Yeast COPII proteins were expressed and purified as previously described22526272829. The H77L mutation was introduced into Sar1p by site-directed mutagenesis with Pfu polymerase (Stratagene) using the coding sequence of Sar1p in the vector pTY4028. Correct DNA sequences were confirmed by sequencing. Green fluorescent Sec13/31 protein was produced by reacting purified Sec13/31p with a 20-fold excess of Alexa 488 maleimide (Invitrogen), followed by purification on a size exclusion column. Alexa 488 labeled Sec13/31p was mixed at 40∶60 molar ratio with unlabeled Sec13/31p.