Lecture 6 - Cell adhesion and communication (part 2) Flashcards
What are integrins?
- non-covalently linked, heterodimeric molecules composed of alpha and beta subunits
- 18 different alpha subunits, 8 beta subunits; 24 different heterodimers known
- bind to ECM components through large extracellular domains and normally short cytoplasmic domains
What are some four classes of integrins and their functions?
RGD-binding integrins
-recognise ligands containging RGD tripeptide active site
LDV binding integrins
-binds LDV ligands, e.g. in type III region of fibronectin, VCAM, MAdCAM
A-domain beta-1 integrins
-bind laminin/collagen
Non-alphaA-domain-containing laminin binding integrins
-bind specific sections of laminin
Leukocyte integrins
-B2
What define the specificity of integrins?
- Common beta subunit
- variation in the alpha subunit defines specificity
What is the structure of the RGD binding integrin?
- alpha and beta heterodimer
- binding site for RGD sequence in larger extracellular domain
- transmembrane domain
- talin binding site on the intracellular domain
What is RGD?
Amino acid recognition motif found on some ECM ligands
- Arginine-glycine-aspartate
- when RDG binding integrin binds to fibronectin
What are the binding features of fibronectin?
- lots of different binding domains
- allow it to interact physically with other member of the ECM
- RGD sequence in the cell binding domain with RGD loop for RGD integrin binding
How are focal adhesions formed?
when integrins cluster on binding ECM to form a plaque-like structure that tethers the cell to the substrate
What are focal adhesions?
juntional complexes that allow cells, via integrins, to interact physically with actin cytoskeleton through anchoring onto stress fibres
Through what proteins do integrins bind to the cytoskeleton at focal adhesions?
Adapter proteins e.g. Vinculin Talin alpha-actinin
What molecules are involved in integrin signalling?
Focal adhesion kinase (FAK)
and other molecules
What can the action of FAK phosphorylation and activation of several down stream signalling pathways result in?
signals for:
cell growth, spreading and movement
What is the signal process through FAK following the interaction with integrins through adapter components? (OUTSIDE IN SIGNALLING)
- ECM (fibronectin) binding leads to integrin clustering (through adapter proteins) and actin polymerisation and formation of focal adhesions
- FAK is autophosphorylated
- P-FAK recruits Src and Fyn (signalling molecules) to the arrangement at the plasma membrane
- Src and Fyn phosphorylate other sites on FAK resulting in additional protein binding to activation divergent signalling pathways e.g. Ras/MAPK, p13k/Akt pathways are activated
- Rho family GTPases activate and regulate actin assembly (Rho - stress fibres, Cdc42 - filopodia, Rac - lamellopodia)
- leads to the rearrangement of cytoskeleton and movement and activity of the cell
- Can lead to an increase in cytoplamsic Ca2+ and activate other downstream signalling events
- >Ca2+ mediated signal inactivates attachment at cell rear
- > rapid assocaition and dissociation results in ‘crawling’
What is outside in signalling?
When ligand binding on the outside of a cell activates intracellular signalling pathways e.g. in integrin signalling through FAK
What is inside out signalling?
where origin of activity is from inside of the cell e.g. binding of a ligand to a G protein coupled receptor or receptor tyrosine kinases
What is the process of Talin (inside out signalling)?
- G protein coupled receptors/receptor tyrosine kinases target inactive talin to activate it and induce dimerisation
- In the correct conformation, talin binds to the cytoplasmic domains of integrins via the beta-integrin subunit, changing their conformation so they can bind to ECM components
- this causes downstream assembly and signalling activities
