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Cell signalling
Uni of Notts, Genes, Molecules and Cells, first year
| Term | Definition |
|---|---|
| Cell signalling | The ability of cells to intercommunicate. Signals are transmitted using either phosphorylation or GTP binding proteins. Each step is a control point |
| Purpose of cell signalling | Integration of information from neighbouring & distant cells to respond to changes in the environment & control growth (homeostasis) |
| Quorum sensing | Bacterial ability to communicate with other cells. Each bacterium secretes a small signalling molecule & when the concentration rises enough bacteria will respond by changing transcription so certain characteristics are expressed/dampened |
| Contact dependent signalling | Cells must be physically touching. 1 cell expresses a target molecule on its membrane which it interacts with a receptor on an adjacent cell (e.g., helper T-cells) |
| Paracrine signalling | Cells secrete molecules into the nearby environment. This only affects local cells since the signal is either rapidly uptaken or quickly degraded (e.g., histamine response) |
| Autocrine signalling | Communication within cells when they produce ligands that bind to receptors on their own cell surface (e.g., cytokines produced by helper T-cells) |
| Synaptic transmission | Very fast, extremely specific (under normal conditions) paracrine (but sometimes autocrine) signalling between neurones |
| Endocrine signalling | Long-distance communication that's slow to reach the target but slow to degrade. Mediated by hormones released in the bloodstream by effector organs to cells all over the body |
| Signal transduction | The process of converting a signal into a response. This is caused by the binding of a signal molecule to a receptor protein causing signal proteins to activate metabolic, gene regulation, or cytoskeletal proteins to cause a response |
| Use of GTP to mediate cellular timeframes | GTP isn't hydrolysed immediately & when it's bound to a signalling protein it serves as an "on" switch until it's hydrolysed. Once the signal has caused an effect, the cell can hydrolyse the GTP to degrade the signal (disregulation can lead to cancer) |
| Fast vs slow signalling responses | Signals eventually alter cell behaviour by changing proteins in the cell at different speeds. A fast response directly alters the function of a protein whereas a slow response would be altering DNA transcription which would have a larger impact |
| Integration of signal pathways (cross-talk) | Many signal pathways simultaneously occur in cells. Some act on the same proteins or upregulate receptors for others (amplification) or degrade the products of a pathway to dampen the signal (attenuation) |
| Hydrophilic first messengers & receptors | Hydrophilic proteins or smaller peptides bind to hydrophilic ligands on the cell surface to cause a conformational shape change in their intracellular domain |
| Hydrophobic first messengers & receptors | Hydrophobic co-enzymes (such as vitamins ADEK) can dissolve through the membrane simply & reach intracellular targets creating a shorter pathway. They must be transported to the cell by carriers such as human serum albumin (fatty acid carrier) |
| How effects of ligands differ in the body | The same ligand can cause different outcomes depending on where it is in the body. This demonstrates different downstream transduction components (even if same receptor) from different genes coming from different evolutionary pathways |
| How GPCRs help transmit signals | Binding to 1 domain either opens an ion channel to affect certain processes with changes in conformation or activates a heterotrimeric G-protein to dissociate & activate a 2nd messenger enzyme |
| How enzyme-linked receptors become catalytically active | Dimer ligand could bind to 2 extracellular domain to make the intracellular space catalytic or a monomer could bind to 2 domains simultaneously to recruit & activate a catalytic enzyme without being catalytic themselves |
| How nuclear receptors bind hormones | A hydrophobic ligand binds to an intracellular protein, causing the corepressor to drop off the ligand binding domain which binds to the ligand binding domain |
| How receptors are activated & work | A coactivator protein binds around the ligand binding domain causing a conformational change in the DNA-binding domain upstream of the affected sequence |
| G-protein regulatory proteins | GTPase Activate Proteins (GAPs) speeds the internal clock of G-proteins by increasing hydrolysis. Guanine nucleotide Exchange Factors (GEFs) promote G-protein activation by swapping GDP with GTP |
| Adaptor proteins | Molecular links connecting GPCRs to their downstream effectors to organise & amplify signalling pathways. They bind to specific targets such as phosphotyrosine or proline |
| Heterotrimeric G-proteins structure & function | Intracellular transducers composed of an α GTP-active subunit & can sometimes dissociate when active. The β & γ subunits are more closely associated & activate channels or downstream effectors |
| C3 loop | Part of the GCPR structure between the 3rd & 4th cytosolic domain (out of 7) which (when a ligand is bound) can act as its own GEF to promote activation |
| How ligand binding changes metabotropic receptors | Causes conformational shape change in the helices, domain 6 swings outwards while domain 5 slightly moves inwards, causing a receptor to distort open & the C3 GEF loop to activate |
| cAMP synthesis | Adenyl cyclase cleaves pyrophosphate from ATP & forms an ester bond between the remaining phosphate & deoxyribose to form 5' AMP. It can bind to Gs (stimulatory) or Gi (inhibitory) |
| How cAMP activates PKA | PKA is a tetramer made of 2 inactive catalytic & 2 regulatory subunits. cAMP binds to regulatories causing them to dissociate & activates catalytics & start phosphorylating (such as a cAMP degrading enzyme) |
| Single-pass transmembrane proteins | Membrane protein which spans both leaflets of the cell membrane with a single α-helix, an extracellular domain for ligand binding & an intracellular domain to transmit signals |
| Receptor Tyrosine Kinases (RTKs) | 2 monomers which dimerize following the binding of a ligand, their catalytic domains cross-phosphorylate (trans-autophosphorylation) to make a catalytic domain proteins can bind to & be activated |
| Why RTKs are used almost universally for growth factor binding | Cell growth is irreversible & costly needing high precision. They're only active as dimers when ligands bind meaning few false positives & tyrosine phosphorylation is very rare normally |
| MAPK (Mitogen-Activated Protein Kinase) pathway | Multi-tiered kinase cascade pathway starting when a growth factor binds to RTK, recruits an adaptor bound to a GEF, this stimulated a Ras G-protein causing a cascade that enters the nucleus |
| MAPK pathway cascade stages in depth | Ras-GTP activates Raf (MAPKKK for serine/threonine) Raf activates MEK (MAPKK for serine/threonine & tyrosine) MEK activates ERK (the true MAPK) which enters the nucleus to phosphorylate transcription factors |
| How signals are removed | Ligands are removed or degraded, receptor is absorbed & production of it is downregulated, GTP is hydrolysed, phosphatases dephosphorylate, & second messengers are degraded |
Created by:
Beech47
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