Get out! And don't come back!

An active mechanism keeps vesicles that are departing the ER from turning round and fusing back to it, say Faustin Kamena and Anne Spang (Max Planck, Tübingen, Germany).

Directionality is a challenge for vesicle traffic. Although vesicles traveling from ER to Golgi and from Golgi to ER have very different job descriptions, recycling of transport proteins ensures that the compositions of the two vesicle types are similar if not identical. The two vesicle types start off with different coats (COPII for ER to Golgi and COPI for Golgi to ER) but shed them soon after departure.

Kamena and Spang figured that vesicles departing from the ER might be prevented from doing a U-turn by the same ER machinery that welcomes fusion of Golgi-derived vesicles. Sure enough, a mutant of the...