Receptor Tyrosine Kinases

Question 1

Functions that RTKs tend to regulate

Answer 1

Growth, proliferation, differentiation, apoptosis

Question 2

RTK ligands are usually. . .

Answer 2

proteins

Question 3

GPCR ligands are usually. . .

Answer 3

small molecules

Question 4

Basic structure of a receptor tyrosine kinase

Answer 4

Extracellular ligand binding domain

Transmembrane helix

Intracellular kinase domain

Question 5

How is the signal of ligand binding tranduced across the membrane by RTKs?

Answer 5

By dimerization and transphosphorylation of the cytoplasmic kinase domains.

Note that this means, practically by definition, that substantial upregulation of these domains will lead to ligand-independent activation.

Question 6

RTK dimerization loop

Answer 6

Domains on the extracellular side of the protein that dimerize to facilitate bringing together the intracellular domains of the RTKs.

Question 7

Insulin receptor family-RTKs are. . .

Answer 7

pre-dimerized on the extracellular side

Binding just moves the cytoplasmic regions closer together so that they may interact by making the angle between them more acute.

Question 8

FGFR family-RTKs. . .

Answer 8

multimerize along a carbohydrate chain (specifically heparan sulfate)

This heparan sulfate is often attached to a protein forming heparan sulfate proteoglycan (HSPG). This also serves to localize the signal.

Question 9

Difference between the terms “cross-phosphorylation” and “auto-phosphorylation”

Answer 9

“cross-phosphorylation” refers specifically to the phosphorylation of one receptor by another

Autophosphorylation may refer to self phosphorylation or to cross-phosphorylation.

Question 10

Activation loops on RTKs

Answer 10

Question 11

How does cross-phosphorylation lead to downstream effects?

Answer 11

Cross-phosphorylation enables the kinase domains to phosphorylate multiple sites on the juxtamembrane domain of the receptor. These sites, when phosphorylated, serve as docking sites for other proteins, such as GRB2. The active kinase domains may also, of course, phosphorylate other proteins to propagate the signal, and some of this activity is mediated by these docking site which serve a recruitment role for substrates.

Question 12

Common domains in RTK signal propagation

Answer 12

Question 13

Although all SH2 domains recognize phosphotyrosine, . . .

Answer 13

they also recognize adjacent amino acids.

This is true for almost all of these binding domains, they are slightly different in order to control specificity.

Question 14

Ras

Answer 14

Question 15

Grb2

Answer 15

Question 16

MAPK Cascade

Answer 16

“Mitagen Associated Protein Kinase”

Question 17

KSR Scaffold

Answer 17

Question 18

MAPK Cascade downstream effects

Answer 18

Question 19

Ras is a _____ G protein.

Answer 19

monomeric

Question 20

PI3K recruitment

Answer 20

Question 21

Reaction Catalyzed by PI3K

Answer 21

Question 22

PI3K Activation of Akt/PKB

Answer 22

Question 23

PDK1

Answer 23

Recruited to the PM by PIP₃

Phosphorylates and stably activates Akt when it is brought into proximity. Akt is then active and may diffuse into the cell to exert its effects.

Question 24

Major Akt targets

Answer 24

BAD and Caspase 9 are proteins that mediate apoptosis, otherwise known as programmed cell death

FOXO is a transcription factor that upregulates expression of pro-apoptotic genes.

Inhibition of certain GAPs leads to increased GLUT4 insertion into the membrane and thus increased glucose uptake.

Question 25

mTOR

Answer 25

mTOR is a regulator of protein synthesis. The best characterized way it does that is by **phosphorylating a protein called 4E-BP**. In its unphosphorylated state, 4E-BP binds and inhibits the initiation factor eIF4E (an initiation factor that binds the mRNA cap). Because 4EBP inhibits eIF4E, and mTOR inhibits 4E-BP, the net effect is that **mTOR promotes translation**.

Question 26

AMP Kinase

Answer 26

AMP kinase responds to the AMP/ATP ratio in the cell. When the **AMP/ATP ratio is high**, that activates AMP kinase, which **phosphorylates and activates TSC2**. Since **TSC2 is a GAP for Rheb**, that **inactivates Rheb**, and the net effect is to inhibit translation when energy stores are low: **if the cell is running out of energy, it doesn’t want to be using the large amount of energy needed to make a protein.** Note that phosphorylation of TSC2 by AMPK activates it, whereas phosphorylation by AKT at different sites inactivates it, and that’s how these two inputs can have opposite effects on translation.

Question 27

Turing off RTKs

Answer 27

Question 28

Protein Tyrosine Phosphatases (PTPs)

Answer 28

Question 29

PTEN

Answer 29

Dephosphorylates PIP₃

Question 30

HER-family RTKs

Answer 30

Her1 = EGFR

Question 31

Her2

Answer 31

Her2 has **no known ligand**. However, **even without a ligand Her2 is in a conformation that allows it to dimerize**: either homodimerizing with itself, or by forming heterodimers with other members of the Her family. Its activation loop doesn’t need to be phosphorylated to be in the open state: **kinase domain dimerization triggers a conformational change that is sufficient to open up the loop and make the kinase catalytically active**

Question 32

Her3

Answer 32

Her3 has an inactive kinase. It actually does have a full length intracellular region, but the kinase domain lacks catalytic activity