Beyond vesicles and tubules: What matters at ER exit sites?

Export of secretory proteins at ER exit sites (ERES) has been described to occur via COPII-coated vesicles, tubules, or tunnels. These models have not been considered to reflect the flexibility of ERES but rather as mutually exclusive mechanisms. Here, we overview these export mechanisms, discuss why they coexist, and hypothesize about the biological relevance of carrier geometry. Finally, we suggest experimental avenues to resolve the disagreement.