21 - RTKs and MAP Kinase Signalling

Question 1

MAP kinase signalling is a _______ component signalling cascade

Answer 1

MAP kinase signalling is a three component signalling cascade involving MAPKKK, MAPKK, and MAPK

Question 2

What is MAP kinase coupled to?

Answer 2

This is couple to RTK activity is by the Grb2SOS complex which activates the oncogene Ras

Question 3

What is Grb2 recruited to?

Answer 3

So Grb2 is recruited to phosphorylated tyrosine receptors
o Grb2 is linked to SOS
o SOS causes exchange of GDP on Ras to GTP and in its active form it can trigger MAPK

Question 4

What are the components of the MAP kinase pathway like in a RESTING STATE?

Answer 4

In the resting state the components of the pathway are in a dissociated form so we have a scaffold protein which is separate from the RAF, MEK, and ERK kinases and Grb2 and SOS are separate as well
o Following ligand binding to RTKs, leads to recruitment of the individual components and the assembly of the signalling cascade uses the scaffolding proteins

Question 5

What does mitogenic signalling trigger?

Answer 5

Mitogenic signalling triggers activation of the ERMAP kinase pathway by recruiting and activating Ras at the cell membrane
- this leads to activation of Raf (a Ser/Thr kinase) which activates MEK (a dual specificity kinase)
- leading to phosphorylation of Erk, and then Erk can phosphorylate transcription factors

Question 6

What has been discovered since the discovery of the three major MAPK pathways

Answer 6

Since the discovery of these three major MAP kinase pathways sequence homology lead to the identification of further MAP kinase pathways in humans
* 16 different MAPKKK, Raf being the first, 9 MAPKK, and MAPK including ERK in humans
* Also some found in yeast and plants
* Yeast was the first model organism that lead to the discovery in humans

Question 7

MAPK discovery in yeast

Answer 7

When they were first discovered in yeast, three major pathways were discovered: a pheromone pathway a starvation pathway, and a high osmolarity pathway
* the pathways are involved in adaptation to these different responses, again with MAPKKK, MAPKK, and MAPK
* MAPK pathway creates an amplification effect

Question 8

How is the MAPK pathway assembled?

Answer 8

• MAPK pathway is assembled using scaffold proteins
• There’s a protein called Ste5 and this causes assembly of the MAPKKK next to the MAPKK

Question 9

Why is scaffolding important in the MAPK pathway?

Answer 9

• The scaffolding is important is it increases the specificity of the MAPKKK for the MAPKK and phosphorylation of the MAPK
• Since the discovery of yeast scaffold proteins, scaffold proteins have been discovered in humans so we have a protein that assembled the junk pathway called KIP, and the KSR is involved in the ERKMAP kinase pathway

Question 10

How is further specificity Brough about in the MAPK pathway?

Answer 10

• Further specificity is brought about by determining what the active MAPK can phosphorylate and activate
• So in the case of ErkMAP kinase as part of its amino acid sequene it will recognise docking domains on target proteins

Question 11

What do targets for MAPK phosphorylation have?

Answer 11

o Targets for MAPK phosphorylation have a special sequence of amino acids which allow the MAPK to dock with it and once it is docked it can then phoshorykate and activate it
o There is a specific proline rich recognition sequence for MAPKs called proline directed kinases so MAPK will only phosphorylate Ser/Thr that have proline next to it

Question 12

What happens in the absence of a docking domain in the target protein?

Answer 12

In the absence of a docking domain in the target protein, scaffold proteins can be used to link the MAP kinase once its activated to phosphorylate/ activate/ regulate its substrates

Question 13

How can a further level of control of MAP kinase signalling be introduced?

Answer 13

A further level of control is by limiting the subcellular distribution of MAP kinase
o E.g. the ERK MAP kinase pathway

Question 14

How does the phosphorylation of Erk come about?

Answer 14

Raf phosphorylates MEK, MEK phosphorylates Erk- this occurs in the cytoplasm
o However many of the substrates for Erk are found in the nucleus because they are transcription factors so the Erk will enter the nuclear envelope through nuclear pore
o Once in the nucleus it will interact with binding proteins which will allow it to phosphorylate target transcription factors which have docking domains or will use the scaffold proteins

Question 15

What was the first example of a transcription factor that is phosphorylated by MAPK?

Answer 15

The first example of a transcription factor that is phosphorylated by MAPK was the ELK transcription factor which undergoes multiple phosphorylation’s in response to ERK activation

Question 16

What happens during the phosphorylation of ELK?

Answer 16

• This occurs in the nucleus
• The phosphorylation of Elk starts at the EGF receptor leading to a signal to the nucleus controlling changes in gene expression leading to cell proliferation
• It then interacts with sequences of DNA called the serum response element, recruits other transcription factors , leading to the induction of many genes, including cfos