Cell cycle control: many ways to skin a cat

Triggering the all-or-nothing switch into mitosis

The past decade of cell cycle investigations has identified many roads not taken. The kinase that drives mitosis can be modulated by cyclins, by activating phosphorylation, by inhibitory phosphorylation and by binding of inhibitors, but one of these regulatory options controls the transition from G2 phase to mitosis in most circumstances. A switch-like mechanism integrates signals of cellular status and commits the cell to mitosis by abruptly removing inhibitory phosphate from preformed cyclin:Cdk1 complexes. The pathways that flip this switch alter the balance of modifying reactions to favor dephosphorylation, thereby generating a flood of mitotic kinase.

Identification of sugarcane cDNAs encoding components of the cell cycle machinery

Data on cell cycle research in plants indicate that the majority of the fundamental regulators are conserved with other eukaryotes, but the controlling mechanisms imposed on them, and their integration into growth and development is unique to plants. To date, most studies on cell division have been conducted in dicot plants. However, monocot plants have distinct developmental strategies that will affect the regulation of cell division at the meristems. In order to advance our understanding how cell division is integrated with the basic mechanisms controlling cell growth and development in monocots, we took advantage of the sugarcane EST Project (Sucest) to carry an exhaustive data mining to identify components of the cell cycle machinery. Results obtained include the description of distinct classes of cyclin-dependent kinases (CDKs); A, B, D, and H-type cyclins; CDK-interacting proteins, CDK-inhibitory and activating kinases, pRB and E2F transcription factors. Most sugarcane cell cycle genes seem to be member of multigene families. Like in dicot plants, CDKa transcription is not restricted to tissues with elevated meristematic activity, but the vast majority of CDKb-related ESTs are found in regions of high proliferation rates. Expression of CKI genes is far more abundant in regions of less cell division, notably in lateral buds. Shared expression patterns for a group of clusters was unraveled by transcriptional profiling, and we suggest that similar approaches could be used to identify genes that are part of the same regulatory network.

Cloning and expression analysis of a Petunia hybrida flower specific mitotic-like cyclin

A cyclin cDNA clone (Pethy;CycB1;1) was isolated from a Petunia hybrida ovary specific cDNA library. Sequence comparison revealed that Pethy;CYCB1;1 protein is highly homologous to mitotic B1 cyclins. Northern analysis and in situ hybridisation experiments showed that its expression is developmentally regulated and restricted to flower organs. We have attempted to define some of the cell division patterns which contribute to shaping each floral organ by analysing Pethy;CycB1;1 expression on Petunia flower sections. While in sepals, epidermis and parenchyma cell division patterns were comparable, there were two distinct cell division patterns in petals. In the epidermis, Pethy;CYCB1;1 expression was found both at the petal tip and along epidermis, whereas in the parenchyma only at the petal tips. In reproductive organs cell divisions were detected only in sporophytic tissues. No signals were detected inside meiotic cells.

Drosophila myb is required for the G2/M transition and maintenance of diploidy

The myb proto-oncogenes are thought to have a role in the cell division cycle. We have examined this possibility by genetic analysis in Drosophila melanogaster, which possesses a single myb gene. We have described previously two temperature-sensitive, recessive lethal mutants in Drosophila myb (Dm myb). The phenotypes of these mutants revealed a requirement for myb in diverse cellular lineages throughout the course of Drosophila development. We now report a cellular explanation for these findings by showing that Dm myb is required for both mitosis and prevention of endoreduplication in wing cells. Myb apparently acts at or near the time of the G2/M transition. The two mutant alleles of Dm myb produce the same cellular phenotype, although the responsible mutations are located in different functional domains of the gene product. The mutant phenotype can be partially suppressed by ectopic expression of either cdc2 or string, two genes that are known to promote the transition from G2 to M. We conclude that Dm myb is required for completion of cell division and may serve two independent functions: promotion of mitosis, on the one hand, and prevention of endoreduplication when cells are arrested in G2, on the other.

Cdks and the Drosophila cell cycle

Cyclin-dependent kinases play essential roles in driving the cell cycle. Much progress has been made in Drosophila over the past year in identifying the specific requirements for individual cyclins in particular cell cycle events. These studies encompass many aspects of the cell cycle, from the addition of a G1 phase to the cell cycle during embryogenesis to the role of cyclin degradation in progression through anaphase.

