20 - Signal Propagation from RTKs

Question 1

What classes of RTKs are there?

Answer 1

Three classes: tyrosine-specific, Ser/Thr specific, and histidine specific

Question 2

Give examples of organisms that have RTKs with INTRINSIC PROTEIN KINASE ACTIVITY

Answer 2

• histidine specific: prokaryotes, unicelular eukaryotes, plants
• serine/threonine-specific: plants, animals: TBFbeta receptor family
• tyrosine specific: animals: receptors of insulin, growth factors, morphogens

Question 3

Give examples of organisms that have RTKs with ASSOCIATED PROTEIN KINASE

Answer 3

• histidine-specific: prokaryotes: (chemo)taxis receptors
• serine/threonine-specific: animals: receptors of IL1, TNFalpha, bacterial compounds, morphogens
• tyrosine specific: animals: receptors of cytokines, antigens, extracellular matrix components

Question 4

What happens when a receptor binds to a RTK?

Answer 4

• When the ligand binds this recruit’s dimerization of receptors, so two monomers are recruited linked together by the activating ligand
• This triggers phosphorylation of the intracellular domain or the kinase domain itself
• The phosphates on tyrosine serve as docking sites for signalling proteins
• So, we have recruitment of signalling proteins and further phosphorylation of signalling proteins
• To switch it off, we have dephosphorylation (activation of phosphatases)

Question 5

Steps of RTKs:

Answer 5

1. Dimerization
2. Phosphorylation
3. Recruitment of signalling proteins
4. Activation of downstream signalling
5. Dephosphorylation to reset the receptor to its resting state

Question 6

Src-homology 2 (SH2) domains

Answer 6

• Respresentation of a complex between a SH2 and a peptide of an RTK containing a P-Tyr
• The P-Tyr is enveloped within the interior of the SH2 domain
• This class of protein domain was first discovered in the Src oncogene after the discovery of the Src tyrosine kinase activity

Question 7

SH3 domains

Answer 7

• The SH3 domains bind to specific short proline rich sequences with a Pro-Xaa-Xaa-Pro core in their partner (Xaa means any amino acid)
• SH3 proteins are involved in signal transduction but also in cytoskeletal proteins
• Representation of a complex between the SH3 domain of GRB2 and a peptide of SOS, rich in prolines
• Tends to zig zag due to the structure of proline
• Hydrophobic interaction

Question 8

SH2 and PTB (phosphotyrosine-binding) domains

Answer 8

• both domains consist of two b-sheets flanked by one or more a-helices
• The orientation of the sheets and helices, and the mechanism of peptide binding, is markedly different for the two domains
• The third domain discovered was PTB- interact with phosphorylated tyrosines
  
  • some anti-parallel sheets but the orientation is not the same as SH2
  • example of convergent evolution- similar jobs but different structure

Question 9

What happens after a RTK is activated?

Answer 9

Here we have the activation of a RTK - we have a monomeric inactive RTK, which binds ligands and acts as a crossbridge
- when the two monomers are brought together it triggers activation of the RTK domain and you get autophosphorylation on the cytoplasmic portion of the receptor and also transphosphorylation where one receptor will phosphorylate on the other

Question 10

Why is decorating tyrosines on the receptor important during activation?

Answer 10

We have decorating tyrosine’s on the cytoplasmic portion of the receptor, and these tyrosine’s can serve as a docking sites where SH2 domains can interact, so the types of signalling protein that can interact is Grb and SOS

- Grb has an SH2 domain which interacts with the phosphorylated tyrosine
- SOS and Grb2 are kunked together by an SH3 domain, this recruitment will then triger the activation of the RasMAPK pathway
- Other types include Src, PI3K, GAP, PLC, Shp2

Question 11

Different signalling proteins recruited to activated receptors fall into how many classes?

Answer 11

Group 1: enzymes
- Contain enzyme activity
Group 2: adaptors
- Serve as scaffold proteins to allow formation of signalling complexes at the receptor

Question 12

Features of the different domains

Answer 12

o Protein components have different combinations of SH2 and SH2 domains
o SH2, SH3, and PTB domain-containing signalling proteins
o SH2, SH3 and PTB domains are important in molecular “adhesives”

Question 13

What makes each tyrosine-phosphorylation site unique and how are they classed?

Answer 13

Each tyrosine-phosphorylation site has a unique sequence for interaction with specific signalling molecules containing SH2 or phosphotyrosine binding (PTB) domains
o This provides specificity and diversity for RTK signalling
o Classed by how mitogenic they are (how easily they stimulate mitosis)

Question 14

What are growth factors?

Answer 14

Growth factors themselves are mitogens that stimulate cell growth by being ligands for RTKs

Question 15

Why are some growth factors strong mitogens?

Answer 15

Strong mitogens like PDGF are strong because when they interact with the PDGF receptor it causes phosphorylation of a large number of tyrosines in the chain of the receptor so multiple docking sites are generated whe the receptor is activated
o It is also strong as it has two tyrosine kinase domains
o Full range of phosphorylation of the intracellular domain

Question 16

Give an example of a mid-range mitogen

Answer 16

EGF-mitogen
o It is a mid-range mitogen because it can activate a number of signalling pathways but not as many as the PDGF receptor
o Don’t get PI3K recruitment- limited growth response

Question 17

Give an example of a weak mitogen

Answer 17

TrkA- weak mitogen
o Only binds phospholipase C gamma