Transport of proteins from the ER to the Golgi is mediated by small (60-80 nm) vesicles associated with the coat proteins COPII and COPI — COPII proteins appear to function in budding of these vesicles from the ER, whereas COPI proteins function later on in transit to the Golgi. But how are proteins too large for these vesicles transported? David Stephens and Rainer Pepperkok have examined ER-Golgi transport of procollagen — a rigid molecule>300 nm long and clearly unable to fit into an 80 nm vesicle. Using fusion proteins tagged with GFP variants, they demonstrate that procollagen is carried in transport compartments distinct from those containing typical cargo proteins such as ERGIC-53 (see ). Interestingly, the authors show that the segregation of procollagen and ERGIC-53 into distinct transport compartments does not occur if COPI is inactivated. Their findings thus not only reveal a novel transport pathway but also implicate COPI in control of an early stage of ER-Golgi trafficking at which cargos are segregated into distinct carriers.
