COPII, the coat protein complex II, is the cargo master of the cell. Stationed close to the endoplasmic reticulum (ER), COPII grabs and sorts newly minted proteins as they emerge from the ER, wrapping them into what are called COPII vesicles. These are shipped to the Golgi apparatus for reprocessing as export or internal cargoes. COPII is standard evolutionary equipment from yeast to humans and its five core components have been intensely studied. Yet when looking at COPII vesicle formation in mammals, Pengcheng Zhang and former ASCB president Randy Schekman, both of the University of California, Berkeley, and the Howard Hughes Medical Institute, had the feeling that something else was involved in the more complex protein sorting that mammalian cells require. As reported in the June 15 issue of Molecular Biology of the Cell, Zhang and Schekman say that at least one factor from the cytosol, that is, from outside COPII itself, is required to recruit new ER proteins for COPII wrapping. Zhang and Schekman used the CRISPR/Cas9 gene editing method to set up a model COPII system in a human HeLa cell line and a cargo, the precursor of a transforming growth factor, as bait to identify the cytosolic outsider. This outsider, they say, is the transmembrane protein Cornichon-1 (CNIH), which acts as a cargo receptor for proteins to enter the COPII complex. (A cornichon is also a small, tart French pickle.) Any news from the Schekman lab about vesicle formation is welcome for it was Schekman’s seminal discoveries going back to the 1970s that more or less invented the whole vesicle transport field and led to his share of the 2013 Nobel Prize in Medicine.
