Positioning of ER exit sites around the Golgi depends on BicaudalD2 and Rab6 activity

The endoplasmic reticulum (ER) is involved in biogenesis and transport of secreted and membrane proteins. The ER membranes are spread throughout the cell cytoplasm as well as the export domains known as ER exit sites (ERES). A subpopulation of ERES is centrally localized proximal to the Golgi apparatus. The significance of this subpopulation of ERES on ER-to-Golgi transport remains unclear. In our study we demonstrated that the peri-Golgi concentration of ERES and their processive movements are dependent on cargo departure. Using the Sar1a[H79G]-induced ERES clustering assay we identified BicaudalD2 and Rab6 proteins as possible components of the dynein adaptor complex which drives ERES concentration at the Golgi region. Inhibition of BicaudalD2 function by its N-terminal fragment or inhibition of Rab6 by the dominant-negative mutation did not cause Golgi dispersion yet significantly affected ERES central concentration. We demonstrated that the processive movements of ERES are frequently coupled with the carrier departure. BicD2 partially colocalized with ERES and with a carrier. Golgi accumulation of the model secretory protein – VSVG was delayed by inhibition of Rab6 and BicD2. Thus, we concluded that a BicD2/Rab6 dynein adaptor is required for maintenance of Golgi-associated ERES. We propose that Golgi-associated ERES may enhance the efficiency of the ER-to-Golgi transport.