What Happens When Ran GTPase Binds Tightly to GEF?

[physicisttobe]

Homework Statement

Questions about Ran GTPase

Relevant Equations

...

Hi everyone!

There is a question that I can't solve.
Ran GTPase controls nuclear import of proteins and is present at a much higher concentration than its GAP (GTPase activating protein) and GEF (guanine nucleotide exchange factor) proteins. Imagine a mutation in the Ran GTPase resulting in an extremely tight binding between GTPase and its GEF and a very slow dissociation. What happens as a result?
The answer to this question is: GDP, because the tightly bound GEF's will be unavailable to act on further Ran molecules.

But why GTP? Isn't it GDP? I mean, there is a binding between GDP and GEF, and this binding forms GTP+GEF so that GTP can bind to another cargo protein (NLS). I do not understand this. Could you explain me that?

 

[TeethWhitener]

The answer to this question is: GDP, because the tightly bound GEF's will be unavailable to act on further Ran molecules.

But why GTP? Isn't it GDP?

I'm assuming there's a typo here? Is the answer GTP or GDP? Do you think it should be GTP or GDP? The way you've written this is very confusing.

 

[TeethWhitener]

To actually answer your question, I think you may have a few misconceptions about the Ran cycle.

Homework Statement:: Questions about Ran GTPase
Relevant Equations:: ...

there is a binding between GDP and GEF, and this binding forms GTP+GEF so that GTP can bind to another cargo protein (NLS).

GEF does not bind GDP. GEF binds to the RanGDP complex (which itself consists of Ran bound to a GDP), allowing the release of the GDP molecule and the uptake of a GTP molecule to form a RanGTP-GEF complex. Normally, GEF binds much less tightly to RanGTP than RanGDP. This allows GEF to dissociate from RanGTP and lets uncomplexed RanGTP escape the nucleus and go back into the cytoplasm where it's acted on by GAP, hydrolyzing the bound GTP to GDP and regenerating the RanGDP, setting up the full Ran cycle. However, in your homework question, you're asked to consider a situation where Ran is mutated such that the RanGTP-GEF complex does not dissociate. Can you proceed from here?

 

[physicisttobe]

Unfortunately, I didn't understand it. Is the answer GDP, because the tightly bound GEFs will be unavailable to act on further Ran molecules.

And i had a typo, i mean why GDP? Isn't GTP? But the solution "GDP, because..." is correct, right?

 

[physicisttobe]

And they were several answers like
1) GTP, because the Ran-GAP protein is activated.
2) GDP, because the Ran-GAP protein is activated.
3) GDP, because the tightly bound GEFs will be unavailable to act on further Ran molecules.
molecules.
4) GTP, because the tightly bound GEFs can no longer act on further Ran molecules.
5) GDP, because fewer GAP molecules are available.

So my colleague told me that the right answer is: GDP, because the tightly bound GEF's will be unavailable to act on further Ran molecules. But I'm not sure if this is correct because I can't imagine what it is about. Right now I have some understanding problems.

 

[TeethWhitener]

I'm still having a very hard time understanding what you're saying. GTP and RanGTP are not the same thing. GTP is a small molecule that can lose a phosphate group to form GDP. RanGTP is a complex of the Ran protein and GTP. You've been using the two interchangeably and I can't tell whether you mean GDP/GTP or RanGDP/RanGTP.

Because of this, it's possible that your colleague might mean RanGDP (not GDP) and you mean GTP (not RanGTP) or vice versa. Let's make sure the terminology is clear first before we try to work on the actual question further.

 

[physicisttobe]

All right. Now I know the differences between GTP and RanGTP, but my colleague told me that the right answer is "GDP, because the tightly bound GEF's will be unavailable to act on further Ran molecules".
However, I can't understand the solution. What do you think? Is this third phrase correct?

 

[TeethWhitener]

If your colleague means GDP and not RanGDP, then I think they are incorrect. However, the question itself is worded very weirdly. Here’s what happens:

Once GEF is used up, Ran can no longer exchange GDP for GTP. Once all the Ran-bound GTP is converted to RanGDP by GAP (or slowly without it), then you’re just stuck with Ran-bound GDP which can’t pick up GTP for further conversion. How that corresponds to the possible answers you gave, I’m not really sure. RanGDP will build up, but there won’t be active conversion of GTP to GDP, and GDP won’t be freed from Ran at an appreciable rate.