Whi5

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Whi5 is a transcriptional regulator in the budding yeast cell cycle, notably in the G1 phase. [1] It is an inhibitor of SBF, which is involved in the transcription of G1-specific genes. Cln3 promotes the disassociation of Whi5 from SBF, and its disassociation results in the transcription of genes needed to enter S phase. [2]

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Roles in cell cycle progression

Start of the checkpoints in the cell cycle, which allows the cell to enter S phase from late G1, and has an all-or-nothing response to stimulus from the cell. The checkpoint allows the cell to either enter G0 or G1 phase and cell conditions must be sufficient to enter the cell cycle; for example, if the cell is starving, or if there is nutrient depletion, then it will halt progression in the cell cycle. However, if the start checkpoint is satisfied then the cell can begin DNA replication and the cell will halt growing. In the cascade of events that leads to the transcription of G1-specific genes, Whi5 is involved in the regulation of transcription. [2]

According to David Morgan, SCB-binding factor (SBF) and MCB-binding factor (MBF) are transcription factors that bind to SCBs and MCBs respectively. SCBs and MCBs are in promoter regions upstream of key genes expressing G1-specific proteins, which signal the transition from G1 to S phase. [1] The transcription factors are heterodimers, which contain a DNA-binding unit (Swi4 and Mbp1) and a regulatory sub-unit (Swi6). SCBs contain Swi4 and Swi6, while MCBs contain Mbp1 and Swi6. [2] Therefore, activation of SBF and MBF will result in the transcription of G1-specific genes.

Interactions with SBF

In a study done by Robertus de Bruin et al. (2004), researchers found that Whi5 is an important regulatory protein that binds to SBF. Therefore, G1-specific SCB-controlled genes are regulated upstream by Whi5, suppressing their transcription. [2] It is a stably-bound protein that binds to promoters via SBF in early G1 phase and, before transcriptional activation is cued, Whi5 dissociates from SBF. Thus, its activity supports the biological definition of Whi5 being an inhibitor of SBF-controlled genes. [2] Additionally, another study by Michael Costanzo et al. (2004) explains that SBF is needed to recruit Whi5 to the G1/S promoter because their interaction is stable. [3]

Whi5 regulation

According to David Morgan, Cln3/Cdk1, a cyclin-CDK complex unit, promotes the dissociation of Whi5 from SBF through inhibitory hyperphosphorylation. [1] Additionally, according to de Bruin, Cdc28 CDK, is believed to be involved in the phosphorylation of Whi5. [2] Cdc28 is activated by Cln 1, Cln2, and Cln 3, and is an important part of cell cycle progression. [3] Once activated, the association of Whi5 and its eventual dissociation from SBF results in activation of the transition to S phase. It is phosphorylated in many positions in G1, like the metazoan Retinoblastoma protein (Rb), but only certain phosphor-residues correlate with the transition from G1 to S phase. [2] Additionally, de Bruin explains that Whi5 phosphorylation determines the timing of SBF-dependent transcriptional activation and cell cycle progression. For example, in a cln3Δ and whi5Δ mutant, cells will enter S phase sooner, because the absence of whi5 bypasses the need for Cln3 activation. Therefore, in a cln3Δ and whi5Δ cell, the timing of cell cycle progression is not regulated by inhibitory phosphorylation by Cln3/Cdk1 and other cyclins, which results in smaller cell size. Thus, Cln3/Cdk1 is important for the dissociation of Whi5 and the timing of when it should dissociate. [2] Whi5 alone cannot determine the correct timing for cell cycle events, but it does affect the onset to begin the transition. [3]

According to Costanzo et al. (2004), Whi5 is believed to change its localization depending on CDK phosphorylation of Whi5. [3] Like transcription factors, it will localize to either the nucleus or outside of the nucleus. When CDK is active and it associates with Whi5, then Whi5 will dissociate from SBF, and it will exit the nucleus. However, when CDK is not present or active, then Whi5 will localize back into the nucleus. Whi5 is in the nucleus in late mitosis and G1 phase. Once the mitotic exit network is activated and CDK activity is reduced, Whi5 enters the nucleus. And, when Cln3 activates CDK, then it will cause the dissociation of Whi5 and its concomitant exit from the nucleus. [3]

Whi5 and Cln3 dilution

A study done by Kurt Schmoller et al. (2015) shows that with increasing concentration of Cln3, there is also increasing cell size. Therefore, the total concentration of Cln3 is constant until pre-Start G1 is reached. [4] Additionally, in the same respect, Whi5 amount does not increase or decrease, but with increasing cell size, total Whi5 concentration decreases. Thus, with Whi5 total concentration decreasing and Cln3 total concentration remaining constant, Whi5 dilution via cell growth results in the control of proliferation. Researchers found that in S/G2/M phases, Whi5 is synthesized in a size-dependent manner. When the daughter cell is born, the small cell has a high concentration of Whi5, which keeps the cell in pre-Start phase. As the cell size increases, the preliminary Whi5 amount will be diluted in the larger cytosol volume, and the constant Cln3 concentration will be greater than the concentration of the Whi5 inhibitor. Therefore, the concentration of Whi5 and Cln3 can explain why there are timing standards for when the cell will enter S phase. [4] Thus, the Whi5 inhibitor and its coordination with Cln3 are critical proteins that control cell size.

