Laufman et al. reveal how a membrane-tethering complex promotes the transport of vesicles from endosomes to the trans-Golgi network (TGN).
The conserved oligomeric Golgi (COG) complex helps to recycle proteins back through the Golgi apparatus by tethering retrograde transport vesicles to their target membranes. Whether the complex has a similar function in docking endosome-derived vesicles to the TGN has remained unclear, however.
Laufman et al. found that endosome-to-TGN trafficking was impaired in cells lacking the COG subunit Cog6. Many of the SNARE proteins that promote the fusion of endosome-derived vesicles with the TGN were mislocalized in Cog6’s absence.
In particular, the SNARE Syntaxin 6 no longer localized to the Golgi, and its expression level was reduced by proteasomal degradation. Cog6 bound directly to Syntaxin 6, suggesting that the tethering complex stabilizes the SNARE, perhaps by promoting its association with other Golgi-localized SNARE proteins.
Endosome-to-TGN transport was restored in Cog6-depleted cells by boosting levels of the vesicle SNARE VAMP4. Because vesicle SNARE overexpression often compensates for the loss of upstream tethering factors, this suggests that the COG complex does in fact tether endosome-derived vesicles to the TGN. The complex therefore appears to control transport to the TGN in two ways—by tethering vesicles and by stabilizing Syntaxin 6—as well as controlling retrograde trafficking to earlier Golgi compartments. Senior author Sima Lev says this probably allows the two transport steps to be coordinated. She now wants to investigate whether COG subunits interact with additional components of the TGN fusion machinery.