Question

When the fungal metabolite brefeldin A is added to cells, the Golgi apparatus largely disappears and the Golgi proteins intermix with those in the ER. Brefeldin A treatment also causes the rapid dissociation of some Golgi-associated peripheral membrane proteins, including subunits of the COPI coat. This implies that brefeldin A prevents transport involving COPI-coated vesicles by blocking the assembly of coats and thus the budding of transport vesicles. In principle, brefeldin A could block formation of COPI-coated vesicles at any point in the normal scheme for assembly, which is shown in the figure below. The following observations identify the point of action of brefeldin A.

1. ARF with bound GTPS (a nonhydrolyzable analog of GTP) causes COPI coated vesicles to form when added to Golgi membranes. Formation of vesicles in this way is not affected by brefeldin A.
2. ARF with bound GDP exchanges GDP for GTP when added to Golgi membranes. Trypsin-treated (protease) Golgi membranes do not stimulate GTP-for GDP exchange. The exchange reaction with normal Golgi membranes does not occur in the presence of brefeldin A.
3. ARF with bound GTP can be made to exchange GDP for GTP in the absence of Golgi membranes by treatment with phosphatidylcholine and cholate. This artificial exchange is not affected by brefeldin A.

GTP ARF-GDP (cytosol) COPI ARF-GTP (membrane) COPI-coated vesicles GDP GEF-catalyzed

Given these experimental observations, how (at the molecular level) do you think brefeldin A blocks formation of COPI-coated vesicles?

Answer 1

Here ARF act as a molecular switch ,

It is present in two form ARF GDP - soluble form

AFT GTP - membrane bound form

This membrane biund form, insert in to membrane and Crest a ficalmpoint where rest of the coated protein cargo molecules binds and form cop vesicles . So insertion of ARF GTP to membrane is essential for formation of vesicles. This exchange of GDP to GTP is done with help of GEF ( guanine exchange factor). With GEF , no formation of ARF GTP and no vesicles formation.

So this brefeldin A inhibits GEF , thus prevent formation of ARF GTP and its insertion in to membrane and prevent COP vesicles formation.

Hope this will help