11 - JAK/STAT RTK System - Williamson

Question 1

what is the function of the JAK/STAT signalling system?

Answer 1

allow for cytokine signalling (cytokine receptors)

• cytokines are involved in signalling and communication between cells of the immune system
• the same cytokine can have different effects on cells depending on the cell type and the environment

Question 2

how many cytokines can bind to a cell at once? how is this regulated?

Answer 2

up to 2 signals can bind to a cell. need a way of integrating these signals ie do we produce smaller/bigger signal when there are different inputs. also needs to have specificity in order to respond appropriately to correct signal

Question 3

describe the Rs invovled in the JAK/STAT pathway and state some Ls they bind

Answer 3

• 2 identical Rs -> homodimer
• intracellular domain has an additional kinase JAK domain bound via a protein (this protein attached to both R and membrane)
• L = growth hormone

Question 4

state some differences and similarities between STAT signalling and SMAD signalling

Answer 4

SIMILARITIES; (not all)
- both have TM domains within their receptors
DIFFERENCES;
- SMAD = Ser P, STAT = Tyr P
- SMAD kinase domain is part of the receptor intracellular cytoplasmic domain whereas STAT = additional JAK kinase domain bound to the cytoplasmic domain
- SMAD Rs are not identical -> heterodimers, STAT = 2 identical Rs -> homodimers

Question 5

draw a diagram and explain how binding of a growth hormone to its R leads to differential expression of genes

Answer 5

• binding of GH to one R promotes dimerisation to the other R (this searching isn’t too difficult because both on the membrane)
• cytoplasmic domains are now in close proximity so can crossP/autoP each other on the activation lip resulting in ACTIVE JAK kinases
• active JAK kinases now P the R producing docking sites for adaptor proteins eg SH2, SH3 to bind . signal NOW ON
• the SH2 domain bound to DNA binding domain now binds to the phosphotyrosine residues and becomes P
• SH2 + DBD dissociates from R and dimerises. the pY residues end us in the SH2 domain site.
• dimerisation creates intact NL, moves into the nucleus where it can tightly bind to DNA and act as a TF
• DBDs can bind to DNA cooperatively @ 2 repetitive DNA sequences

Question 6

what is the SH2 domain? describe it and state why it is useful

Answer 6

small set of protein domains used widely in signalling pathways and for recognising motifs
- recognises pY residues 1000x better than Tyr

Question 7

how many times will SH2 domains create a signal when the R actually hasn’t been phosphorylated? what failsafe mechanisms does the cell have to stop this occurring?

Answer 7

1/1000 times, the SH2 domains will produce a signal when R not activated
- to stop the cell responding to this, nonspecific phosphatases exist to remove Ps from things that have been incorrectly P

Question 8

what are adaptor proteins? give an example and state the general use of adaptor proteins in signalling

Answer 8

adaptor Proteins can be used to link a specific signal to a more general pathway. or allow multiple inputs into the same pathway (think about foreign adaptor plus)
eg SH2 domains

Question 9

describe some other domains similar to the SH2 domain and state what specific things they recognise

Answer 9

PTB - also recognises pY but differently - recognises 3 aa residues BEFORE pY
PH domains - recognises lipids and parts of membrane eg phosphoinositides eg IP3 binds to the PH domain
SH3 - recognises polypro line helices (PXXP - pro has to be arranged in certain configurations)
PDZ - recognises C terminal peptides

Question 10

draw a simple diagram of the SH2 domain. state notable features, their uses and what the SH2 domain recognises

Answer 10

• SH2 recognises 3 residues AFTER the pY (in contrast to PTB)
• SH2 has +vely charged pocket that recognises the -vely charged pY
• 2 prongs that extend out. 2nd prong provides specific recognition
• N/C termini linked closely together for easy genomic incorporation into new protein domain eg if SH2 domain needs to be linked to signalling enzyme

Question 11

can these protein domains interact? if so, what is the function of this interaction and give an example

Answer 11

yes many protein domains can interact to link to > downstream signalling events.
many proteins consist entirely of these domains
can be used to bring protein complexes together and to assemble large complexes
eg Grb2 protein brings together to form an adaptor complex
OR eg2 - phosphatase called Shp2 has 2 SH2 domains so we get cooperative binding. stronger binding overall mediated through weaker interactions

Question 12

where does the specificity of the kinase come from?

Answer 12

specificity comes from the biding of the signalling proteins to their target NOT the specificity of the Kinase itself

Question 13

how is the JAK/STAT signal turned OFF?

Answer 13

• SOCS protein (suppressor of cytokine signalling). SOCS has an SH2 domains which binds R and recruits E3 Ub ligase via a 2nd domain called a SOCS box. ubiquitinylated protein then degraded by proteasome. this is a form of -ve feedback because signalling from this cytokine receptor induces expression of SOCS
• phosphatase that uses an SH2 domain to bind to R and deP JAK therefore turning R off

Question 14

overall, what does this system control? what do defects in the JAK/STAT system lead to?

Answer 14

controls growth and development

defects lead to cancer / developmental abnormality

Question 15

give a specific example of a JAK/STAT ligand, state its function and its medical consequence

Answer 15

• L = erythropoietin
• stimulates RBCs to proliferate and differentiate (this is the reason why it is illegal in sports)
• Olympic gold medailst for cross country skiing found to have faulty erythro. R which meant signal not turned off properly therefore had higher conc of RBCs