Your friend is trying to create an in vitro system for studying the formation of COPII vesicles. He finds that to get efficient 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. Explain why your friend needs to add GMP-PNP instead of GTP.