Lecture 9 - Tyrosine Kinase Signaling

Question 1

What are the 2 types of tyrosine kinases?

Answer 1

1. Receptors

2. Receptor-associated

Question 2

List 5 receptor tyrosine kinases.

Answer 2

1. Insulin receptor
2. IGF-1 receptor
3. Nerve growth factor receptor
4. Epidermal growth factor receptor
5. Platelet derived growth factor
   + many more growth factor receptors

Question 3

What must be true of receptor associated tyrosine kinases?

Answer 3

They are soluble in the cytoplasm

Question 4

What do the receptor tyrosine kinases have in common? 3 parts

Answer 4

1. 1 single transmembrane domain
2. Elaborate, large, and globular extracellular domain
3. Intracellular tyrosine kinase catalytic domain

Question 5

Where is the catalytic domain of receptor tyrosine kinases located?

Answer 5

Proximal to the membrane on the cytosolic side

Question 6

Describe the 4 steps of receptor tyrosine kinase activation.

Answer 6

1. Ligand binding induces receptor dimerization through various mechanisms
2. Dimerization activates the tyrosine kinase domain of the receptor by moving the regulatory binding from the active site
3. Tyrosine kinase INTERmolecularly trans-autophosphorylates the receptor mostly OUTSIDE of the catalytic domain
4. Substrate phosphorylation

Question 7

Which ONLY receptor tyrosine kinase is a dimer? How?

What is also unique about this receptor’s functioning?

Answer 7

Insulin receptor is covalently dimerized

Also phosphorylates insulin receptor substrates (IRS) on tyrosines, creating even more phosphotyrosine binding sites, on top of the usual binding sites on the receptor itself

Question 8

Is the trans-auto-phosphorylation of receptor tyrosine kinase necessary for proper substrate phosphorylation?

Answer 8

YUP

Question 9

What is the most fundamental difference between tyrosine kinases signaling strategies and Ser/Thr kinases signaling strategies?

Answer 9

• Ser/Thr kinases: presence of P on substrates results in a conformational change which activates or inactivates(couple of exceptions)
• Tyr kinases: some small conformational changes, but mainly the cytosolic phosphotyrosines serve as binding sites for phosphotyrosine binding proteins

Question 10

What are 3 examples of phosphotyrosine binding proteins?

Answer 10

1. PI3-K using a p85 adaptor
2. GAP = GTPase Activating Protein
3. PLPC-gamma

Question 11

Describe the specificity of the phosphotyrosine binding sites and give a supporting argument.

Answer 11

Each binding protein has a VERY specific recognition 5 AA sequence including the phosphotyrosine

Synthetic peptides, as short as 5 AAs, containing a P-tyrosine can block binding to SPECIFIC binding sites on the receptor

Question 12

Why did researchers originally think the tyrosine kinases had low affinity and specificity? How is this overcome by the tyrosine kinase?

Answer 12

Because they have low affinity and specificity at their catalytic site, not their phosphotyrosine binding sites:

• Deal with lack of specificity by having the high specificity at the 5 AA sequences
• Deal with lack of affinity by bringing in the substrates to the enzyme, creating an artificially high substrate concentration to work with the high Km of the catalytic site

Question 13

What parts of the phosphotyrosine binding proteins bind to the binding sites?

Answer 13

Src homology 2 domains (SH2)

Question 14

2 types of receptor associated protein tyrosine kinases?

Answer 14

1. Src

2. Jak/STAT

Question 15

To what kind of receptors do receptor associated protein tyrosine kinases bind? 6 examples?

Answer 15

Cytokine receptors

Eg: GH receptor, prolactin receptor, interferon receptor, erythropoietin receptor, G-CSF receptor, IL receptors

Question 16

To what do the receptor associated protein tyrosine kinases bind on the receptors?

Answer 16

Tyrosine kinase binding domain on the cytosolic side

Question 17

Do receptors that receptor associated protein tyrosine kinases bind to also form dimers?

