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.
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