Q. Your friend is trying to generate an in vitro system for studying the formation of COPII vesicles. He finds that to get well-organized vesicle budding from artificial membranes, he needs to add the coat proteins, the Sar1 GTP-binding protein, the GTPase-activating protein (GAP) for Sar1, and GMP-PNP (a nonhydrolyzable analog of GTP) to synthetic membranes; his experiment will not work in the presence of GTP. Give details why your friend needs to add GMP-PMP instead of GTP.