Binding of β/δ-COP dimer to Golgi membranes. Native coatomer or isolated β/δ-COP dimer (Fig. 5, fraction 14) after dialysis against buffer pH 6.7 was incubated with ARF1 without membranes in the presence of GTP[γS] or with ARF1 and Golgi membranes in the presence of GDP[βS] or GTP[γS]. After incubation the samples were layered on a sucrose cushion and centrifuged. The supernatant was removed and the precipitate was analyzed by SDS/PAGE on 7.5–16.5% gradient gels and Western blotting with antibodies against β-, α-, β′-, γ-, and δ-COPs. As a control Golgi membranes were incubated with ARF1 and GTP[γS]. Lane i contains 20% of the sample.