MCBL: Cell Signaling & Communication II

Question 1

How many classes of enzyme-linked receptors are there and what are their properties?

Answer 1

1) Tyrosine kinase - Have tyrosine kinase activity
2) Tyrosine kinase associated receptors - Associate with tyrosine kinases
3) Serine/threonine kinases - Serine/threonine kinase activity
4) Guanylyl cyclases - Guanylyl cyclase activity
5) Histidine kinase associated receptors - Activate a two component signaling system in which the kinase phosphorylates its own histidine and then transfers the same phosphate to a 2nd intracellular signaling molecule
6) Receptor-like tyrosine phosphatases - Remove phosphate groups from tyrosines on specific intracellular signaling proteins. Ligands for these have not been identified.

Question 2

How many genes code for receptor tyrosine kinases?

Answer 2

60 human genes

Question 3

What are the signal molecules for receptor tyrosine kinases?

Answer 3

Proteins of the growth factor and hormone families

Question 4

What activates tyrosine kinase receptors?

Answer 4

Dimerization activates the tyrosine kinase receptors.

When ligands bind, the receptor dimerizes and they receptor than autophosphorylates on the tyrosine residues.

Phosphorylation within the kinase domain will enhance receptor activity

However, if there is a mutant receptor with an inactive kinase domain, the receptor can not autophosphorylate though it can dimerize. This blocks receptor activity.

Question 5

Picture of inactive tyrosine kinase receptor.

Answer 5

Question 6

Picture of active tyrosine kinase receptor.

Answer 6

Question 7

What do phosphorylated tyrosine residues on the tyrosine kinase receptor serve as?

Answer 7

These phosphorylated residues on the tyrosine kinase receptor serve as docking sites for other intracellular signaling proteins.

Question 8

Picture of intracellular signaling proteins bound to phosphorylated tyrosines on the tyrosine kinase receptor.

Answer 8

The binding of these additional cell signaling proteins allows cell signaling to be broadcast on multiple pathways.

Question 9

True or false: Proteins with SH2 domains bind to phosphorylated tyrosines on tyrosine kinase receptors.

Answer 9

True; proteins with Sh2 domains (phosphotyrosine binding domains or PDBs) do bind at specific phosphorylated tyrosine residues on the receptor.

Question 10

Picture of protein with Sh2 domain binding to phosphorylated tyrosine receptor.

Answer 10

Question 11

What type of protein is Ras?

Answer 11

Ras is a member of a large family of monomeric GTPases.

Question 12

How is Ras activated?

Answer 12

Receptor tyrosine kinases activate Ras via the activities of an adaptor protein that has its Sh2 binding region bound to the receptor and its Sh3 domanin bound to a Ras-GEF

Ras is bound by GDP in its inactive form

Ras-GEF exchanges GDP for GTP on the Ras protein

Ras-GTP is now active and it causes some downstream signal cascade

Question 13

Picture showing Ras and GDP/GTP.

Answer 13

Question 14

Picture of Ras activation by tyrosine kinase receptor.

Answer 14

Question 15

How does Ras activate MAP kinase?

Answer 15

Ras activates MAP kinase via the phosphorylation of threonine and tyrosine

MAPK then phosphorylates other proteins

MAPK-induced genes are called immediate early genes

NOTE: Ras - MAPK mediated responses are short-lived and are terminated by dephosphorylation by dual-specificity phosphatases

Question 16

Picture of Ras activation of MAP Kinase.

Answer 16

Question 17

Another picture showing the activation of MAP kinase by Ras.

Answer 17

Question 18

What are scaffold proteins used for in relation to MAP kinase?

Answer 18

Scaffold proteins help to prevent cross-talk between parallel MAP kinase modules.

Question 19

Picture of MAP Kinase bound to scaffold proteins.

Answer 19

Question 20

What do Rho family GTPases do?

Answer 20

The Rho family of GTPases regulate the cytoskeleton.

These proteins are monomeric.

Inactive Rho is bound to guanine nucleotide dissociation inhibitors in the cytosol.

Rho-GEF is stimulated by receptor tyrosine kinases. This leads to the activation of Rho proteins.

Question 21

How is the inositol phospholipid docking site generated?

Answer 21

Phosphatdylinositol 3-kinase (PI 3K) phosphorylates inositol phospholipids

Activated by receptor tyrosine kinases, GPCRs

PI#K catalyzes the phosphorylation of inositol phospholipids at the 3 position of the inositol ring This becomes PIP3 and serves as the docking site and brings proteins to the membrane.

Proteins bind to PIP3 via the Pleckstrin homology domain

Question 22

Picture of the PI3-Kinase-AKT signaling pathway.

Answer 22

Pathway is involved in blocking apoptosis and promotes cell survival.

Question 23

True or false; Downstream pathways activated by receptor tyrosine kinases and GPCRs overlap.

Answer 23

True; there is a fair amount of overlap in these downstream signaling pathways.

Question 24

How do tyrosine kinase-associated receptors respond?

Answer 24

These receptors respond in much the same way that tryosine kinase receptors respond. However, they depend on a seperated protein for tyrosine kinase activity.

Two major families:

1) Src family - All contain Sh2 and Sh3 domains on the cytoplasmic side of the membrane. Can bind to activated tyrosine kinase receptors in a mutually agonistic way. To strengthen and prolong signals.
2) Focal adhesion kinases (FAKs) - Bind the cytosolic tail of integrins and bring about adhesion. FAKs cross phosphorylate one another and can also bind and phosphorylate Src.

Question 25

What kind of receptors are cytokine receptors?

Answer 25

These are tyrosine kinases-associated receptors.

They are associated with Janus kinases (Jaks) which are tyrosine kinases.

Jaks phosphorylate & activate genes called STATs.

STATs have an Sh2 domain that allows binding to the phosphotyrosines on the cytokine receptor and to each other

STATs then move to the nucleus to stimulate gene transcription

More than 30 cytokines activate the Jak-STAT pathway

Question 26

Picture showing tyrosine kinase-associated receptor, Jak-STAT pathway.

Answer 26

Question 27

TGF beta receptor serine/threonine kinase SMAD pathway picture.

Answer 27

Transforming growth factor beta (TGF-Beta) uses two classes of receptor serine/threonine kinases (Type I & II)

Ligand first binds & activatesto a Type II receptor

This results in the recruitment, phosphorylation and activation of a Type I receptor

Type I receptor then phosphorylates a latent gene regulatory SMAD protein

Once phosphorylated, SMAD proteins complex and move into the nucleus to activate a set of genes on DNA

Question 28

Describe receptor guanylyl cyclases.

Answer 28

These receptors are single-pass transmembrane proteins with an extracellular binding site and an intracellular guanylyl cyclase catalytic domain

Activated cyclase produces cGMP which then activates a cGMP dependent protein kinase (PKG)

PKG phosphorylates specific proteins on serines or threonines

The ligands for these receptors are natriuretic peptides