More vesicles bud from liposomes when Sec16p is present (bottom).
The protein in question, yeast Sec16p, is an ER resident required in vivo for COPII-dependent vesicle budding. In vitro, Sec16p is not necessary for budding from liposomes reconstituted with pure cytosolic COPII proteins. However, this in vitro reaction depends on a nonhydrolyzable form of GTP, probably because the COPII coat falls apart when Sar1p (the initiator of coat assembly) hydrolyzes GTP. Until now, the function of Sec16p in liposome budding could not be tested, because the protein was difficult to purify
Supek et al. report conditions that stabilize Sec16p and have purified enough protein for in vitro studies. Microsomal membranes stripped of endogenous Sec16p were stimulated in vesicle budding by the purified....
