Cell Signalling Flashcards
Overview of the signalling pathway
- Ligand
- Receptor
- Intracellular signalling proteins
- Effector proteins
- Functions (metabolic enzyme/gene expression/altered cell shape or movement)
Cell Receptors
- Cell signalling is diverse
- Receptors are signal transducers that bind low concentrations of ligand with high affinity
- Bind hydrophilic ligands while small hydrophobic signals can pass through the bilayer directly
Signalling Cascade
- Primary Transduction
- Relay (scaffold proteins)
- Transduce and amplify
- Integrate/spread/anchor/modulate
- Effector protein activation
- Gene transcription`
Types of Receptors
Cell Surface Receptors
- Ion channel coupled receptors
- G protein coupled receptors
- Enzyme coupled receptors
Nuclear Receptors
- bind to intracellular ligands to alter gene transcription like steroids or vitamins
Phosphorylation Dependent Signalling Switch
- Serine/threonine kinases that phosphorylate molecules to create cascades (signal amplification)
1. Signal in causes ATP hydrolysis so kinase phosphorylation
2. Loss of P by protein phosphatase when signal leaves causes switch ‘off’
GTP Binding Protein
- Signal causes release of GDP and binding to GTP - switch ‘on’
- GTP hydrolysis via GTPase activity when signal leaves causes GDP binding and inactivation
Two Types of GTP binding proteins
- G proteins (trimeric gtp binding proteins)
2. monomeric GTPase1
G protein coupled receptor Signalling
- Single polypeptide folded into a globular shape and embedded in the cell membrane
- 7 looping segments forming a cylindrical structure
- GPCR interacts with g proteins.
- G protein has an alpha, beta, gamma subunit. The alpha unit binds to either GTP or GDP depending on active state
- Ligand binding leads to a conformational change activating a trimeric GTP binding protein coupling the receptor to a membrane channel
GPCR Process
- Ligand binding to GPCR causes a conformational change in a subunit that triggers GPCR and G protein interaction. GTP replaces GDP in the alpha unit.
- GCPR acts as a GEF to make the alpha unit release its GDP - The G protein dissociates into the GTP bound alpha subunit and a beta/gamma dimer which diffuses laterally to interact with other membrane proteins
- Activated complex regulates membrane target protein activity
- When the GTP is hydrolyzed to GDP (alpha unit is a GTPase), the subunits become inactive and the GPCR associates with the g protein (molecular switch)
- G proteins can affect ion channels in the membrane to alter ion permeability (eg. acetylcholine affecting heart rate)
Regulating GTP binding proteins
GAP (gtp-ase activating proteins) = switch off
GEF (guanine exchange factors) = switch on
1. GEF exchanges GDP for GTP and activates monomeric GTPase
2. GAP hydrolyzes GTP so GDP binds again, leading to inactivated monomeric GTPase
GTPase RAS
- GTPase switch
- Family of monomeric GTPases
- switches between 2 conformational states depending on whether GTP or GDP binds
- has covalently attached lipid groups anchoring protein to the cytoplasmic face of the membrane
- this change allows other proteins to distinguish between active/inactive RAS
- active RAS binds and activates downstream pathways.
RAS Gef = uptake of GTP and activation
RAS Gap = GTP hydrolysis and inactivation - the indirect coupling of a self-phosphorylating RTK receptor to a RAS gef drives Ras into the active state.
The short lived RAS signals must be converted to long ones to promote longer lasting effects, and this is done through the MAP kinase module.
Cell Signalling
Receptors are often transmembrane proteins and a signal can affect different cells in different ways. This is because of differences in intracellular signalling proteins, effector proteins, activated genes, etc.
Induces cell response according to predetermined state.
Intracellular signalling proteins relay signals inside the cell and alter effector proteins that alter cell behaviour. They act like switches - go from an inactive to active state when signal is received.
How to signals stay strong?
- high affinity and specificity of interactions between signalling molecules and receptors
- downstream target proteins can ignore background signals
- robust signalling systems in the face of signal variability
Signalling Complexes
Signalling molecules are often localised by scaffold proteins into scaffolding complexes that lead to higher local concentrations so are faster and more efficient.
Induced proximity = a signal triggers signalling complex assembly commonly used to relay signals along a pathway.
Induced proximity = a signal triggers signalling complex assembly/commonly used to relay signals along a pathway.
1. preformed signalling complex on scaffold protein
2. assembly of signalling complex on activated complex
3. assembly of signalling complex on phosphoinositide docking sides
Adaptation
Enables cells to respond to changes in signal molecule concentrations over a wide range, ie. a strong signal response will modify the cell’s signalling machinery to make it less responsive to the same signal (negative feedback).
This can occur by receptor inactivation or pathway inhibitor production.
