Vesicular and uncoated Rab1-dependent cargo carriers facilitate ER to Golgi transport

Secretory cargo is recognized, concentrated and trafficked from ER exit sites (ERES) to the Golgi. Cargo export from the ER begins when a series of highly conserved COPII coat proteins accumulate at the ER and regulate the formation of cargo loaded, COPII vesicles. In animal cells, capturing live de novo cargo trafficking past this point is challenging; it has been difficult to discriminate whether cargo is trafficked to the Golgi in a COPII coated vesicle. Here, we utilized a recently developed live cell, cargo export system that can be synchronously released from ERES to illustrate de novo trafficking in animal cells. We find that components of the COPII coat remain associated with the ERES, while cargo is extruded into COPII uncoated, non-ER associated, Rab1-dependent carriers. Our data suggest that in animal cells COPII coat components remain stably associated with the ER at exit sites to generate a specialized compartment, but once cargo is sorted and organized, Rab1 labels these export carriers and facilitates efficient forward trafficking.