Regulators of the cell cycle

The C. elegans genome encodes multiple members of the cyclin-dependent kinase (CDK) family. At least two CDKs, CDK-1 and CDK-4, are essential for cell-cycle progression (Boxem et al., 1999; Boxem and van den Heuvel, 2001; Park and Krause, 1999). These CDKs act at distinct times in the cell cycle and use specific cyclin partners, similar to their mammalian orthologs (Table 1).
CDK-1, previously known as NCC-1 for nematode cell cycle, was identified based on its close similarity to the prototypical yeast CDK (Mori et al., 1994). In contrast to yeast, but similar to mammalian Cdk1, cdk-1/(ncc-1) is specifically required for G2/M progression and not for G1 or S phase (Boxem et al., 1999). Maternal cdk-1 product suffices for embryogenesis, and candidate null mutant animals arrest cell division during L1 development. Several observations indicate that the post-embryonic precursor cells in these mutants arrest in G2 phase: such cells show normal expression of the rnr::GFP reporter and BrdU incorporation during S phase, but fail to proceed into mitosis (as indicated by absence of chromosome condensation and nuclear envelope breakdown). Moreover, endoreduplication cycles, which skip M phase, continue in cdk-1 mutants and intestinal nuclei accumulate the normal 32n DNA content. Following RNAi of cdk-1 in adult hermaphrodites, oocytes show delayed meiotic maturation, form an eggshell upon fertilization, but neither align nor segregate homologous chromosomes. Thus, cdk-1 is required for meiotic as well as mitotic M phase.
CDK-1 likely acts with mitotic cyclins of the A and B subfamilies (Kreutzer et al., 1995). A single full-length cyclin A gene (cya-1) is expressed in C. elegans, as well as three typical B-type cyclins (cyb-1, cyb-2.1 and cyb-2.2), and a distinct member of the cyclin B3 subfamily (cyb-3). While cyb-1 and cyb-3 each are individually required during embryonic development, simultaneous inactivation of these mitotic cyclins causes a more severe phenotype: cyb-1;cyb-3(RNAi) embryos arrest at the one-cell stage and resemble cdk-1(RNAi) embryos (our unpublished results). These data support the notion that different mitotic cyclins have functions that are partly distinct and partly overlapping.
The cdk-4 Cdk4/6 kinase and cyd-1 D-type cyclin genes are required for progression through G1 phase during larval development (Boxem and van den Heuvel, 2001; Park and Krause, 1999). CYD-1 and CDK-4 likely act in complex, as indicated by their direct interaction in vitro and close similarity in null phenotypes (Park and Krause, 1999). In contrast to larval divisions, only a few very late embryonic divisions depend on cyd-1/cdk-4 activity (Boxem and van den Heuvel, 2001; Yanowitz and Fire, 2005). Possible explanations for this difference include that the early embryonic divisions lack a G1 phase and therefore will not need a G1 cyclin or CDK. Also, as one of its most important functions, cyd-1 and cdk-4 act to antagonize the transcriptional repressor lin-35 Rb (see below; (Boxem and van den Heuvel, 2001). In the absence of cyd-1/cdk-4 function, lin-35 Rb may inappropriately repress cell-cycle genes, but this cannot prevent divisions that are driven by maternal products. Also, cyd-1 and cdk-4 could primarily promote growth, as in Drosophila (Datar et al., 2000; Meyer et al., 2000), which is not incorporated in the embryonic divisions. However, larval divisions arrest in G1 while cells continue to grow in the mutants, and growth retardation occurs later (Boxem and van den Heuvel, 2001). Therefore, absence of G1 phases and maternal contribution of DNA replication components likely explain the limited requirement for cyd-1/cdk-4 during embryogenesis
Taken together, CDK-1 and CDK-4 act in G2/M and G1, respectively, like their mammalian orthologs Cdk-1 and Cdk4/6. It is currently not clear whether C. elegans also uses a Cdk2 ortholog, which acts subsequent to Cdk4/6 in mammals to promote G1/S and S phase progression. The best candidate is K03E5.3, which shares 43% amino-acid identity with human Cdk2 (Boxem et al., 1999). Inhibition of this gene by RNAi resulted in a variable phenotype, with animals arresting during embryogenesis, during early or late larval development, and as sterile adults.
In other metazoans, Cdk2 acts with Cyclin E to promote S phase entry. C. elegans cye-1 Cyclin E deletion animals show surprisingly normal development until the L3 stage, at which time the VPC divisions proceed slowly and incompletely (Fay and Han, 2000). This modest phenotype apparently depends on long lasting maternal function, as RNAi results in embryonic lethality at approximately the hundred-cell stage (Brodigan et al., 2003; Fay and Han, 2000).
Several other members of the Cdk superfamily are present in C. elegans, including a Cdk7/Mo15 ortholog (Boxem et al., 1999; Liu and Kipreos, 2000). Cdk7 was identified as CDK-activating kinase (CAK) in mammalian cells, but also as part of the TFIIH transcription factor, responsible for phosphorylating the C-terminal domain (CTD) of RNA polymerase II (Fisher and Morgan, 1994; Shiekhattar et al., 1995). The combination of a temperature-sensitive mutation and RNAi of cdk-7 resulted in one-cell arrest similar to cdk-1(RNAi) embryos (Wallenfang and Seydoux, 2002). In addition, partial inactivation of cdk-7 interfered with transcription and phosphorylation of the RNA polymerase CTD. These data support dual activities of CDK-7 as both CDK-activating kinase (CAK) and CTD kinase in vivo.
Several other CDKs are likely to act independent of the cell cycle. CDK-5 is remarkably close to human Cdk5, sharing 74% identity at the amino-acid level, which has neuronal functions (Cruz and Tsai, 2004). Two other CDKs, CDK-8 and CDK-9, likely are involved specifically in regulating transcription (Liu and Kipreos, 2000; Shim et al., 2002).

