Most GTPases are present inside the cell at a much higher concentration than their upstream GAP and GEF proteins. Imagine a mutation in a certain GTPase, such as a Rab protein, resulting in an extremely tight binding between the GTPase and its GEF, and a very slow dissociation. What would you expect to happen in the cell as a result?
A) Rab proteins will be activated, because the tightly bound GEFs will be unavailable.
B) Rab proteins will be inactivated, because the tightly bound GEFs will be unavailable.
C) Rab proteins will be activated, because the Rab-GAPs will become activated.
D) Rab proteins will be inactivated, because their GAPs will become activated.
E) Rab proteins will be activated, because there are fewer GAPs available.

Question

Quizplus · Accepted Answer

The tight binding to the mutant titrates out the available GEFs (guanine nucleotide exchange factors), which are not as abundant as the GTPase itself. Therefore, not enough GEF will be available to activate the rest of the GTPase molecules; thus, compared to the normal situation, the GTPase molecules will become inactivated, with GDP remaining bound.

Most GTPases Are Present Inside the Cell at a Much