Exam #2 Processes ONLY Flashcards
(36 cards)
What are the steps of the structural cap model of the dynamic instability of microtubules?
- a,b- Dimers (GTP form) are added to the + end (forms an open sheet)
- Tube closes spontaneously forcing GTP hydrolysis.
- Tubulin GDP do not favour the straight shape—> mechanical strain —> instability
- Catastrophic shrinkage.
How are intermediate filaments assembled?
- Start off with monomer ( helical regions with globular domains on the end)
- Parallel association of monomers (globular domains match —> dimer
- Antiparallel association of the dimers—> tetramer
- 8 tetramers —> unit lenght
- End-end association forms long IFs
- IFs undergo dynamic remodelling through intercalation of new units into filaments.
How are Actin Microfilaments assembled?
- Starting seed filament forms from monomer nucleation.
- At high {G-actin} monomers are added to both ends
- As [G-actin] drops, monomers are added to the + end.
- Elongation occurs at the + end, and there is shortening at the - end. (monomers move down the growing filament)
- Once [G-actin] stabilizes—> dynamic equilibrium —> no net gain or loss.
How does contraction occur through cross-bridge cycling?
- ATP binds to myosin head which causes it to detach from the actin filament
- Hydrolysis of ATP causes the head to bind weakly to the filament
- Release of Pi causes tighter attachment and power stroke—> contraction
- Release of ADP sets the stage anew.
How does Ca2+ regulate skeletal muscle contractions?
- Signals from the neuromusclar junction trigger release of Ca2+ from smooth ER stores
- At low Ca2+, tropomyosin prevents myosin from interacting with actin
- At high Ca2+, tropomyosin moves allowing cross bridging to occur.
How is smooth muscle contraction regulated?
- Not by Ca2+, but by phosphorylation of the light chains
- Phosphorylation of the light chains moves the myosin head in contact with actin —> cross bridge cycling.
What are the steps of signalling pathways?
1) Cell releases 1st messenger
2) Receptor on responding cell reconizes and binds with the messenger molecule
3) Interaction between ligand and receptor—> signal relayed into cytoplasmic domain
4) One type of receptor will activate effector (an enzyme)
5) produces a second messenger
OR
4a) Another type of receptor transmits a signal by making its cytoplasmic domain a recruiting station for cellular signalling protein.
6) Activation of protein at the top of the pathway
7) Distinct proteins act in sequence by changing confomation of subsequen proteins in the pahway
8) Signals reach target proteins
9) response
How do kinases and phosphatases work in signal transduction?
1) PK1 activates PK2 through phosphorylation
2) PK2 activates PK3
3) PK3 phosphorylates TF—> increased affinity of DNA—> activation of gene expression
What is is the mechanism of GCPR?
1) Ligand binds to receptor—> conformation changes—> better affinity for trimeric G protein—> G protein binds to G protein-receptor complex
2) Alpha subunit of trimeric G protein exchanges its GDP for GTP —> conformational change—> Alpha subunit has low affinity for beta and gamma subunits
3) GTP-alpha suunit dissociates from the rest of the complex and binds to effector (AC)
4) Activated AC produces cAMP (2nd messenger)—> 2nd messenger transmits the signal.
5) GTPase activity of alpha subunit hydrolyzes bound GTP —> deactivates itself.
6) Alpha subunit of G protein dissociates from effector —> reforms trimeric G protein
TERMINATION
7) The receptor phosphorylated by GRK
8) Phosphorylated receptor is bound by Arrestin—> inhibits activation of addition G-proteins.
How does the mobilizaiton of glucose via cAMP work?
1) Hormone (epinephrine or glucagon) binds to receptor—> activates alpha subunit of trimeric G protein
2) Activates effector (AC) to produce cAMP—> cAMP diffuse into cytoplasm
3) cAMP binds to PKA—> activates it
4) Phosphorylation of glycogen synthase —> inactivation—> no conversion of glucose to glycogen
5&6) Phosphorylation of phosphorylase kinase —> activation —> transfer phosphate groups to glycogen phosphorylase molecules.
7) Glycogen breaks down
8) Glucose released into blood stream.
9) Active PKA can also translocate to the nucleus —> phosphorylate CREB
10) phosphorylated CREB binds as a dimer to CRE —> increase transcription of genes (ie. gluconeogenesis)
How is phosphatidylinositol (PI) converted into 2nd messengers?
1&2) phosphate groups added by lipid kinases to PI to form PIP and then PIP2
3) Ligand binding activates heterotrimeric G protein
4) Alpha subunit activates PI-specific phospholipase C beta
5) This splits PIP2 into DAG and diffusive IP3
6) DAG recruit PKC and activates it
7) IP3 diffuses into cytosol
8) IP3 binds to IP3 receptor and Ca2+ channel in membrane of SER —> releases Ca2+ into the cytosol.