Comments

Post a Comment

Popular posts from this blog

Cell Cycle Regulation

Image
In the article on cell cycle checkpoints, we looked at the  why  of cell cycle transitions: the factors that a cell considers when deciding whether or not to move forward through the cell cycle. These include both external cues (like molecular signals) and internal cues (like DNA damage). Cues like these act by changing the activity of core cell cycle regulators inside the cell. These core cell cycle regulators can cause key events, such as DNA replication or chromosome separation, to take place. They also make sure that cell cycle events they take place in the right order and that one phase (such as G _1 ​ 1 ​ ​ start subscript, 1, end subscript ) triggers the onset of the next phase (such as S). In this article, we'll look at a few of the most important core cell cycle regulators: proteins called cyclins, enzymes called Cdks, and an enzyme complex called the APC/C. Cyclins Cyclins  are among the most important core cell cycle regulators. Cyclins ...
Read more

The anaphase-promoting complex

Image
In addition to driving the events of M phase, MPF also triggers its own destruction by activating the  anaphase-promoting complex/cyclosome  ( APC/C ), a protein complex that causes M cyclins to be destroyed starting in anaphase. The destruction of M cyclins pushes the cell out of mitosis, allowing the new daughter cells to enter G _1 ​ 1 ​ ​ start subscript, 1, end subscript . The APC/C also causes destruction of the proteins that hold the sister chromatids together, allowing them to separate in anaphase and move to opposite poles of the cell. How does the APC/C do its job? Like a Cdk, the APC/C is an enzyme, but it has different type of function than a Cdk. Rather than attaching a phosphate group to its targets, it adds a small protein tag called  ubiquitin  ( Ub ). When a target is tagged with ubiquitin, it is sent to the  proteasome , which can be thought of as the recycle bin of the cell, and destroyed. For example, the APC/C attaches a ubiquitin tag...
Read more

Checkpoints and regulators

Image
Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions, but they aren’t always in the driver’s seat. Instead, they respond to cues from inside and outside the cell. These cues influence activity of the core regulators to determine whether the cell moves forward in the cell cycle. Positive cues, like growth factors, typically increase activity of Cdks and cyclins, while negative ones, like DNA damage, typically decrease or block activity. As an example, let's examine how DNA damage halts the cell cycle in G _1 ​ 1 ​ ​ start subscript, 1, end subscript . DNA damage can, and will, happen in many cells of the body during a person’s lifetime (for example, due to UV rays from the sun). Cells must be able to deal with this damage, fixing it if possible and preventing cell division if not. Key to the DNA damage response is a protein called p53, a famous tumor suppressor often described as “the guardian of the genome.”  ^{10} ​ 1 0 ​ ​ start superscript, 10, end...
Read more