• In non-neoplastic cells, is controlled by proteins that intervene at checkpoints to prevent progression to next phase
• Enzymatic activity of protein is activated by phosphorylation, which changes their conformation, and usually leads to more phosphorylation (or kinase) activity
• Phosphorylation also creates docking sites on phosphorylated proteins, especially with tyrosine kinases, which recruits other target proteins
• Cell cycle: G1 to S (DNA synthesis) to G2 to M (mitosis)
• Cells NOT in the cell cycle (in resting phase) are in G0

G1
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1. pRb (retinoblastoma protein) binds to E2F (transcription factor), which blocks transcription of S phase genes; cells stimulated by extracellular signals cause accumulation of CDK 4-6/cyclin D1 complex, which phosphorylates Rb, reducing its affinity for E2F, causing pRb to dissociate, freeing E2F to activate S phase genes
2. Ubiquitin, in response to unknown stimulus, binds to and destroys CDK inhibitors, making CDK active
3. p16 gene (CDKN2) inhibits CDK4-6/cyclin D1 complex, which prevents phosphorylation of Rb, which prevents progression of cell cycle into S phase

S
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• Growth factor binds to receptor on outer membrane
• Receptor on inner membrane dimerizes, which activates tyrosine or serine/threonine kinases
• Dimerized receptor subunits phosphorylate each other on tyrosine residues, creating docking sites for other proteins
• Grb2, an adaptor molecule which transfers the activation state, docks to phosphotyrosine and attracts the Sos protein
• Sos, a nucleotide exchange factor, attracts Ras
• Ras binds to inner membrane by linking to an isoprenyl group (farnesyl): process is called prenylation
• Ras binds GTP and becomes activated
• Activated Ras is a kinase, which activates Raf-1 via GTP
• Somehow Raf-1 is translocated from cytosol to plasma membrane
• Ras is inactivated by GAP (GTPase activating protein), which increases Ras's intrinsic GTPase activity
• Activated Raf-1 is a kinase, which activates MEK
• MEK activates MAPK/ERK (mitogen activated protein kinase / extracellular signal regulated kinase) using ATP
• Activated MAPK/ERK directly activates:
  1. ERK1/2 (extracellular signal regulated protein kinases 1 and 2)
  2. BAD (bcl2 family) by phosphorylation
  3. Ribosomal S6 protein kinase (RSK = pp90rsk), which translocates to nucleus and phosphorylates several transcription factors, including jun
  4. Transcription factors (fos)
  5. (Alternate pathway) CREB kinase, which phosphorylates and activates CREB at serine 133, which activates intermediate early growth and some delayed response genes with CREB binding sites
• Activated MAPK also translocates to nucleus, where it phosphorylates and activates transcription factor ELK1 at Ser 383
• ELK1, with serum response factor, binds to serum response element within IEG promoter to activate IEG genes
• Activated fos and jun bind near myc gene, which initiates gene transcription
• Activated myc activates other genes, including cyclin D1
• Cyclin D1 may initiate progression of cells from S

G2
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• M phase promoting factor (MPF) is CDC2 protein kinase plus cyclin B
• MPF is inactive when Threonine 14 & Tyrosine 15 are phosphorylated (by Wee1 and Myt1 kinases)
• Plx1 extensively phosphorylates CDC25 at mitosis, which activates it
• Activated CDC25 dephosphorylates Thr14 & Tyr15 (antagonistic to Wee1 & Myt1), which activates CDC2

M
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• Cyclin levels increase
• Increased cyclin partially activates CDK (cyclin dependent kinase), which has bound inhibitors
• CDK triggers beginning of mitosis (prophase, metaphase)
• Ubiquitin is passed from enzymes E1 to E2 to E3 (bucket brigade)
• E2: active throughout cell cycle
• E3: temporal specificity; is on mitotic spindle, checks if all chromosomes are on spindle.
  • If so, ubiquitin tags an unknown "tether" protein for proteolysis, which causes anaphase to proceed
• Ubiquitin tags cyclin for proteolysis, which inactivates CDK, and allows mitosis to finish