Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
12.8 Regulation of the Cell Cycle by Protein Kinases
This chapter explains how eukaryotic cells control division through cyclin-dependent protein kinases (CDKs), cyclins, checkpoints, growth-factor signaling, DNA-damage responses, and targeted protein degradation. Accurate cell-cycle regulation is essential for development, tissue maintenance, and prevention of cancer.
Why Cell Cycle Control Is Important
• Regulation of the eukaryotic cell cycle is one of the most dramatic outcomes of signaling pathways.
• During embryonic growth and development, cells divide actively in many tissues.
• In adult organisms, many cells stop dividing and become quiescent.
• A cell’s decision to divide or not divide is critically important.
• Failure of division control causes unregulated proliferation, leading to cancer.
• Proper division requires a precise biochemical sequence so each daughter cell receives essential molecules.
• Studies across many eukaryotes show that core regulatory mechanisms are highly conserved.
The Four Main Stages of the Cell Cycle
• The eukaryotic cell cycle has four major stages.
• G1 phase
• S phase
• G2 phase
• M phase
G1 Phase
• G1 is the gap phase after mitosis and before DNA synthesis.
• RNA synthesis and protein synthesis continue.
• No DNA replication occurs.
• Cells evaluate nutrients, growth signals, and DNA status.
S Phase
• S means synthesis phase.
• DNA is replicated.
• The amount of cellular DNA doubles.
• RNA and proteins needed for replication are also synthesized.
G2 Phase
• G2 is the gap between DNA synthesis and mitosis.
• Additional proteins are synthesized.
• The cell approximately doubles in size.
• The cell prepares for chromosome separation and mitosis.
M Phase
• M phase is mitosis plus cytokinesis.
• The maternal nuclear envelope breaks down.
• Paired chromosomes move to opposite poles.
• New nuclear envelopes form around daughter chromosomes.
• Cytokinesis pinches the cell into two daughter cells.
Typical Timing in Animal Cells
• In laboratory animal cells, a full cycle commonly takes about 24 hours.
• Approximate example durations shown in the text:
• G1: 6–12 h
• S: 6–8 h
• G2: 3–4 h
• M: ~1 h
G0 Quiescent Phase
• After mitosis and entry into G1, a cell may continue cycling or stop dividing.
• Nondividing cells enter G0.
• G0 may last hours, days, or the lifetime of the cell.
• If stimulated again, some cells reenter through early G1.
Examples of Cells in G0
• Differentiated cells such as hepatocytes or adipocytes may remain in G0 while performing specialized functions.
• Stem cells retain the capacity to divide and differentiate into multiple cell types.
CDKs Are the Core Cell-Cycle Clock
• Cell-cycle timing is controlled by cyclin-dependent protein kinases (CDKs).
• CDKs phosphorylate specific proteins at precise times.
• These phosphorylation events coordinate orderly cell division.
Structure of CDKs
• CDKs are heterodimers composed of:
• A catalytic subunit = cyclin-dependent kinase
• A regulatory subunit = cyclin
• Without cyclin, the catalytic subunit is nearly inactive.
How Cyclin Activates CDK
• Cyclin binding opens the catalytic site.
• A residue essential for catalysis becomes properly positioned.
• Kinase activity may increase about 10,000-fold.
Numbers of Cyclins and CDKs
• Animal cells have at least 10 cyclins (A, B, etc.).
• They have at least 8 CDKs (CDK1 through CDK8).
• Different combinations act at specific cell-cycle stages.
Oscillation of CDK Activity
• In synchronously dividing cells, CDK activities rise and fall cyclically.
• These oscillations act as a master clock for the cell cycle.
• They help ensure one stage is completed before the next begins.
Four Major Ways CDKs Are Regulated
• Phosphorylation or dephosphorylation of the CDK
• Controlled degradation of cyclin
• Periodic synthesis of cyclins and/or CDKs
• Binding of specific CDK inhibitors
Activation by Phosphorylation
• Phosphorylation of a Thr residue in the T loop stabilizes an open active conformation.
• The T loop moves away from the substrate-binding cleft.
• Protein substrates can now bind.
Activation by Dephosphorylation
• Removal of phosphate from Tyr15 of CDK2 removes a negative charge that blocks ATP access.
• ATP can bind the active site.
• CDK activity rises further.
Self-Reinforcing Activation
• The phosphatase that removes the inhibitory phosphate is itself activated by phosphorylation from the CDK pathway.
• This positive feedback sharply increases CDK activation.
DNA Damage Arrest at G2
• A single-strand DNA break can arrest the cell cycle in G2.
• ATM and ATR proteins initiate the checkpoint response.
