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Facteurs moléculaires affectant la division cellulaire
Plusieurs facteurs externes et internes influencent l'initiation et l'inhibition de la division cellulaire. Par exemple, la mort des cellules environn…
La division cellulaire, ou phase mitotique du cycle cellulaire, est contrôlée par des stimuli chimiques externes, tels que les facteurs de croissance et les hormones, et des facteurs internes, tels que les régulateurs du cycle cellulaire.
Le cycle cellulaire est régulé par une combinaison de protéines appelées cyclines et kinases dépendantes des cyclines ou Cdk. Les concentrations de différentes cyclines fluctuent avec les différentes phases du cycle cellulaire.
Lorsqu’une cellule approche de la fin de la phase G2, le niveau de cycline B augmente. À des concentrations élevées, la cycline B forme un complexe avec Cdk1 appelé facteur de promotion de la phase M ou MPF.
L’activité MPF est régulée par des événements de phosphorylation et de déphosphorylation qui déterminent si la cellule subit une division ou non.
Jusqu’à la fin de G2, la kinase Wee1 phosphoryle les résidus de tyrosine sur le MPF et le maintient inactif. Cela empêche l’entrée dans le cycle cellulaire et arrête la cellule à la phase G2/M.
Une fois que Cdc25 déphosphoryle le MPF, une kinase activatrice de Cdk phosphoryle une thréonine conservée sur le MPF et l’active.
Maintenant, les cellules subissent une mitose et produisent deux cellules filles.
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Q1: What are cyclins and cyclin-dependent kinases and how do they control cell division?
Cyclins and cyclin-dependent kinases (Cdks) are regulatory proteins that control cell division. While Cdk levels remain constant, cyclin concentrations fluctuate during different cell cycle phases. When cyclins bind to Cdks, they become phosphorylated and activated, driving the cell through specific phases. This dynamic regulation ensures orderly progression through the cell cycle.
Q2: How does the M-phase promoting factor activate to trigger mitosis?
The M-phase promoting factor (MPF) forms when cyclin B binds to Cdk1 as the cell approaches the M phase. Wee1 kinase initially keeps MPF inactive by phosphorylating tyrosine residues. Cdc25 phosphatase then dephosphorylates MPF, and a Cdk-activating kinase phosphorylates a threonine residue, fully activating MPF and triggering mitosis.
Q3: What external factors promote or inhibit cell division?
External factors significantly influence cell division initiation. Growth factors and human growth hormone (hGH) promote cell division, while their absence inhibits it. Cell crowding also suppresses division, and the death of nearby cells can stimulate division. These external chemical stimuli work alongside internal regulators to control when cells divide.
Q4: How do negative regulatory proteins like p53 and p21 halt the cell cycle?
Negative regulators including p53 and p21 halt cell cycle progression. When p53 detects DNA damage, it recruits repair enzymes and triggers apoptosis if damage persists. Elevated p53 levels stimulate p21 synthesis, which binds to Cdk/cyclin complexes and blocks progression to the S phase, preventing damaged cells from dividing.
Q5: When do different cyclin levels peak during the cell cycle?
Cyclin concentrations vary predictably across cell cycle phases. Cyclin D rises in G1 and remains high through most phases. Cyclin E peaks at the G1-S junction, while cyclin A is elevated during S and G2 phases. Cyclin B levels rise as the cell approaches M phase, promoting the transition to mitosis.
Q6: What is the role of the retinoblastoma protein in cell cycle regulation?
The retinoblastoma protein (Rb) functions as a negative regulator that halts the cell cycle. It prevents cells from progressing through critical checkpoints until appropriate conditions are met. By blocking cycle progression, Rb ensures that cells complete each phase properly before advancing, maintaining genomic stability.
Q7: How do phosphorylation and dephosphorylation events control MPF activity?
Phosphorylation and dephosphorylation events precisely regulate MPF activity. Wee1 kinase phosphorylates tyrosine residues on MPF to keep it inactive and prevent G2/M transition. Cdc25 phosphatase reverses this by dephosphorylating MPF, allowing a Cdk-activating kinase to phosphorylate threonine and fully activate MPF for mitosis.