RBF, a novel RB-related gene that regulates E2F activity and interacts with cyclin E in Drosophila

Genetic studies have shown that cyclin E and dE2F are critical regulators of S-phase entry during Drosophila embryogenesis. Whereas the ectopic expression of cyclin E activates dE2F-dependent transcription, it has been proposed that cyclin E does not act directly on dE2F but targets a negative regulator of E2F activity. Such a regulator might be analogous to the family of RB-related proteins (pRB, p107, and p130) that associate with E2F in humans; however, extensive efforts have failed to find such homologs in Drosophila. We have developed a two-hybrid approach that allows transcription activators to be used as bait for interacting proteins. From a screen using Drosophila E2F (dE2F and dDP) as bait, we identified a novel gene, RBF. RBF combines several of the structural features of pRB, p107, and p130, suggesting that it may have evolved from a common ancestor to the three human genes. RBF associates with dE2F and dDP in vivo and is a stoichiometric component of E2F DNA-binding complexes. RBF specifically repressed E2F-dependent transcription and suppressed the phenotype generated by ectopic expression of dE2F and dDP in the developing Drosophila eye. RBF was phosphorylated by a cyclin E-associated kinase in vitro, and loss-of-function cyclin E mutations enhanced an RBF overexpression phenotype, consistent with the idea that the biological activity of RBF is negatively regulated by endogenous cyclin E. The properties of RBF suggest that it is the intermediary factor that was proposed to allow cyclin E induction of E2F activity. These findings indicate that RBF plays a critical role in the regulation of cell proliferation in Drosophila and show that analogous pathways regulate S-phase entry in a diverse range of species.

Quantitative analysis of proliferation and cell cycle length during development of the rat retina

The rat retina has been a useful model system for the study of the development of the central nervous system (CNS). In order to facilitate future studies on the mechanisms that control retinal growth, we have quantified the proliferation of retinal cells and the length of the cell cycle throughout development. For each day during development, the number of mitotic and postmitotic cells per retina, the proportion of cycling cells, S phase length, and cell cycle length were determined through quantification of cell numbers and 3H-thymidine labeling. As retinal development proceeds, the proportion of cycling cells decreases, and cell cycle length increases, in part due to an increase in S phase length.

Quantitative analysis of proliferation and cell cycle length during development of the rat retina

The rat retina has been a useful model system for the study of the development of the central nervous system (CNS). In order to facilitate future studies on the mechanisms that control retinal growth, we have quantified the proliferation of retinal cells and the length of the cell cycle throughout development. For each day during development, the number of mitotic and postmitotic cells per retina, the proportion of cycling cells, S phase length, and cell cycle length were determined through quantification of cell numbers and 3H-thymidine labeling. As retinal development proceeds, the proportion of cycling cells decreases, and cell cycle length increases, in part due to an increase in S phase length.

Drosophila Wee1 kinase rescues fission yeast from mitotic catastrophe and phosphorylates Drosophila Cdc2 in vitro

Cdc2 kinase activity is required for triggering entry into mitosis in all known eukaryotes. Elaborate mechanisms have evolved for regulating Cdc2 activity so that mitosis occurs in a timely manner, when preparations for its execution are complete. In Schizosaccharomyces pombe, Wee1 and a related Mik1 kinase are Cdc2-inhibitory kinases that are required for preventing premature activation of the mitotic program. To identify Cdc2-inhibitory kinases in Drosophila, we screened for cDNA clones that rescue S. pombe wee1- mik1- mutants from lethal mitotic catastrophe. One of the genes identified in this screen, Drosophila wee1 (Dwee1), encodes a new Wee1 homologue. Dwee1 kinase is closely related to human and Xenopus Wee1 homologues, and can inhibit Cdc2 activity by phosphorylating a critical tyrosine residue. Dwee1 mRNA is maternally provided to embryos, and is zygotically expressed during the postblastoderm divisions of embryogenesis. Expression remains high in the proliferating cells of the central nervous system well after cells in the rest of the embryo have ceased dividing. The loss of zygotically expressed Dwee1 does not lead to mitotic catastrophe during postblastoderm cycles 14 to 16. This result may indicate that maternally provided Dwee1 is sufficient for regulating Cdc2 during embryogenesis, or it may reflect the presence of a redundant Cdc2 inhibitory kinase, as in fission yeast.

Cell proliferation and DNA replication defects in a Drosophila MCM2 mutant

The yeast MCM2, MCM3, and MCM5/CDC46 genes are required for DNA replication and have been proposed to act as factors that license the DNA for one and only one round of replication per cell cycle. We have identified a Drosophila gene, DmMCM2, that is highly homologous to MCM2. A P-element insertion into this gene, which prevents its transcription, inhibits proliferation of cells in the imaginal discs and central nervous system (CNS) and causes an apparent prolongation of S phase in the embryonic and larval CNS. DmMCM2 is expressed in the embryo in a pattern corresponding to that of S-phase cells. These results suggest that DmMCM2 plays a role in the regulation of DNA replication analogous to that of its yeast counterpart.