• They inactivate the phosphatase that removes inhibitory phosphate from the CDK.
• The CDK remains inactive.
• The cell cannot divide until DNA is repaired.
Cyclin Destruction Controls Timing
• Precisely timed proteolysis of mitotic cyclins regulates CDK activity.
• Cyclin accumulation first activates the cyclin-CDK complex.
• Later, the same active complex helps trigger cyclin destruction.
Destruction Box and Ubiquitin
• Cyclins contain a destruction box sequence.
• Destruction box recognizing protein (DBRP) and ubiquitin ligase attach ubiquitin molecules to cyclin.
• Polyubiquitinated cyclin is targeted to proteasomes.
• Proteasomes degrade cyclin.
• Loss of cyclin inactivates the CDK.
Proteasome Importance
• Ubiquitin-proteasome systems regulate cyclins.
• They also carry out general cellular protein turnover.
Growth Factors Promote Cell Division
• Extracellular growth factors and cytokines stimulate cell division.
• They activate signaling cascades such as MAPK pathways.
• Jun and Fos transcription factors are phosphorylated and activated in the nucleus.
Role of E2F
• Jun and Fos help induce expression of cyclins, CDKs, and transcription factor E2F.
• E2F promotes production of enzymes required for deoxynucleotide and DNA synthesis.
• This helps drive passage from G1 into S phase.
Specific CDK Inhibitors
• Dedicated inhibitory proteins can bind and inactivate specific CDKs.
• One major example is p21.
Cell Fate Outcomes Controlled by CDKs
• Depending on signals, CDK networks help determine whether a cell will:
• Divide
• Differentiate
• Become permanently quiescent
• Reenter the cycle after quiescence
How CDKs Control Cell Events
• CDKs regulate division by phosphorylating critical target proteins.
• Many targets are known, and many more remain to be fully understood.
Target 1: Lamin and Nuclear Envelope Breakdown
• Nuclear envelope structure depends partly on lamin intermediate filaments.
• Before chromosome segregation, CDKs phosphorylate lamin.
• Lamin filaments depolymerize.
• The nuclear envelope breaks down during mitosis.
Target 2: Myosin and Cytokinesis Machinery
• Actin-myosin contractile machinery pinches a dividing cell into two during cytokinesis.
• After division, a CDK phosphorylates a regulatory myosin subunit.
• Myosin dissociates from actin.
• The contractile machinery is inactivated.
• Later dephosphorylation allows reassembly for the next cycle.
Target 3: Retinoblastoma Protein (pRb)
• pRb is a crucial CDK substrate involved in G1 checkpoint control.
• It was named after the retinal tumor cell line where it was discovered.
• pRb helps regulate division in many cell types.
Unphosphorylated pRb Blocks S Phase
• Unphosphorylated pRb binds transcription factor E2F.
• Bound E2F cannot activate genes required for DNA synthesis.
• These genes include DNA polymerase, ribonucleotide reductase, and others.
• Therefore the cell cannot pass from G1 to S phase.
Phosphorylated pRb Releases E2F
• Cyclin E–CDK2 phosphorylates pRb when division should proceed.
• Phosphorylated pRb releases E2F.
• E2F activates DNA-synthesis genes.
• The cell commits to S phase and later mitosis.
DNA Damage Checkpoint in G1
• Double-strand DNA breaks are detected by MRN protein complexes.
• ATM and ATR are activated.
• They phosphorylate and activate p53.
p53 and p21 Pathway
• Activated p53 acts as a transcription factor.
• It stimulates synthesis of p21.
• p21 inhibits cyclin E–CDK2.
• pRb remains unphosphorylated.
• pRb continues binding E2F.
• The cell cycle arrests in G1.
Why G1 Arrest Matters
• G1 arrest gives the cell time to repair DNA before replication.
• This prevents transfer of defective genomes to daughter cells.
If Damage Is Too Severe
• If DNA cannot be effectively repaired, the same machinery can trigger apoptosis.
• Apoptosis removes the dangerous cell and helps prevent cancer.
Universal Importance
• Exact numbers of cyclins and CDKs vary among species.
• However, the basic cell-cycle control logic is conserved throughout eukaryotes.
In a Nutshell
The eukaryotic cell cycle is controlled by cyclin-dependent protein kinases whose activities oscillate through cyclin binding, phosphorylation, inhibitor proteins, and cyclin destruction. CDKs drive orderly progression through G1, S, G2, and M phases by phosphorylating key proteins such as lamin, myosin regulators, and pRb. DNA-damage checkpoints involving ATM, ATR, p53, and p21 can halt the cycle for repair or trigger apoptosis, preventing uncontrolled growth and cancer.