Final answer:
ER-resident proteins are packaged in vesicles in the RER and transported to the ERGIC before fusing with the Golgi apparatus for further processing and sorting. This step is part of the proteins' journey through the endomembrane system on their way to their final destination.
Step-by-step explanation:
In cellular biology, the traffic of proteins through the endomembrane system is a critical aspect of cell function. When ER-resident proteins travel through the system, they typically follow a path that begins in the rough endoplasmic reticulum (RER), where they are synthesized and may undergo certain modifications, such as the addition of carbohydrates. Proteins not destined to remain in the RER are packaged within vesicles and transported from the RER by budding from the membrane.
These vesicles then move to the ER-Golgi intermediate compartment (ERGIC), and subsequently fuse with the Golgi apparatus on its receiving or cis face. The proteins are processed within the Golgi stack, receiving further modifications and eventually are sorted for their final destination, which may include incorporation into the plasma membrane or delivery to other organelles. In this way, the proteins complete their journey through the endomembrane system.
If a vesicle containing the ER-resident proteins reaches the ERGIC, it is indicative of the ongoing cellular process where proteins are trafficked to their required location. Specifically, this step lies between the proteins exiting the RER and being further processed and sorted in the Golgi apparatus.