Question

Imagine that Arf1 protein was mutated so that it could not hydrolyze GTP, regardless of its binding partners. Would you expect COPI-coated vesicles to form normally? How would you expect transport mediated by COPI-coated vesicles to be affected? If this were the only form of Arf1 in a cell, would you expect it to be lethal? Briefly explain your answers.

Answer 1

Part 1) Would you expect COPI-coated vesicles to form?

Hydrolysis of GTP is an important step in the release of transportation vehicles. But on the other hand, GTP hydrolysis isn't required for the formation of vesicles.

What this mean is that, if ARF is bound to GTP (which is independent of its hydrolysis) it will not hydrolyse the GTP. When ARF hydrolyse the GTP it becomes GDP and ARF-GDP is an inactive form of ARF. Now since it's bound to ARF-GTP and it can not hydrolyse it, theoretically it will be activated all the time.

Activation of ARF by GTP initiate the formation of COPI coated vesicles. So inability of ARF to hydrolyse the GTP will not affect the formation of COPI coated vesicles.

To answer the question, COPI coated vesicles will form in this cell but ARF will be activated all the time, so vesicle formation will be higher than normal.

Part 2) How would you expect transport mediated by COPI coated vesicles to be affected

Now, Hydrolysis is the most important factor for the release of COPI coated vesicles from the membrane. Since this ARF can not hydrolyse the GTP, it will not stimulate the release of the vesicle from the membrane and the transport will be affected as no protein will be transported from the Golgi network.

Part 3)

Yes, this mutation will be lethal if there's only one copy of ARF in that cell.

Transportation of proteins is necessary for the cell survival as even though proteins will be synthesised by the cell, they will not reach their destination, i.e, Plasma membrane or Endoplasmic reticulum and this will effect the cell as these proteins are involved in every function of the cells. So in simpler terms, there will be no expression of those proteins which are transported by COPI coated vesicles and this may result in the death of the cell as some of those proteins maybe vital for the cell survival.