Final answer:
COPII coat proteins are key to the transport of vesicles from the RER to the Golgi apparatus, where they facilitate vesicle movement and are involved in the fusion process with the cis Golgi vesicles. After delivering their cargo, COPI proteins are recycled.
Step-by-step explanation:
COPII coat proteins are essential in the transport of vesicles within cells. They facilitate the movement of transition vesicles from the rough endoplasmic reticulum (RER) to the Golgi apparatus. These vesicles carry a mix of proteins, and after budding off from the RER with the help of COPI and COPII, they dissociate from any attached ribosomes but remain associated with the COP proteins. These vesicles then fuse with the cis Golgi vesicles, a process also mediated by COP proteins, where the COPI proteins detach after the fusion, to be recycled back to the RER. The proteins contained within the vesicles undergo further processing in the Golgi vesicle stack, including terminal glycosylation. Finally, at the trans face of the Golgi vesicles, specific cargo proteins are sorted into separate vesicles with different cellular or extracellular destinations, and these vesicles acquire membrane V-SNARE proteins for targeted delivery.