Control of cell proliferation during plant development

Knowledge of the control of cell division in eukaryotes has increased tremendously in recent years. The isolation and characterization of the major players from a number of systems and the study of their interactions have led to a comprehensive understanding of how the different components of the cell cycle apparatus are brought together and assembled in a fine-tuned machinery. Many parts of this machine are highly conserved in organisms as evolutionary distant as yeast and animals. Some key regulators of cell division have also been identified in higher plants and have been shown to be functional homologues of the yeast or animal proteins. Although still in its early days, investigations into the regulation of these molecules have provided some clues on how cell division is coupled to plant development.

A novel mammalian protein, p55CDC, present in dividing cells is associated with protein kinase activity and has homology to the Saccharomyces cerevisiae cell division cycle proteins Cdc20 and Cdc4

A novel protein, p55CDC, has been identified in cycling mammalian cells. This transcript is readily detectable in all exponentially growing cell lines but disappears when cells are chemically induced to fall out of the cell cycle and differentiate. The p55CDC protein appears to be essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which exhibited a compensatory increase in p55CDC transcripts in the sense orientation. Immunoprecipitation of p55CDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune complex. The highest levels of protein kinase activity were seen with alpha-casein and myelin basic protein as substrates and demonstrated a pattern of activity distinct from that described for the known cyclin-dependent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven repeats homologous to the beta subunit of G proteins, and the highest degree of homology in these repeats was found with the Saccharomyces cerevisiae Cdc20 and Cdc4 proteins, which have been proposed to be involved in the formation of a functional bipolar mitotic spindle in yeast cells. The G beta repeat has been postulated to mediate protein-protein interactions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a component of the mammalian cell cycle mechanism.

A novel mammalian protein, p55CDC, present in dividing cells is associated with protein kinase activity and has homology to the Saccharomyces cerevisiae cell division cycle proteins Cdc20 and Cdc4

A novel protein, p55CDC, has been identified in cycling mammalian cells. This transcript is readily detectable in all exponentially growing cell lines but disappears when cells are chemically induced to fall out of the cell cycle and differentiate. The p55CDC protein appears to be essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which exhibited a compensatory increase in p55CDC transcripts in the sense orientation. Immunoprecipitation of p55CDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune complex. The highest levels of protein kinase activity were seen with alpha-casein and myelin basic protein as substrates and demonstrated a pattern of activity distinct from that described for the known cyclin-dependent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven repeats homologous to the beta subunit of G proteins, and the highest degree of homology in these repeats was found with the Saccharomyces cerevisiae Cdc20 and Cdc4 proteins, which have been proposed to be involved in the formation of a functional bipolar mitotic spindle in yeast cells. The G beta repeat has been postulated to mediate protein-protein interactions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a component of the mammalian cell cycle mechanism.

Nuclear concentration and mitotic dispersion of the essential cell cycle protein, p13suc1, examined in living cells

Stamen hair cells of Tradescantia virginiana have been microinjected with p13suc1 labeled with carboxyfluorescein (CF) and studied throughout the division cycle in living cells by using the confocal laser scanning microscope. The protein, p13suc1, is essential for the rapid inactivation of the key mitotic catalyst, p34cdc2 kinase, at anaphase and for completion of nuclear division. During interphase or prophase, CF-p13suc1 concentrates quickly (< 2 min) in nuclei, reaching levels that are approximately 2-fold greater than those in the cytoplasm. At nuclear envelope breakdown, CF-p13suc1 permeates throughout the entire spindle and nonspindle cytoplasm. The protein is excluded from the tightly condensed chromosomes but otherwise no regions accumulate or exclude the protein. It remains evenly distributed throughout metaphase, anaphase, and well into cytokinesis; however, during telophase CF-p13suc1 reconcentrates in the daughter nuclei.

Cell division in plants

The past couple of years have seen the isolation and characterization of many of the regulatory genes from plants that are thought to be intimately involved in regulation of the cell division cycle. In addition, characterization of plant-specific aspects of the cell division cycle has provided insight into how spatial and temporal controls may be linked. The comparative lack of cell mobility means that plant organs are historic records of the cell cycles that occurred during their evolution. Differentiated cells retain a capacity for re-entry into the cell cycle, which is probably an adaptation to compensate for the damage that they must tolerate because of a sedentary lifestyle. Understanding how plants cope with such damage and manage to generate such an array of diverse multicellular structures will require a basic comprehension of cell division.