9) Binding of IP3 to its receptor causes release of Ca2_ into the cytosol —> various effects
How is Calcium transported (released) in the cell?
1) Voltage-gated channel on PM opens —> entry of small amount of Ca2+
2) Ca2+ binds to RyRs in ER
3) Release of Ca2+ (Calcium induced calcium release, CICR)—> triggers contraction
4) Low [Ca2+] in ER restored by SERCA pump on ER and by secondary active transport via Na+/Ca2+ exchanger
How are intracellular calcium stores in SER replenished (SOCE)?
1) When Ca2+ depleted in SER—> signalling between membranes —> 2 proteins cluster —> Orai1 opens and Ca2+ enters cytosol —> Ca2+ can be pumped into ER to replenish stores.
How are RTK’s activated (ligand mediated activation)?
Bivalent ligand binds –> dimerization of receptor —> active dimer
How are RTK’s activated (receptor mediated activation)?
Binding of monovalent ligand to each monomer—> conformational change —> dimerization interface—> active dimer.
What happens after RTK’s for active dimers?
1) Dimerization brings 2 kinase domains close to each other
2) kinase on one receptor phosphorylates Tyr residues in the cytoplasmic domain of the other and vice versa (rans auto phosphorylation)
3) New phosphoTyr resideues becoming binding sites for target proteins with SH2 or PTB domains
How do neutrophils invade endothelial cells during inflammation?
1) chemical signals from damaged tissues activate endothelial cells
2) Selectins are displayed on activated cells —> more adhesive to circulating neutrophils (neutrophils roll along the wall of the vessel)
3) Integrins on neutrophils become activated
4) Integrins on neutrophils bind to IgSF (ICAMs) on the entothelial cells —-> neutrophils become firmly adhered
5) Bound neutrophils change shape and squeeze between adjacent endothelial cells —> immune response.
How does the Ras-MAP kinase pathway work?
1) Growth factor binds to its RTK —> activation
2) Autophosphorylation of receptor dimer
3) Receptor binds Grb2 —> binds Sos (a GEF for RasGTP)
4) Sos catalyzes GDP for GTP of Ras—> Ras binds Raf
5) Raf recruited to the membrane from the cytoplasm where it is phosphorylated —> activated
6) Raf phosphorylates MEK
7) MEK can add phosphate to both Tyr and Ser/Thr (on MAPK there is Thr-X-Tyr) —> MEK phsophorylates both residueces —> activates ERK through phosphorylation.
8) Activated ERK goes to the nucleus —> phosphorylates transcripion Factors (ex. Elk1)
9_ Phosphorylation increases affinity to regulatory regions on DNA —> transcription of genes such as cyclin D1 —> cell growth
10) MAPK phosphatase MKP-1 ranscription is induced
11) MAPK can dephosphorylate activated ERK shutting down the pathway.
How does IR signalling work?
Binding of insulin –> conformational change —> brings TK domains close —> autophosphorylation —> activation.
How does the PI3 Kinase pathway work?
1) Activation of PI3K —> PIP3
2) PKB interacts with PIP3 at plasma membrane through its PH domain.
3) PKB changes conformation —> is now a substrate for PDK1 which phosphorylate PKB
4) Another kinase adds phosphate at 2nd site —> PKB activated
5) Activated PKB dissociates from PM and moves to the cytosol and nucleus.
6) PKB mediates insulin response, lead to transporters of glycose transporters to PM and glycogen synthesis.
How doe CDK-cyclins regulate the cell cycle in yeast?
G1—> S
CDC2 is activated by G1/S cyclins —> synthesis
G2 —> M
CDC2 is activated by mitosis cyclins —> mitosis
How does CDK phosphorylation by Wee1 work in yeast (phosphorylation of CDK)?
- cdc2 interacts with mitotic cyclin in G2
- CAK phosphorylates Thr on cdc2—> activation
- Wee1 phosphorylates Tyr on cdc2 —> inactivation
- After cell grows to a certain extent cdc25 phosphotase removes phosphate on Tyr —> activation
- Mitosis occurs.
How does controlled proteolysis of Cdk’s work?
- SCF & APC (ubiquitin ligases) recognize proteins to be deraded —> ligate polyUbiquitin chains to the protein —> protesosome —> degradation
How does the spindle checkpoint work?
Occurs during metaphase-anaphase transition.
Triggered when one or more chromosomes fail to align at metaphase plate.
1) Mad2 inds to unattached kinetochores —> sends “wait” signal
2) Mad2 interacts with Cdc20 (prevents APC breaking down securin)
3) Delays anaphase.
