins are key regulators of cytoskeletal dynamics. Deregulated activity of Rho GTPases is associated with cancers and neurodegenerative diseases, and their potential as drug ...targets has long been recognized. Using an economically effective drug screening workflow in fission yeast and human cells, we have identified a Rho GTPase inhibitor, O1. By a suppressor mutant screen in fission yeast, we find a point mutation in the
gene that confers resistance to O1. Consistent with the idea that O1 is the direct inhibitor of Rho1, O1 reduced the cellular amount of activated, GTP-bound Rho1 in wild-type cells, but not in the O1-resistant mutant cells, in which the evolutionarily conserved Ala62 residue is mutated to Thr. Similarly, O1 inhibits activity of the human orthologue RhoA GTPase in tissue culture cells. Our studies illustrate the power of yeast phenotypic screens in the identification and characterization of drugs relevant to human cells and have identified a novel GTPase inhibitor for fission yeast and human cells.
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