Newly synthesized secretory proteins leave the endoplasmic reticulum (ER) in COPII-coated vesicles and pass through the tubulovesicular membrane clusters of the ER-Golgi intermediate compartment (ERGIC) to reach the Golgi. But is the ERGIC a pre-existing stationary structure (the stable compartment model), or is it formed by the fusion of COPII vesicles (the maturation model)? Hans-Peter Hauri and co-workers use live-cell imaging to provide strong evidence in favour of the stable compartment model (see ). The authors compare the trafficking of GFP-labelled ERGIC-53, a membrane lectin that recycles from the ERGIC to the ER, with the movement of the anterograde marker protein VSV-G-GFP and the fluorescent protein dsRed. Their results indicate that GFP-ERGIC-53 mainly localizes to long-lived stationary elements plus some short-lived highly mobile elements, which may act as fast carriers between individual ERGIC clusters. The authors conclude that anterograde and retrograde cargos are repeatedly sorted in the ERGIC structures through a dissociative process, the molecular mechanism of which remains to be determined.
