Question

For eạch of the following, indicạte whạt the effect would be on the EFG/Rạs signạling pạthwạy (increase or decrease) and why.

a. A destạbilizing mutạtion in the SH2 domain of Grb2.

b. Overexpression of Rạs associated GAP’s.

c. Introduction of GTPγS (a GTP ạnalog that is difficult to hydrolyze)

d. Overexpression Sos

Answer 1

a) Destạbilizing mutạtion in the SH2 domain of Grb2 will decrease the Ras signalling response. Because, SH2 domain of Grb2 binds to phosphorylated tyrosine residue containing motifs of EGFR and promotes activation of Ras pathway. Destabilizing mutation in SH2 domain inhibits proper binding to specific phospho tyrosine sequences and decreases the Ras signalling.

b) Overexpression of Rạs associated GAP’s will decrease the Ras signalling response. Because, GAPs (GTPase activating proteins) binds to the activated G proteins and promote GTPase activity, there by turns off Ras signalling. Overexpression of GAPs will produce large numbers of GTPase activating proteins that strongly binds to and activates GTPase to inhibit Ras pathway.

c) Introduction of GTPγS will decrease the Ras signalling pathway. Because, many GTPases are inactivated by GTP hydrolysis and resulting binding of GDP. GTPγS is non-hydrolyzable analog that binds strongly and persistently to GTPases and promotes their prolonged activation. This results in turning off or inhibition of Ras signalling.

d) Overexpression Sos results in decrease in Ras pathway. Sos actually helps to exchange nucleotide binding in GTPases, thereby facilitates release of inhibiting GDP and binding of GTP to GTPases. This increases GTPase activity and inhibits Ras pathway.