Export out of the endoplasmic reticulum (ER) involves the Sar1/COPII-machinery acting at ER exit sites (ERES). Whether and how cargo proteins are recruited upstream of SarI/COPII is unclear. Two models are conceivable, a recruitment model where cargo is actively transported via a transport factor and handed over to the Sar1/COPII in ERES, and a capture model, where cargo freely diffuses into ERES where it is captured by the Sar1/COPII. Using the novel secretion inhibitor FLI-06, we show that recruitment of the cargo VSVG to ERES is an active process upstream of Sar1 and COPII. Applying FLI-06 before concentration of VSVG in ERES completely abolishes its recruitment. In contrast, applying FLI-06 after VSVG concentration in ERES does not lead to dispersal of the concentrated VSVG, arguing that it inhibits recruitment to ERES as opposed to capture in ERES. FLI-06 also inhibits export out of the TGN, suggesting that similar mechanisms may orchestrate cargo selection and concentration at ER and TGN. FLI-06 does not inhibit autophagosome biogenesis and the ER-peroxisomal transport route, suggesting that these rely on different mechanisms.
- Received January 15, 2016.
- Accepted August 25, 2016.
- © 2016. Published by The Company of Biologists Ltd