Question

In neurons, treatment with nerve growth factor (NGF) increases cell survival, which is mediated by activation of the Ras/Raf/Erk pathway. To further determine the role of the Ras/Raf/Erk pathway, you decided to over-express a Grb mutant that does not have SH2 domain.

Answer the following questions.

A. What is Grb? Describe its role in activation of the Ras/Raf/Erk pathway.

B. What is SH2 domain?

C. What effect would over-expression of the Grb mutant have on NGF-induced cell survival? Explain your answer.

Answer 1

A) GRB is growth factor receptor bound protein. It has one SH2 domain and two SH3 domains. It acts as a linking protein between the receptor kinases and Ras. When a growth ligand (like NGF ) binds to NGFR, a tyrosine kinase receptor, the receptor autophopshorylates it cytosolic domain, the the phosphorylated tyrosine residues recruits Grb. Grb binds the tyrosine using it's SH2 domains and the two SH3 domains , guanine exchange nucleotide factor (SOS) which then binds to RAS protein , removes GDP to GTP and activates Ras, Ras then further activates the signal Cascade leading to expression.

2) Src homology domain 2 is a protein domain which binds to phosphorylated tyrosines in protein domains. It is composed of a beta sheet flanked by two alpha helices. PhospjPhosphoryof tyrosines act as a switch to recruit proteins with Src 2 domain.

3) NGF induced cell signalling won't happen. Grb2 without SH2 domain cannot bind with phosphorylated tyrosines residues in the receptor and thus cannot activate Ras protein for signal transduction.