In the first step of the mammalian secretory pathway, proteins are exported from the ER in vesicles coated with the COPII complex. Vesicle budding occurs at discrete ER-exit sites [ERES; also known as the transitional ER(tER)] – but how are these sites defined in metazoans? To address this question, David Stephens and colleagues () investigate the ER localisation of the COPII component Sec16A in human cells. Using confocal microscopy, immunogold EM and electron tomography, the authors show that – in the steady state – Sec16A localises to cup-like structures at the tER. Importantly, Sec16A localisation is spatially distinct from that of the COPII components Sec23-24 and Sec13-31, which localise to structures formed later in the pathway. The authors next disrupt the association of Sec23-24 and Sec13-31 with ERES, and show that Sec16A localises at tER independently of either Sec23-24 or Sec13-31. Moreover, the kinetics of Sec16A-tER association are unaffected by the loss of these other COPII subunits. Finally, the authors define the region of Sec16A that mediates its localisation to the tER, and show that the central conserved domain of Sec16A interacts with Sec13. The authors conclude, therefore, that Sec16A functions early in vesicle formation, and might act as a platform for COPII assembly at ERES. Their data shed light on how ERES are defined.
