III. Signal transduction and cell cycle | 44. Structure and function of tyrosine kinase receptors, the Erkl / Erk2 MAP kinase cascade

Question 1

I. Basics
1. What are Tyrosine kinase receptors (RTKs)?

Answer 1

• Tyrosine kinase receptors (RTKs) are transmembrane receptors which bind and respond to growth factors (EGF, NGF) and other locally released proteins that are present at low concentrations.
• These receptors play important roles in the regulation of cell growth, differentiation and survival.

Question 2

I. General features of the RTK
2. What are General features of Tyrosine kinase receptors (RTKs) ?

Answer 2

• One extracellular ligand-binding domain
• One hydrophobic transmembrane domain
• Intracellular part: tyrosine kinase domain with enzymatic activity. This can phosphorylate tyrosine side chains of their substrates. Can also catalyze receptor auto-phosphorylation
• C-terminal with tyrosine

Question 3

II. Receptor activation
1. Describe the steps of Receptor activation?

Answer 3

• In the absence of extracellular signals, the RTKs are not dimerized and exist separately as monomers
• Upon binding of an extracellular ligand, the two monomers go together and form a dimer
• The two RTKs will then phosphorylate each other (auto-phosphorylation)
• The receptor will get activated, since now thetyrosine residues (on the intracellular surface) are phosphorylated
• These tyrosine residues will serve as ‘’docking sites’’ for intracellular proteins, and by this the RTKs can bind several signaling molecules, creating a signaling cascade

Question 4

II. Receptor activation
2. The intracellular signaling proteins that bind to the phosphorylated tyrosines have different structures and functions
=> T/F?

Answer 4

TRUE!!!!

Question 5

II. Receptor activation
3. The intracellular signaling proteins that bind to the phosphorylated tyrosines have different structures and functions
=> Explain

Answer 5

• The intracellular signaling proteins that bind to the phosphorylated tyrosines have different structures and functions.
• These proteins can be either SH2 (Src homology 2 – bind to phosphorylated tyrosine) domains or PTB (phosphotyrosine-binding) domains.
• Proteins with SH2 domain can be either enzymes (PLCγ or PI 3-kinase) or adaptor proteins without enzymatic activity (Grb2 – growth factor receptor-bound protein 2).

Question 6

III. Activation of Ras by growth factors
1. What does the Ras superfamily consist of?

Answer 6

The Ras superfamily consists of different families of monomeric GTPases.

Question 7

III. Activation of Ras by growth factors
2. How do we activate Ras by growth factor?

Answer 7

The Ras superfamily consists of different families of monomeric GTPases. By interacting with different intracellular signal proteins, a single Ras protein can spread the signal along several downstream signaling pathways. In inactive form, Ras is GDP- bound and GTP-bound in active form
- The Grb2 is an adaptor protein which contains a SH2 domain that can bind to phosphorylated tyrosine residues. Grb2 also has a SH3 domain which binds to proline-rich motifs
- Grb2 will through SH3 bind to the SOS protein and this Grb2-SOS complex will then bind to the tyrosine residues (via the SH2)
- SOS is a proline-rich guanine nucleotide exchange factor (GEF) and will activate Ras by exchanging GDP with GTP

Question 8

IV. Ras effects
1. What are Ras effects?

Answer 8

To stimulate cells to proliferate or differentiate, the short-lived Ras signaling events must be converted into longer-lasting ones that can sustain the signal and relay it downstream to the nucleus to alter the pattern of gene expression.

Question 9

IV. Ras effects
2. How do we achieve Ras effects?

Answer 9

One of the key mechanisms used is a system of proteins known as the (Erk1/2) MAP kinase cascade (mitogen-activated protein kinase cascade).
- The GTP-bound Ras activates Raf, the first serine-threonine kinase (MAP3K) in the cascade
- Each of the 3 kinases in the cascade then activate the next by phosphorylating it
- Since all the 3 kinases in this pathway phosphorylate multiple substrates, the initial signal is amplified at each step (MAP3K -> MAP2K -> MAPK/Erk)
- Then, the final enzyme in the pathway phosphorylates transcription factors which
can stimulate transcription of a set of genes needed for transcription
=> This can accelerate translation and start growth and cell cycle