Cell Signalling Flashcards
Describe how cell activity is affected.
- Most things which affect cell activity or function do not enter the cells.
- They act on membrane-bound receptors that control signalling proteins via the production of second messengers.
- These mediate cell activity.

What are the general principles of signal trasduction?
- Many signalling proteins act as molecular switches.
- There are two common ways to activate / deactivate signalling proteins.
- Human genomes encodes ~520 kinases and ~150 phosphatases.
- There are two main types of kinases.
- There are two types of GTP-binding proteins.
What are the 2 main ways of switching signalling proteins on/off?
- Phosphorylation
- GDP binding
What are the 2 main types of kinases?
- Tyrosine kinase
- Serine / threonine kinase
What are the 2 types of GTP-binding proteins?
- Trimeric G proteins
- Monomeric GTPases
Explain the role of G-protein-coupled receptors in signal transduction.
- Ligand binding activates a G-protein which in turn activates or inhibits another protein.
- Often this is an enzyme that generates a specific second messenger.

What is a G-protein-coupled receptor?
- All G-protein-coupled receptors have 7 membrane spanning regions with their amino termini on the extracellular face and their carboxy termini on the cytoplasmic face of the plasma membrane.
- The 7 membrane-spanning regions are alpha helices.
- A ligand binding to a G-protein-coupled receptor activates the associated G-protein which in turn inhibits / activates a downstream enzyme to generate an intracellular second message.
- G-protein activation and complex formation are part of a cycle.
- Trimeric: composed of 3 different subunits (α, β and Ɣ)

What is the mechanism of action of a G-protein-coupled receptor?
- Binding of the ligand to the receptor changes its conformation, causing it to bind to the Gα protein in such a way that GDP is displaces and GTP is bound.
- This triggers Gβɣ dissociation activating downstream pathways.
- Activation is short-lived, as GTP bound to Gα hydrolyses to GDP in seconds, leading to the re-association of Gα with Gβɣ and inactivation of adenylate cyclase.

Describe how adenylate cyclase is stimulated.
- GTP is required for the ligand-induced stimulation of adenylate cyclase.
- Glucagon receptor couples to GαS.
- Overall, the system needs:
- A receptor
- A transducer (G-protein)
- An amplifier (adenylate cyclase) that generates large amounts of a second messenger.
- cAMP is the second messenger made by adenylate cyclase.
Describe the structure of a G-protein-coupled receptor.
- Receptors consist of 7 transmembrane helices (hydrophobic amino acids) that reside in the plasma membrane.
- Interacts with heterotrimeric G-protein complex on the extracellular side.

Which type of G-protein stimulates phospholipase C?
Gαq
Which type of G-protein stimulates adenylate cyclase and increases cAMP?
Gs
Which type of G-protein inhibits adenylate cyclase and decreases cAMP?
Gi
What are the functions of the Gβ/ɣ dimer?
- Gate ion channels
- Stimulates adenylate cyclase
- Stimulates PLA2
- Stimulates PLC-β, PLC-ε and PLC-𝜼
How does phospholipase C become activated?
Its substrate is phospholipis which resides in the cell membrane.
1-2% of the cell membrane is this phospholipid so it is not abundant.

What are phospholipase C isoforms?
- Proteins which possess distinct domain structures but catalyse the same reaction (liberation of IP3 and DAG from PIP2).
- Some domains are common (catalytic, membrane localisation).
- Some domains are uique (regulatory).
- Acivated by different pathways.
Describe calcium signalling.
- Cytosolic calcium levels are dynamic.
- Calcium can enter from intracellular stores or from outside the cell via calcium channels.
- Channels may be receptor or voltage operated.
- Calcium can exit via other channels present in plasma or organelle membranes.
What is the resting calcium concentration of a cell?
~100nM
What is the activated calcium concentration of a cell?
0.5-1µM.
Cellular response depends upon the duration of the signal.
What is the effect of the calcium wave on an egg cell?
- Calcium wave does 2 things:
- Tells the cell to start dividing
- Prevents any more sperm coming in because an egg should not be fertilised
Describe protein kinase C.
- A large family: at least 12 different isoforms.
- Most are present as catalytically inactive, soluble proteins in the cytoplasm.
What happens to PKC in the cell?
- Rise in cytosolic calcium levels causes PKC to bind to the cytosolic leaflet of the plasma membrane, where it can be activated by the membrane-associated DAG and or Ca2+.
- PKC then phosphorylates
Protein kinase C functions ‘indirectly’ to do what?
To alter gene expression.
The genes can phosphorylate kinases.
Binding of adrenalin to the β2 adrenergic receptor is another type ot G-protein-coupled receptor signalling.
Describe what happens.
- Mediates the body’s response to stress/fear (fight or flight).
- Release of glucose and fatty acids from liver/fat cells
- Increased contraction of cardiac muscle
- Bindind of adrenaline to β2 adrenergic receptor increases the intracellular concentration of cAMP (cyclic AMP) as receptor couples to Gαs.
- cAMP is synthesised within cells from ATP by the enzyme adenylate cyclase.
- cAMP is degraded by the enzyme cAMP phosphodiesterase.
- Different receptors utilise a common adenylate cyclase (ie each receptor does not have its own intrinsic adenylate cyclase).






