Lecture 3 - Cell communication II - tyrosine kinase-associated signalling pathways Flashcards
What can cross the plasma membrane and interact with intracellular receptors?
Small hydrophobic molecules - such as steroid hormones
Can large hydrophilic signalling molecules - such as proteins- cross the plasma membrane?
no, they ineract with extracellular portion of transmembrane receptors. Molecules can cross plasma membrane via gated membrane channels (e.g. Ca2+ ions)
Describe the properties of a hydrophilic extracellular domain that interacts with a ligand
often contains CONSERVED ELEMENTS:
- immunoglobin (Ig)-like domains
- epidermal growth factor (EGF)- like domains
- fibronectin type III repeats
- cysteine-rich regions
Describe the properties of the transmembrane segment
usually 20-25aa of alpha-helical structure made up of hydrophobic amino acids which stabilised via interactions with fatty acid chains of the lipid bilayer
Describe hydrophilic signalling machinery
In the case of receptor tyrosine kinases (RTKs), this involves a tyrosine kinase domain
How is a protein inserted through the plasma membrane?
Genes encoding transmembrane proteins contain a single-peptide which directs the newly synthesized protein to the endoplasmic reticulum
How is information transmitted from extracellular ligand to intracellular ‘effector’?
- Conformational changes to multi-pass transmembrane receptors which are then often associated with G-proteins
- The dimerisation/multimerisation of single pass transmembrane receptors
What are transmembrane receptors important for?
Family of 58 single pass transmembrane receptors that include a tyrosine kinase domain. They are important for developmental/homeostatic roles.
Explain enzyme-coupled receptors
Dimerisation of extracellular ligand (e.g. platelet-derived growth factor). Each ligand bind to a receptor, which are then bought together and activate each other.
Other mechanisms include monomeric ligand with 2 ligand receptor binding sites. Binding of a ligand alters their affinity for one another (e.g. Epidermal growth factor - EGF)
Do receptor tyrosine kinase have enzymatic activity?
Yes - receptors are bought together and phosphorylate/activate each other
How many ommatidia per eye?
800 ommatidia per eye
- 8 photoreceptors
- 4 cone cells
- 2 primary pigment cells
- 6 secondary pigment cells
- 3 tertiary pigment cells
Describe the development of photoreceptors
- imaginal discs are developing adult structures within the Drosophila larva
- Differentiation of photoreceptors in the eye disc occurs over the course of 3rd install larval development
-First photoreceptor cell, the R8, differentiates within the morphogenetic furrow - Rows added approximately every 2 hours with progressive recruitment of photoreceptors from the surrounding pool of undifferentiated cells
What signaling pathway is required in R-cell recruitment?
EGFR-RAS/RAF-MEK-ERK signaling pathway
Describe the EGFR-RAS/RAF-MEK-ERK pathway
- Dimerised PDGF (platelet-derived growth factor) ligands have a high affinity for receptors
- Receptors brought together due to ligand binding, leads to phosphorylation of p-Y residues in the cytosolic tail by intrinsic kinase domain of the opposite receptor.
- p-Y residues act as docking sites for the SH2 domain of adaptor protein Grb2
- SH3 domains of Grb2 bind proline-rich regions of Sos - a GDP to GTP exchange factor (GEF)
- Sos GEF activity swaps GDP to GTP in membrane associated Ras molecule activating it.
- Active ras-GTP binds to inhibitory domain of Raf
- Raf is a serine/threonine kinase that then activates itself
- Downstream pathway components MEK and Erk are activated in sequence and are all S/T kinases
- pErk translocates to the nucleus, interacts with Tis and triggers gene expression.
How is R-cell recruitment regulated?
- Raf is negatively regulated by its own N-terminal region, the 14-3-3 protein and its own phosphorylation - all which need to be removed before the protein can be activated
- Erk contributes to multiple levels of negative regulation including including the ‘negative phosphorylation’ of Sos to prevent its binding to Grb2
- The activated PDGFR also recruits multiple negative regulators of the pathway including the RasGAP, phosphates and triggering endocytic internalisation.