SBF-controlled genes

Once Whi5 is dissociated from SBF-controlled genes, it results in the transcription of major genes that allow the cell to enter S phase. These genes include G1/S and S cyclins, which are crucial for the onset of the next phase. [1] According to Vishwanath Iyer et al. (2001), SBF-controlled genes are important for budding and for membrane and cell-wall biosynthesis. Therefore, Whi5 is an important regulator for eventual cell cycle events. [5]

Related Research Articles

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Cyclin-dependent kinase complex

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Restriction point

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Cell cycle checkpoint

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Cyclin D

Cyclin D is a member of the cyclin protein family that is involved in regulating cell cycle progression. The synthesis of cyclin D is initiated during G1 and drives the G1/S phase transition. Cyclin D protein is anywhere from 155 to 477 amino acids in length.

Cyclin-dependent kinase 6

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Sic1, a protein, is a stoichiometric inhibitor of Cdk1-Clb complexes in the budding yeast Saccharomyces cerevisiae. Because B-type cyclin-Cdk1 complexes are the drivers of S-phase initiation, Sic1 prevents premature S-phase entry. Multisite phosphorylation of Sic1 is thought to time Sic1 ubiquitination and destruction, and by extension, the timing of S-phase entry.

A series of biochemical switches control transitions between and within the various phases of the cell cycle. The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases. The phases include the G1 and G2 phases, DNA replication or S phase, and the actual process of cell division, mitosis or M phase. During the M phase, the chromosomes separate and cytokinesis occurs.

The Start checkpoint is a major cell cycle checkpoint in yeast. The Start checkpoint ensures irreversible cell-cycle entry even if conditions later become unfavorable. The physiological factors that control passage through the Start checkpoint include external nutrient concentrations, presence of mating factor/ pheromone, forms of stress, and size control.

Cln1, Cln2, and Cln3 are cyclin proteins expressed in the G1-phase of the cell cycle of budding yeast. Like other cyclins, they function by binding and activating cyclin-dependent kinase. They are responsible for initiating entry into a new mitotic cell cycle at Start. As described below, Cln3 is the primary regulator of this process during normal yeast growth, with the other two G1 cyclins performing their function upon induction by Cln3. However, Cln1 and Cln2 are also directly regulated by pathways sensing extracellular conditions, including the mating pheremone pathway.

Clb5 and Clb6 are B-type, S-phase cyclins in yeast that assist in cell cycle regulation. Clb5 and Clb6 bind and activate Cdk1, and high levels of these cyclins are required for entering S-phase. S-phase cyclin binding to Cdk1 directly stimulates DNA replication as well as progression to the next phase of the cell cycle.

Mitotic Exit is an important transition point that signifies the end of mitosis and the onset of new G1 phase for a cell, and the cell needs to rely on specific control mechanisms to ensure that once it exits mitosis, it never returns to mitosis until it has gone through G1, S, and G2 phases and passed all the necessary checkpoints. Many factors including cyclins, cyclin-dependent kinases (CDKs), ubiquitin ligases, inhibitors of cyclin-dependent kinases, and reversible phosphorylations regulate mitotic exit to ensure that cell cycle events occur in correct order with fewest errors. The end of mitosis is characterized by spindle breakdown, shortened kinetochore microtubules, and pronounced outgrowth of astral (non-kinetochore) microtubules. For a normal eukaryotic cell, mitotic exit is irreversible.

WHI3 is a developmental regulator in budding yeast. It influences cell size and the cell cycle by binding CLN3 mRNA and inhibiting its translation. This, in turn, inhibits the G1/S transition.

BCK2, also named CTR7, is an early cell cycle regulator expressed by the yeast Saccharomyces cerevisiae. It was first discovered in a screen for genes whose overexpression would suppress the phenotypes of PKC1 pathway mutations. Though its mechanism is currently unknown, it is believed to interact with Swi4 and Mcm1, both important transcriptional regulators of early cell cycle.

References

  1. 1 2 3 4 Morgan, David (2007). The Cell Cycle: Principles of Control. Oxford: New Science Press.
  2. 1 2 3 4 5 6 7 8 Bruin, Robertus (2004). "Cln3 Activates G1-Specific Transcription via Phosphorylation of the SBF Bound Repressor Whi5". Cell. 117 (7): 887–898. doi: 10.1016/j.cell.2004.05.025 . PMID   15210110.
  3. 1 2 3 4 5 Costanzo, Michaeal; Nishikawa, Joy; Tang, Xiaojing; Millman, Jonathan; Schub, Oliver; Breitkreuz, Kevin; Dewar, Danielle; Rupes, Ivan; Brenda, Andrews; Mike, Tyers (June 25, 2004). "CDK Activity Antagonizes Whi5, and Inhibitor of G1/S Transcription in Yeast". Cell. 117 (7): 899–913. doi: 10.1016/j.cell.2004.05.024 . PMID   15210111.
  4. 1 2 Schmoller, KM; Turner, JJ; Koivomagi, M; Skotheim, JK (October 8, 2015). "Dilution of the cell cycle inhibitor Whi5 controls budding-yeast cell size". Nature. 526 (7572): 268–272. doi:10.1038/nature14908. PMC   4600446 . PMID   26390151.
  5. ↑ Iyer, V. R.; Horak, C. E.; Scafe, C. S.; Botstein, D.; Snyder, M.; Brown, P. O. (2001). "Genomic binding sites of the yeast cell-cycle transcription factors SBF and MBF". Nature. 409: 533–538. doi:10.1038/35054095.
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