Identification of a novel, N-ethylmaleimide-sensitive cytosolic factor required for vesicular transport from endosomes to the trans-Golgi network in vitro.

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Abstract
We have recently described a cell-free system that reconstitutes the vesicular transport of 300-kD mannose 6-phosphate receptors from late endosomes to the trans-Golgi network (TGN). We report here that the endosome----TGN transport reaction was significantly inhibited by low concentrations of the alkylating agent, N-ethylmaleimide (NEM). Addition of fresh cytosol to NEM-inactivated reaction mixtures restored transport to at least 80% of control levels. Restorative activity was only present in cytosol fractions, and was sensitive to trypsin treatment or incubation at 100 degrees C. A variety of criteria demonstrated that the restorative activity was distinct from NSF, an NEM-sensitive protein that facilitates the transport of proteins from the ER to the Golgi complex and between Golgi cisternae. Cytosol fractions immunodepleted of greater than or equal to 90% of NSF protein, or heated to 37 degrees C to inactivate greater than or equal to 93% of NSF activity, were fully able to restore transport to NEM-treated reaction mixtures. The majority of restorative activity sedimented as a uniform species of 50-100 kD upon glycerol gradient centrifugation. We have termed this activity ETF-1, for endosome----TGN transport factor-1. Kinetic experiments showed that ETF-1 acts at a very early stage in vesicular transport, which may reflect a role for this factor in the formation of nascent transport vesicles. GTP hydrolysis appears to be required throughout the transport reaction. The ability of GTP gamma S to inhibit endosome----TGN transport required the presence of donor, endosome membranes, and cytosol, which may reflect a role for guanine nucleotides in vesicle budding. Finally, ETF-1 appears to act before a step that is blocked by GTP gamma S, during the process by which proteins are transported from endosomes to the TGN in vitro.