COATOMER, ARF1P, AND NUCLEOTIDE ARE REQUIRED TO BUD COAT PROTEIN COMPLEX I-COATED VESICLES FROM LARGE SYNTHETIC LIPOSOMES

Synthetic coat protein complex I (COPI)coated vesicles form spontaneou sly from large (approximate to 300 nm in diameter), chemically defined liposomes incubated with coatomer, Arf1p, and guanosine 5'-[gamma-thi o]triphosphate. Coated vesicles are 40-70 nm in diameter, approximatel y the size of COPI vesicles formed from native membranes. The formatio n of COPI-coated buds and vesicles and the binding of Arf1p to donor l iposomes depends on guanosine 5'-[gamma-thio]triphosphate. In contrast to the behavior of the COPII coat, coatomer binds to liposomes contai ning a variety of charged or neutral phospholipids. However, the forma tion of COPI buds and vesicles is stimulated by acidic phospholipids. In the absence of Arf1p, coatomer binds to liposomes containing dioleo ylphosphatidic acid as a sole acidic phospholipid to form large coated surfaces without forming COPI-coated buds or vesicles. We conclude th at Arf1p-GTP and coatomer comprise the minimum apparatus necessary to create a COPI-coated vesicle.