Week 6 - Receptors in cell signalling, principles of receptor-mediated endocytosis Flashcards
How can chemical signals be classified?
According to their function
- Hormones (signalling between cells in different tissues via the circulation)
- Neurotransmitters (signalling at specialised cell junctions in the nervous system, synapses)
- Local chemical mediators (signalling between adjacent cells in the same tissue)
A single molecule may fall into more than 1 of these categories depending on where it is synthesised and released, and it’s site of action
What is a ligand?
Any small molecule that binds specifically to a receptor site
What is an antagonist?
A ligand that does not cause activation upon binding
- It opposes the action of the ligand
What are partial agonists?
Agonists which stimulate a receptor but are unable to elicit the maximum cell response possible
What is a receptor?
A molecule that recognises specifically a second molecule (ligand) or family of molecules, and which in response to ligand binding brings about regulation of a cellular process
- In the unbound state, it is functionally silent
What are some examples of the roles of receptors in cellular physiology?
- Signalling by hormones/local chemical mediators
- Neurotransmission
- Cellular delivery
- Control of gene expression
- Cell adhesion
- Modulation of the immune response
- Sorting of intracellular proteins
- Release of intracellular calcium stores
What is the difference between the binding affinity of enzymes and that of ligands?
Affinity of ligand binding at receptor sites is generally much higher than binding of substrates and allosteric regulators to enzyme
How can receptors be classified?
According to the specific physiological signalling molecule (agonist) that they recognise
- Sub-classification: affinity of a series of antagonists
What is an acceptor?
A receptor whose basic function can be carried out without the interaction of a ligand
- Ligand binding alone produces no response
How do cells respond to a chemical messenger?
It must produce specific receptor proteins
- These recognise and produce a response to the signalling molecule
- Interaction of the signalling molecule with its specific receptor must then result in the activation of a cellular process
What is signal transduction?
Most signalling molecules are unable to cross the plasma membrane, so there are some common mechanisms to transduce an extracellular hydrophilic signal into and intracellular event:
- Membrane-bound receptors with integral ion channels
- Membrane-bound receptors with integral enzyme activity
- Membrane-bound receptors which couple to effectors through transducing proteins
- Intracellular receptors for hydrophobic ligands
What are some similarities between receptor binding sites and the active sites of enzymes?
- Binding at both receptor sites and enzyme sites is specific
- The specificity of binding is governed by the shape of the binding cleft in the receptor/enzyme site
- The specificity of binding confers specificity to the regulation of processes in which receptors are involved or the specificity for substrate of an enzyme
- Binding is most often reversible
- Binding induces a conformational change and a change in the activity of the molecule
- No chemical modification of the ligand
What is a difference between receptor binding sites and the active sites of enzymes?
The affinity of ligand binding at receptor sites is generally much higher
What are the 4 major classes of receptors involved in cellular signalling?
- Membrane-bound receptors with integral ion channels
- Membrane-bound receptors with integral enzyme activity
- Membrane-bound receptors with no integral enzyme or channel activity
- Intracellular receptors
How do membrane-bound receptors with integral ion channels work?
Agonist binding to ligand-gated ion channels results in a change in conformation and opening of a gated channel
- This permits the flow of ions down an electrochemical gradient
- This transduces the signal into an electrical event at the plasma membrane
What is the subunit structure of the ‘classical’ ligand-gated ion channel family?
They have similar pentameric subunit structures
- Subunits have 4 transmembrane domains, 1 of which forms the lining to the channel pore
What are some examples of ligand-gated ion channels?
- Nicotinic acetylcholine receptors
- Gamma amino-butyric acid receptors
- Glycine receptors
How do membrane-bound receptors with integral enzyme activity work?
Agonist binding to the extracellular domain of these receptors causes a conformational change
- This activates an intrinsic enzyme activity contained within the protein structure of the receptor
Give an example of a membrane-bound receptor with integral enzyme activity and describe how it works
Tyrosine kinase-linked receptors
- Binding of hormone to extracellular binding sites activates protein kinase activity in the cytoplasmic domain of the receptor protein
- This autophosphorylates tyrosine residues on the cytoplasmic domain of the receptor
- These are recognised by transducing proteins or directly by enzymes containing phosphotyrosine recognition sites
- On association with receptor or transducing protein, effector enzymes become activated allosterically
- This transduces the message into an intracellular chemical event
How do membrane-bound receptors with no integral enzyme or channel activity work?
