Protein transport by vesicles and tunnels

Abstract

Palade's corpus placed small vesicles as the sole means to transport proteins across stable distinct compartments of the secretory pathway. We suggest that cargo, spatial organization of secretory compartments, and the timing of fission of cargo-filled containers dictate the design of transport intermediates that can be vesicles and transient direct tunnels.

Figure 1.

Cargo, the timing of fission, and spatial compartmental organization dictates the nature of transport intermediates. (A) Budding vesicles can fuse to the target membrane, creating a tunnel between donor and acceptor. However, if fission precedes fusion, this will generate a stand-alone vesicle. (B) Recruitment of a downstream compartment to the budding site creates the exit route via a tunnel. For cargoes like procollagens and lipoprotein particles, TANGO1 family proteins capture cargo and tether post-ER membranes. Tethered membranes fuse with each other and to the nascent bud at the ER. In this case, the “transporting intermediate” is, in fact, the downstream compartment of the secretory pathway. In Drosophila melanogaster, the tethered membrane is likely Golgi, while in mammalian cells the membrane would be the ERGIC. At this point, a further fusion of the ERGIC to the Golgi could even result in a long ER–ERGIC–Golgi tube.