Answer 17

YUP

Question 18

Describe the 4 steps of receptor associated tyrosine kinase activation. (2 options for the first 2 steps)

Answer 18

1. Ligand binding induces receptor dimerization through various mechanisms
2. Dimerization recruits a tyrosine kinase binding to the receptor
   OR
3. Tyrosine kinases already bound to each receptor
4. Ligand binding induces receptor dimerization
5. Tyrosine kinases INTERmolecularly trans-autophosphorylate each other and then the receptor mostly OUTSIDE of the catalytic domain
6. Substrate phosphorylation (same process as receptor tyrosine kinases)

Question 19

What is another name for the JAK kinases?

Answer 19

Janus kinases

Question 20

How many kinase catalytic domains do Janus kinases have? What to note though?

Answer 20

2 (only 1 is particularly active though)

Question 21

Do the receptor associated tyrosine kinases have an SH2 domain? What to note though?

Answer 21

Yes, but that is not what they use to bind the receptor (use dimerization type domains instead)

Question 22

What are the 4 subtypes of JAK kinases?

Answer 22

1. Tyk2
2. Jak1
3. Jak2
4. Jak3

Question 23

What is particular about Jak3?

Answer 23

Tissue specific: expressed in myeloid and lymphoid tissues

Question 24

What does substrate specificity for receptor associated tyrosine kinases depend on?

Answer 24

The nature of the 2 tyrosine kinases in the dimer

Question 25

What is the main difference between the activity of the receptor tyrosine kinases and the receptor associated tyrosine kinases?

Answer 25

The receptor associated ones first trans-auto-phosphorylate each other before trans-auto-phosphorylating the receptor

Question 26

What are STAT proteins?

Answer 26

Signal Transducers and Activators of Transcription

Question 27

How do STAT proteins work? 4 steps

Answer 27

1. SH2 domain containing proteins and are recruited to phosphotyrosines binding sites created by Jaks or srcs
2. Tyrosine kinases phosphorylate the STAT proteins
3. STATs dissociate from the receptor and dimerize via their SH2 domains binding to the phosphotyrosine on the other STAT
4. STAT dimer migrates to the nucleus and other gene regulatory proteins bind to it

Question 28

What are srcs? How do they work?

Answer 28

Same role and functioning as Jak

SH2 domain containing tyrosine kinases that are implicated in many cancers

Question 29

Can Jak and src dimerize together?

Answer 29

YUP

Question 30

Can srcs also recruit STATs?

Answer 30

YUP

Question 31

What is the built in OFF mechanism of the src and JAK signaling pathways? What is this called?

Answer 31

Negative feedback: STAT activates SOCS = Suppressors of Cytokine Signaling = will block further activation at the receptor by either binding at the SH2 binding sites or interfering with phosphorylation

Question 32

What is a built in OFF mechanism of all tyrosine kinases? 2 types? Eg?

Answer 32

Tyrosine phosphatases

2 Types:

1. Bound to the receptor (eg: binding site on platelet derived growth factor receptor for an oncogene called Syp = tyrosine phosphatase)
2. Soluble phosphatase with SH2 binding site to remove phosphates from either the receptor itself, on substrates of tyrosine kinases still bound to the receptor, or substrates of tyrosine kinases that are soluble in the cytoplasm

Question 33

Describe the PI 3 K cascade once activated.

Answer 33

PI3K phosphorylates PIP2 => PIP3 => activated PDK 1 => activates PKB

Question 34

Other than tyrosine kinase, what else can activated PI 3 kinase?

Answer 34

Ras

Question 35

How does STATs dissociating from the receptor and dimerizing via their SH2 domains binding to the phosphotyrosine on the other STAT allows them to go the nucleus?

Answer 35

Dimerization reveals its nuclear localization signals

Question 36

What does trans-autophosphorylation mean?

Answer 36

The phosphorylation by a protein of a residue on an identical protein