- 7 transmembrane domain receptors couple to effectors molecules via a transducing molecule (a G-protein)
- This family of receptors are also know as the G-protein coupled receptor family)
- Effectors may be enzymes or ion channels
- A wide variety of extracellular signalling molecules utilise specific 7 transmembrane domain receptors
- Receptor binding results in a conformational change which activates GDP/GTP exchange in GTP-binding regulatory proteins
- This transduces the message onto an enzyme or channel in the membrane
Give some examples of membrane-bound receptors with no integral enzyme or channel activity
- Muscarine ACh receptor
- Adrenoceptors
- Dopamine receptors
How do intracellular receptors work?
- Hydrophobic ligands penetrate the plasma membrane
- They bind to monomeric receptors and are stabilised by association with heat shock or chaperone proteins
- The activated receptor dissociates from the chaperone protein and translocates to the nucleus
- It then binds to control regions in DNA defined by specific sequences, thereby regulating gene expression
- The effects of intracellular activation are slow in onset as transcription and translation are required
How does phagocytosis occur?
In response to the binding of a particle to receptors in plasma membrane
- The cell extends pseudopods
- These permit further receptor interactions and membrane evagination and particle internalisations
- Internalised phagosomes fuse with lysosomes to form phagolysosomes in which the particulate material is degraded
What is pinocytosis?
Invagination of the plasma membrane to form a vesicle
- Permits uptake of extracellular salutes and retrieval of plasma membrane
What are the 2 forms of pinocytosis?
- Fluid-phase
- Receptor-mediated endocytosis
What is receptor-mediated endocytosis?
Specific binding of molecules to cell surface receptors
What is an example of receptor-mediated endocytosis and explain how it occurs?
Uptake of cholesterol
- Low density lipoproteins originate in the liver
- Animal cells that require cholesterol synthesise cell surface receptors that specifically recognise apoprotein B (found on LDL)
- LDL receptors are localised in clusters over coated pits
- Coated pits invaginate and pinch off from the plasma membrane to form coated vesicles
- Coated vesicles are quickly uncoated (driven by ATP)
- The uncoated vesicles then fuse with larger smooth muscle vesicles called endosomes
- The pH of the endosomes is maintained between approximately 5.5-6.0 by an ATP-dependent proton pump
- At this pH, the LDL receptor has low affinity for the LDL particle and the 2 dissociate
- The transmembranous receptors are sequestered to a domain within the endosome membrane which buds off as a vesicle (this recycles the LDL-receptor to the plasma membrane)
- The endosomes containing the LDL fuse with lysosomes such that the cholesterol can be hydrolysed from the esters and released into the cell
- Hence the receptor is recycled and the ligand is degraded
How is the clathrin coat attached to the plasma membrane?
By a number of integral membrane adaptor proteins
- These form associations with both the clathrin and receptors
What is another name for endosomes?
CURL
- Compartment for the uncoupling of receptor and ligand
How does coated pit formation occur?
Spontaneously and non-specifically
What mutations are there that can affect the LDL receptor?
- Receptor deficiency: mutations that prevent expression of LDL receptor
- Non-functional receptor: no binding of LDL, normal coated pits and internalisation
- Normal receptor binding but no internalisation: due to deletion in the C-terminal of the receptor that makes interaction with the coated pits. LDL receptors are found distributed over the whole call surface in these patients
How are ferric (Fe3+) ions taken up?
- 2 Fe3+ ions bind to apotransferrin forming transferrin in the circulation
- Transferrin binds to the transferrin receptor at neutral pH and is internalised in the same way as LDL
- On reaching the acidic endosome, the Fe3+ ions are released from the transferrin
- Apotransferrin remains associated with the transferrin receptor
- The complex is sorted in the CURL for recycling back to the plasma membrane, where at pH 7.4, the apotransferrin dissociates from the transferrin receptor again
- The ligand and receptor are hence recycled
How does the uptake of occupied insulin receptors occur?
- The insulin receptor only congregates over coated pits when the agonist is bound
- Insulin binding probably induces a conformational change in the insulin receptor, allowing it to be recognised by the coated pit
- In the endosome insulin remains bound to the receptor
- The complex is targeted to the lysosomes for degradation
- This mechanism allows down-regulation of insulin receptors on the membrane surface, desensitising the cell to a continued presence of high-circulating insulin concentrations
- The ligand has been degraded, as has the receptor
What is transcytosis?
When ligands that remain bound to their receptors are transported across the cell
How can membrane-enveloped viruses and toxins enter cells using endocytic pathways?
They exploit endocytic pathways to enter cells after adventitious binding to receptors in the plasma membrane
- Once in the endosome, where the acid pH is favourable, the viral membrane is able to fuse with the endosomal membrane
- This releases viral RNA into the cell
- It can then be translated and replicated to form new viral particles
