ER-to-Golgi carriers arise through direct en bloc protrusion and multistage maturation of specialized ER exit domains.

Protein transport between the ER and the Golgi in mammalian cells occurs via large pleiomorphic carriers, and most current models suggest that these are formed by the fusion of small ER-derived COPII vesicles. We have examined the dynamics and structural features of these carriers during and after their formation from the ER by correlative video/light electron microscopy and tomography. We found that saccular carriers containing either the large supramolecular cargo procollagen or the small diffusible cargo protein VSVG arise through cargo concentration and direct en bloc protrusion of specialized ER domains in the vicinity of COPII-coated exit sites. This formation process is COPII dependent but does not involve budding and fusion of COPII-dependent vesicles. Fully protruded saccules then move centripetally, evolving into one of two types of carriers (with distinct kinetic and structural features). These findings provide an alternative framework for analysis of ER-to-Golgi traffic.