You are studying the activity of the G-protein RAS in a cell line derived from a colonic adenoma. You find that the signal-transduction pathway induced by the activation of RAS is continuously active in these cells and that this correlates to a mutation in the RAS gene. This mutation in RAS most likely results in the inhibition of which of the following activities of this G-protein?
A) GTPase activity
B) guanylate cyclase activity
C) guanine nucleotide binding
D) phosphoprotein phosphatase activity
E) protein kinase activity

Question

Quizplus · Accepted Answer

The mutation in the RAS gene likely inhibits its GTPase activity, preventing RAS from hydrolyzing GTP to GDP, thus keeping it in an active state and continuously signaling.  The activity state of G-proteins is regulated both by the rate of GTP exchange for GDP and by the rate at which the GTP is hydrolyzed to GDP. The activity of G-proteins with respect to GTP hydrolysis is regulated by a family of proteins termed GTPase-activating proteins, GAPs. The inability to hydrolyze GTP bound to a G-protein, such as RAS, results in constant activation of the G-protein leading to inappropriate continuous activation of the growth signals transmitted by that G-protein. This can result in aberrant cell growth associated with cancer.

You Are Studying the Activity of the G-Protein RAS in a Cell