9. Lymphocyte signalling 1 Flashcards
Why is learning about T cell signalling important?
- Most signalling pathways are not restricted to T cells.
- It is a big part of drug development as they are developed against individual molecules to change a function, and it is important to understand how the interactions of the molecules affect each other.
What is the basic function of a receptor?
- A receptor takes extracellular information and uses it to cause a biochemical change that can alter cell behaviour.
- The binding to a receptor will trigger some sort of conformational change.
- This change triggers a biochemical change through enzymes. eg phosphorylation of tyrosine.
- this can be done with 1 molecule like EGFR or 10 like TCR.
What do the protein chains in the TCR contain?
- Signalling motifs that allow the signal from the a/ß chains to be transmitted across the plasma membrane.
- As antigen recognition and signal transduction are on separate proteins, there needs to be something that holds them together.
- The a/ß chains contain positively charged residues in the transmembrane domain.
- The other chains contain negatively charged residues in the transmembrane domain.
- This does have a charge imbalance but this holds the complex together.
- Having charged residues in the membrane is a huge energy burden.
What protein chains make up a TCR?
- a/ß chains that are different in every TCR and bind the MHC complex for antigen recognition.
- CD3yε dimer. This is the same in every TCR.
- CD3δε dimer. This is the same in every TCR.
- Zeta dimer. This is the same in every TCR.
What other complex does the TCR work with?
- the MHC peptide complex
- The peptide and the MHC is recognised by the TCR.
- The invariant chains in the MHC bind the co-receptors associated with the TCR.
- CD4 co-receptor binds MHC2
- CD8 co-receptor binds MHC1
- Basic T cell activation needs antigen recognition and co-receptor binding.
What are the critical signalling motifs in the TCR/BCR?
- ITAMs: immunoreceptor tyrosine-based activation motif.
- These have 2 tyrosine residues that can be phosphorylated by kinases.
- It has a tyrosine and then 3 positions later the must be a large aliphatic amino acid. This is a leucine or isoleucine.
- This is repeated after about 10 amino acids.
How many ITAMs are in a TCR?
1.δ, y and e chains have 1 ITAM each
2. z chains have 3 ITAMs each
3. Totals 10 ITAMs and 20 tyrosine residues
Where is the enzymatic activity in the TCR?
- None of the protein chains in the TCR have enzymatic domains.
- It needs to recruit enzymes to the complex
What enzymes do the TCR recruit for initial signalling?
Tyrosine kinases Lck and ZAP-70 provide the enzyme activity needed for the TCR to generate a signal.
What is Lck?
- A Src family kinase
- It is always associated with co-receptors CD4 and CD8.
- If an MHC-peptide binds the TCR, Lck. is bought into the complex.
- Lck then phosphorylates the ITAMs in the TCR.
What are Src family kinases?
- Originally found in viruses
- They are a large family of kinases
- Include Src, Lck, Csk, Fyn and Lyn
Src family kinase domains: Lipid tail
- Lipid modification of the protein
- Helps it localised to the plasma membrane and stay there for longer.
- Very common in proteins working close to the membrane.
Src family kinase domains: Unique domain
Along with the lipid tail the unique src domain control the interaction with CD4 and CD8.
Src family kinase domains: SH2 domain
- A protein recruitment domain.
- It binds to phosphorylated tyrosines
Src family kinase domains: SH3 domains
- A protein recognition domain
- IT binds to a proline X X proline motif
Src family kinase domains: kinase domain
- The enzymatic domain with some protein protein interaction domains.
- A tyrosine residue in the kinase domain is an activating tyrosine that can activate itself through autophosphorylation.
- A c-terminal tyrosine residue that is inhibitory when it is phosphorylated.
What are the 2 main functions of signalling?
- Causing a change in localisation
- Enzymatic activity
What happens if a signal is always on?
Oncogenic transformation
What needs to happen when a signal turns on?
A method of turning it off is upregulated
How are Src family kinases inhibited/turned off?
- Through proline X X proline motif in the linker-SH3 region that the SH3 can bind.
- C-terminal tyrosine being phosphorylated.
- Through a physical blockade that is often intramolecular.
- This is a different domain in the same protein that binds the functional domain to prevent it eg the SH2 binding the C-terminal tyrosine
What is different about the regulation of Lck in T cells?
- Evidence suggests Lck has similar activity levels in inactive and active T cells.
- The key element of the regulation of Lck and other Src kinases is the localisation and the bringing of Lck to the TCR and plasma membrane.
- CD4/8 is heavily involved in this.
How do different phosphorylation patterns change the activity of Lck?
- If the Inhibitory tyrosine is phosphorylated by another src family kinases called Csk the activity is inhibited.
- If the stimulatory tyrosine is phosphorylated by Lck itself, its activity is activated.
- Both can be dephosphorylated by the transmembrane phosphatase CD45.
What does Lck’s change in location allow it to do?
Allows the Lck to phosphorylate the ITAMs on the TCR and recruit ZAP-70.
What is ZAP-70?
- A kinase that carries out the enzymatic activity needed to activate the TCR.
- Binds to ITAMs to become active.