17.10:
Vesicular Tubular Clusters
COPII vesicles bud from specialized regions in the ER membrane that lack bound ribosomes. These regions are called ER exit sites and are dispersed throughout the ER membrane.
After budding, some transport vesicles shed their coat and fuse with one another to form larger vesicles. Fusion between identical membrane types is called homotypic fusion.
In contrast, heterotypic fusion occurs when non-identical membranes fuse, such as a Golgi-derived vesicle fusing into the ER membrane.
In homotypic fusion, N-ethylmaleimide sensitive factor or NSF proteins force t-SNAREs and v-SNAREs apart from each other. Complementary SNAREs on the adjacent vesicle interact, and the identical membranes fuse.
Such fusion between COPII vesicles forms an extended membranous structure called a vesicular tubular cluster or VTC.
This occurs in a region between the ER and Golgi, called the ER-Golgi intermediate compartment or ERGIC.
The VTC moves along microtubule tracks, carrying ER proteins that are not fully folded and functional, cargo receptors, and SNAREs.
As it moves, new COPI vesicles spontaneously bud off from the tubular clusters. The mechanism of switching the coat protein is yet unknown.
These new transport vesicles carry escaped cargo involved in the ER budding and vesicle fusion back to the ER in a process called the ER retrieval pathway.
17.10:
Vesicular Tubular Clusters
After budding out from the ER membrane, some COPII vesicles lose their coat and fuse with one another to form larger vesicles and interconnected tubules called vesicular tubular clusters or VTCs. These clusters constitute a compartment at the ER-Golgi interface known as ERGIC (Endoplasmic Reticulum Golgi Intermediate Compartment). The ERGIC is a mobile membrane-bound cargo transport system that sorts proteins secreted from ER and delivers them to the Golgi.
With the help of motor proteins such as kinesin and dynein, the VTCs move along microtubule tracks to unload their cargo at the appropriate destination– the Golgi apparatus. As the VTC moves along the tracks, COPI vesicles bud off, carrying escaped ER proteins that need to be taken back to the ER. The composition of the VTC gradually changes as it moves towards the Golgi. The cisternal maturation model, which explains protein traffic through the Golgi, hypothesizes that such VTCs become the cis Golgi network as they arrive from the ER. Eventually, they mature into medial cisterna and finally the trans-cisterna.
Suggested Reading
- Martínez-Menárguez, J. A., Geuze, H. J., Slot, J. W., & Klumperman, J. (1999). Vesicular tubular clusters between the ER and Golgi mediate concentration of soluble secretory proteins by exclusion from COPI-coated vesicles. Cell, 98(1), 81-90.
- Appenzeller-Herzog, Christian, and Hans-Peter Hauri. "The ER-Golgi intermediate compartment (ERGIC): in search of its identity and function." Journal of cell science 119, no. 11 (2006): 2173-2183.