Signal Transduction and Cell Cycle Exam Notes

Signal Transduction

• Juxtacrine signaling requires physical contact between cells.
• Antagonists inhibit receptors by preventing ligand binding.
• When a G-protein-coupled receptor (GPCR) activates a G protein, the GDP bound to the α subunit is replaced by GTP.
• Adenylyl cyclase is NOT a second messenger.
• Cholera toxin targets Gs protein and inhibits its GTPase activity, locking it in the GTP-bound form.
• Protein kinase A (PKA) is activated when the regulatory subunits bind to cAMP, releasing the active catalytic subunits.
• Adrenaline binding to adrenergic receptors activates the Gs-signaling pathway, leading to the breakdown of muscle glycogen and increased intracellular glucose levels.
• Muscle cells treated with cholera toxin in the presence of adrenaline will have higher intracellular glucose levels compared to normal cells treated with adrenaline because cholera toxin inhibits the GTPase activity of Gs protein, leading to prolonged activation of adenylyl cyclase and increased cAMP production.
• Muscle cells overexpressing cAMP phosphodiesterase in the presence of adrenaline will have lower levels of intracellular glucose compared to normal cells treated with adrenaline because cAMP phosphodiesterase degrades cAMP, reducing PKA activation and glycogen breakdown.
• Muscle cells overexpressing a mutant form of PKA, where catalytic subunits cannot associate with regulatory subunits, will have higher levels of intracellular glucose compared to normal cells treated with adrenaline.
• A GPCR in the brain enhances learning and memory via the Gq pathway, activating CaM-kinase in neurons.
• Mice lacking IP3 receptors in neurons will exhibit similar learning and memory defects as CaM-kinase mutant mice.
• In thyroid cells, adrenergic receptor stimulation activates intracellular Gq proteins, resulting in thyroid hormone (thyroxin) secretion and thyroid cell proliferation.
• Thyroxin secretion is induced by DAG production.
• Cell proliferation is induced by increased intracellular Ca^{++}. It is not induced by DAG.
• The thyroxin secretion is mediated by PLC activation.
• IP3 receptors are associated with ER membranes.
• Mutant male cockroaches with a loss-of-function mutation in the RTKX gene (receptor tyrosine kinase) are oblivious to female pheromones.
• Mutant males with a loss-of-function mutation in Ras protein are also unable to respond to females.
• Additional mutations that increase the function of protein Z restore the ability of Ras mutant males to respond to females. Protein Z is likely to be Ras-GAP, as increasing its function would counteract the loss of function of Ras, thus restoring the signal.
• The PI3-kinase pathway mediates EGF-induced cell survival.
• PI3-kinase binds to phosphorylated tyrosine 134 on the EGF receptor.
• To block PI3-kinase pathway activation, perform an amino acid substitution on tyrosine 134 of EGFR, preferably to alanine.
• A mutation that causes RTK to spontaneously dimerize would be expected to promote uncontrolled cell proliferation leading to tumor formation.
• PI3-kinase is likely to contain a PH domain.
• PDK is a kinase that phosphorylates Akt.
• A mutant PI3-kinase protein that binds to active RTKs but has lost the kinase function is expected to have a dominant negative effect when over-expressed in normal cells because it will prevent endogenous PI3K from binding to the RTKs.
• Phosphoinositide B is the preferred substrate for Phospholipase C.
• Phosphoinositide D is the preferred substrate for PI3-kinase.
• Phosphoinositide E is the preferred substrate for PTEN.
• PTEN is a phosphatase.
• Growth factor Z binds to its receptor tyrosine kinase and activates Protein X and Protein Y in fibroblasts, resulting in cell proliferation. Protein X and Protein Y work sequentially.
• Following growth factor Z receptor activation, Protein X activates Protein Y to induce cell proliferation.
• Introducing a dominant negative mutant version of Protein X into cells that express a constitutively active Protein Y will cause the fibroblasts to proliferate even in the absence of growth factor Z.
• Introducing a mutant Z receptor that does not have the ligand-binding domain and over-expressing it in normal fibroblasts will prevent proliferation even when growth factor Z is added.

Cell Cycle

• Cyclins modulate the progression of cells by directly activating protein kinases (Cdks) that are critical regulators of cell division.
• Cyclin presence is required for Cdk activation.
• Levels of Cdk activity change during the cell cycle, in part because cyclin levels change during the cycle.
• Mutations in p53 and Rb are commonly found in cancers and are expected to be loss-of-function mutations in both genes.
• The Retinoblastoma (Rb) protein blocks cells from entering the cell cycle by inhibiting G1/S- and S-cyclin expression.
• Rb will be phosphorylated under Erk activation.
• The order of events following UV irradiation:
  • Activation of ATM/ATR
  • Activation of Chk1/Chk2
  • p53 accumulation
  • Expression of a Cdk inhibitor
• Gain-of-function mutations can result in an increase in cell proliferation if the mutant gene is G1-cyclin.
• In the cell cycle, a mitotic Cdk (M-cdk) is present during M phase.
• Cdk synthesis and degradation is NOT involved in regulating mitotic Cdk (M-cdk) activity.
• Mitotic cdk-cyclin activity is inhibited by Wee1.
• A mutation resulting in loss of function of Mad/Bub proteins would result in uneven distribution of chromosomes after cell division.
• DNA contents in terminally differentiated cells in G0 phase are 2n.
• The yeast cells that go through G1, S and G2 but fails to transit from G2 into the M-phase due to inactivation of cdc25.
• Securin is directly targeted by APC for degradation.
• Cells treated with nocodazole (prevents spindle formation) would arrest in metaphase and APC would not be activated.
• Cut1 corresponds to separase.
• Cut2 corresponds to securin.
• Cut4 corresponds to APC.
• Normal cells treated with growth factors will predominantly be in G1/S phase at 6 hours.
• Normal cells treated with growth factors and UV irradiation will predominantly be in G1 phase at 6 hours.
• Mutant cell line A (p53 mutation, cannot be phosphorylated by CK1/CK2) treated with growth factors and UV irradiation will predominantly be in G1/S phase at 6 hours.
• Mutant cell line B (cannot express p21) treated with growth factors and UV irradiation will predominantly be in G1/S phase at 6 hours.
• Drug X is an activator of PI3-kinase. PI3-kinase normally phosphorylates PIP2 to PIP3.
• Drug Y is an inhibitor of PTEN. PTEN normally dephosphorylates PIP3 to PIP2.