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Lecture 12

BIO1140 Lecture 12: Signal Transduction + Transcription

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Caroline Petit- Turcotte

Transduction and Transcription -Must be able to integrate all topics into one nice story -way to go everything talked about -preparation for Midterm II The Ras (MAPK) Pathway -ligand and Tyr-K receptors Key Hallmarks -growth factors will bind to each monomer of the receptor and the receptors will dimerize -what does this do? --the dimer auto-phosphorylates; this is crucial and tract receptors are not active until this happens --they need this (phosphorylated tyrosines) to interact with various proteins to relay the message --RAS is the key receptor; but what does it lack? It does not have an SH2 domain—these are key, must have the specific shape in the protein to interact. To overcome this, we have bridge proteins. GRP2 has an SH2 domain, which comes with SOS. --when RAS is inactive, it binds with GDT, and binding to SOS reduces the affinity for GBT and increases it to GTP. It’s active and it can now do its job as a second messenger and activate downstream affecters (proteins that carry messengers) --the bridge proteins are still functional, can attract and bring more Ras, all with the same 2 ligands -RAS keeps activating downstream pathways, but what are we trying to achieve? TRANSCRIPTION FACTORS!! Proteins with the ability to interact with sequences of DNA initiate or to stop to transcription of specific genes. Genes that have those sequences under the control of these ligands. -Pathway activated by things like growth factors. What are the key proteins with genes under control of growth factors? Tubulin, to build microtubules. Cell to grow, microtubules to grow, tubulin is needed. Actin would be needed, what else? RNA (transcribe to mRNA and to proteins) -Tyr-K receptors lead or promote cellular survival; what proteins would you need? Anti- apoptotic proteins, i.e. BCL2 If the Ras pathway were to become dysregulated, what could be a potential outcome? Be sure to include the step that is affected in your proposal? If the Ras pathway were to become dysregulated, transcription will not properly occur. GBT will not be dysregulated, and GDP won't be prompted. - A few key elements go wrong and it’s not controllable, a few cells cannot control their life span. There are a few mechanisms to bring the cell back to a controlled state or simply get rid of it? Could trigger APOPTOSIS - The Ras pathway ensures that you do not produce too much bax and bad stays phosphorylated. Phospholipase C: - SH2 domain - Cleaves phospholipids - Phosphatidylinositol, cut the polar head group, release IP3, relive the backbone with fatty acid tails diacylglycerol, - IP3 (in mitochondria) is responsible for activating ligand-gated Ca+ channels on mitochondria and ER. - Once activated, won’t have RAS, and phosphorylation of downstream effectors. - but IP3 that will rise other concentrations and diacylglycerol - Good or bad? -could lead to apoptosis, and amplification -one cell releasing growth factor, picks it up, trigger events and calcium is released—good or bad? -Could be good for muscle cells. It is essential for several physiological events, i.e. muscle contractions, metabolism. The balance is important to understand what is going on inside the cells. IP3 is involved with G-proteins, rather than Tyr-K receptors. Dysregulation on one side can lead to too much calcium and then cell death. Issues in the membrane with diacylglycerol. Anything dependent on Ca and IP3 would be affected. Growth factor: NGF: Growth vs. death Growth vs. Death Another example of a growth factor; same pathway to activate path receptors. It lays down the link with apoptosis and cell survival. If the cell is deprived of growth factor, what happens? It won’t grow. No growth factors, does not dimerize, not autophosphorylates—the cell will then die. How? Too much calcium, and how will it die? Without apoptosis inhibitors being inhibited, BAD is no longer phosphorylated, and bad things will happen—apoptosis will occur. THIS IS A POSSIBLE TRIGGER OF APOPTOSIS. When that signal no longer happens, it serves as a signal that is not wanted. DRAW APOPTOTIC CASCADE, TYR-K RECEPTORS AND SEE HOW THEY HAPPEN TOGETHER. GPRCs G protein cascade receptors interact with several ligands -once they bind to their ligand, it introduces a change in confirmation and shifts domains that span the membranes and recruits heterotrimeric G proteins -heterotrimeric G proteins—alpha, beta and gamma -all have a role to play and are all important focus on alpha: to activate alpha, the receptor must have bound it’s ligand and change confirmation, recruits 3 proteins together (alpha, beta and gamma), interact with receptor, alpha exchanges GDP to a GTP - Similar to Ras - Once exchange, no longer needs beta and gamma, and dissociated from them - Activates an amplifier enzyme G-proteins are wide, will focus on 2 main ones CAMP and IP3 Pathway What determines the duration of activation of PKA? A- Binding affinity for PKA to cAMP B- Binding affinity of catalytic subunit of PKA for cAMP C- Binding affinity of regulatory subunit of PKA for cAMP D- Availability of ATP Answer: C Animation: - Alpha subunit, active GDP from GTP - Activates adenylate cyclase - When activated, enzyme is amplifier enzyme and as long as alpha-subunit is bound, it’s going to take ATP and concert to cyclic AMP - Level of amplification and availability of cAMP will activate the downstream, kinase A - Made of 2 regulatory and catalytic subunits - As cAMP binds to the regulatory subunits, it dissociates from the catalytic subunits, not catalytic units are active - When this happens the catalytic subunits can phosphorylate - Kinase A has a particularity compared to Ras Kinase A will phosphorylate like Ras and ADDITIONALLY can translocate to the nucleus, act as a transcription factor, bind to a protein in the nucleus, and recognise sequences on genes that are dependent on cyclic AMP --what if you had an alpha subunit that didn’t activate adenylate cyclase? No cyclic AMP production. What will happen in the cell? -can have ligands that bind to GPRCs, interact with inhibitory a-subunits than stimulatory a- subunits. -can have ligands that do this, activates the same way. When interacts with amplifier enzyme, instead of producing second messengers, you are shutting them down the amplifier messenger. This now now it prevents the cell from producing a second messenger. - it is a way to communicate to a cell, it tells it to shut down something. Why would you need to inhibit a message? — you’re a muscle or a liver cell: receive glucagon, activates a cascade of events through a GPRC, leads to activating enzymes that break glycogen to glucose and release glucose in the cell, will have enough at glucose at some point; 2 ways to stop it—remove glucagon from the receptor, cascade is no longer active, or can have something else inhibit the amplifier enzyme. Even if glucagon is bound, no longer have phosphorylation and releasing more glucose. Nice to have more than one things to put an end to things. Make connections with the previous topics Activating GPCR often leads to a release of Calcium in the cytoplasm. This will always result in initiating the intrinsic apoptotic pathway. A- True B- False Answer: A- True Calcium is essential, but it just needs to be controlled. This is why mitochondria and ER contribute so strongly to controlling the concentration Phosphatidylinositol IP3—animation on smart biology --same mechan
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