Purification of three related peripheral membrane proteins needed for vesicular transport.

We report conditions under which Golgi membranes depleted of peripheral membrane proteins can be reconstituted for intra-cisternal vesicular transport. Analysis of the reconstitution reveals requirements for N-ethylmaleimide-sensitive fusion protein, a purified peripheral protein involved in the fusion stage of vesicular transport, as well as other peripheral protein activities which can be provided by mammalian cytosol but not yeast cytosol. The restorative activity in bovine brain cytosol is found in two broad and complementing fractions, of average native molecular masses of about 500 and 40 kDa, termed Fr1 and Fr2, respectively. This resolved transport system was used to develop a purification scheme for Fr2. Three proteins of apparent molecular masses of 35, 36, and 39 kDa (Fr2-alpha, -beta, and -gamma, respectively) were found to be responsible for Fr2 activity and were purified to homogeneity. Each Fr2 protein has activity by itself in the reconstituted in vitro Golgi transport assay, although each exhibits a different specific activity and plateau value. No synergy of the three Fr2 proteins was observed during mixing experiments. The three Fr2 proteins seem to be closely related based on size, in vitro activities, chromatographic properties, and peptide maps and may comprise a new family of proteins involved in vesicular transport.