Question

ORDER the following events during COPII vesicle formation?
- Sec24 binds cargo.
- Sar1 inserts its amphipathic helix into the cytoplasmic leaflet of the ER.

- Sec12 GEF binds Sari.
- Sec 13/31 forms outer coat.

-Sar1 exchanges GDP for GTP.

Answer 1

Ans) 1) Sec12 GEF binds Sar1

2) Sar1 exchanges GDP to GTP

3) Sar1 inserts its amphipathic helix into the cytoplasmic leaflet of the ER.

4)Sec 24 binds cargo

5) Sec 13/31 forms outer coat.

(The Sec12 is an ER integral membrane protein which will bind to Sar1-GDP.Sec12 is GEF which converts Sar1-GDP to Sar-1-GTP. In GTP form the hydrophobic N terminus is extended outwards and thus inserts into the amphipathic helix into the cytoplasmic leaflet of the ER.This serves as binding site for Sec23/24 coat protein complex and then the cargo proteins binds to this sec23/24.The next step is formation of outer coat by Sec13/31and after that the Sec23 will stimulate GTP hydrolysis).