Vesicle-associated membrane protein 4 is implicated in trans-Golgi networkvesicle trafficking

The trans-Golgi network (TGN) plays a pivotal role in directing proteins in the secretory pathway to the appropriate cellular destination. VAMP4, a re cently discovered member of the vesicle-associated membrane protein (VAMP) family of trafficking proteins, has been suggested to play a role in mediat ing TGN trafficking. To better understand the function of VAMP4, we examine d its precise subcellular distribution. Indirect immunofluorescence and ele ctron microscopy revealed that the majority of VAMP4 localized to tubular a nd vesicular membranes of the TGN, which were in part coated with clathrin. In these compartments, VAMP4 was found to colocalize with the putative TGN -trafficking protein syntaxin 6. Additional labeling was also present on cl athrin-coated and noncoated vesicles, on endosomes and the medial and trans side of the Golgi complex, as well as on immature secretory granules in PC 12 cells. Immunoprecipitation of VAMP4 from rat brain detergent extracts re vealed that VAMP4 exists in a complex containing syntaxin 6. Converging lin es of evidence implicate a role for VAMP4 in TGN-to-endosome transport.