Epidermal growth factor receptor and EGFR signaling cascade
Epidermal growth factor receptor is an example for enzyme linked receptor
Other names: ErbB1 and HER1
Structure of EGFR: Transmembrane protein,Consists of 3 major regions
Extracellular domains: Contain binding site for the cognate ligand (EGF) and cysteine rich regions
Intracellular domains: Contain region with tyrosine kinase activity + tyrosine rich cytoplasmic tail
Membrane spanning region: Single alpha-helical region of the peptide strand
EGFR signaling cascade
Step 1: Binding of ligand to EGFR
Ligand for EGFR: Epidermal growth factor
- A polypeptide hormone that stimulates cell proliferation especially of epithelial cells and fibroblast cells by binding to receptor proteins on the cell surface
- EGF consists of a family of peptides, of which EGF and transforming growth factors alpha (TGF-α) are two of the most studied members.
- EGF is produced by cells located in the duodenum, while TGF-α is produced by epithelial cells lining the small intestine.
Step 2: Dimerization of EGF monomers
- Prior to binding with a ligand on the extracellular side, EGFR exists as an enzymatically inactive monomer.
- Ligand binding induce receptor dimerization.
- Upon ligand binding, two monomers come together to form a homodimer and the EGFR is then considered activated.
Step 3: Autophosphorylation of cytosolic receptor tyrosine residues
- EGFR dimerization stimulates its intrinsic intracellular protein-tyrosine kinase activity.
- As a result, autophosphorylation of several tyrosine (Y) residues in the C-terminal domain of EGFR occurs.
- This autophosphorylation elicits downstream activation and signaling by several other proteins that associate with the phosphorylated tyrosines through their own phosphotyrosine-binding SH2 domains.
(Phosphotyrosines on the phosphorylated receptors can act as docking sites for SH2 domains on other proteins)
How is the signal transferred beyond the receptor tyrosine kinase?
4 main downstream signaling pathways are induced by EGFR signaling
1) Ras/MAPK pathway
2) Akt pathway
3) CaM and DAG/IP3 pathway
- The SH2 domains of PLC bind phosphotyrosines in the activated receptor.
- The PH domains of PLC bind phosphoinositides at the plasma membrane (PIP2).
- Once the PLC enzyme is active, it catalyses the hydrolysis of PIP2 to IP3 and DAG (second messengers)
Refer the following link for more details on DAG,IP3 signaling cascade
4) JAK/STAT pathway
The SH2 domain of STAT binds to phosphotyrosines in the activated receptor.
Step 4: Binding of Grb2 (growth factor receptor bound protein) to phosphorylated tyrosine residues
- GRB2 is the cytosolic adaptor protein in EGFR signaling pathway (Adaptor proteins contain protein-binding motifs, which facilitate interactions between protein-binding partners and the generation of bigger signaling complexes).
- The adaptor protein GRB2 binds to specific phosphorylated tyrosine residues on the activated receptor via its SH2 domain.
SH2 domain (Src homology 2): SH2 domains allow proteins containing those domains to dock to phosphorylated tyrosine residues on other proteins.
SH3 domains: domains that bind proline-rich stretches of polypeptide
Step 5: Recruitment of Sos protein to the membrane by Grb2 and coupling of inactive Ras.GDP to activated EGFR
- Since Grb2 contain both SH2 and SH3 domain, it play major role in coupling
- SH3 domain of Grb2 binds cytosolic Sos (GEF) and allows them to bind to the membrane complex (bring them near to its substrate Ras.GDP).
(Ras is tethered to the membrane by a hydrophobic farnesyl anchor)
- When the Grb2-Sos complex docks to phosphorylated EGFR, Sos becomes activated
Step 6: Ras activation by Sos protein
- Sos means Son of sevenless
- Sos is a guanine nucleotide exchange factor for Ras.
- Sos promotes formation of active Ras.GTP by exchanging GDP with GTP.
- The activated Ras-GTP complex then dissociates from Sos.
Step 7: Activation of MAP kinase signaling pathway
The active form of Ras then activates the MAP kinase portion of the signaling pathway