Final answer:
Proteins from the Rough Endoplasmic Reticulum are packaged into transport vesicles, which fuse with the cis face of the Golgi apparatus. There, the proteins undergo modifications, such as glycosylation, essential for their final function. Modified proteins are then routed to their designated locations, including the plasma membrane.
Step-by-step explanation:
The default transport of proteins from the Rough Endoplasmic Reticulum (RER) through vesicles begins when proteins are entered into the RER and packaged. These vesicles, coated with COPI and COPII proteins, then pinch off from the RER, dissociating from ribosomes but remaining associated with the coat proteins. The transport vesicles then fuse with the cis face of the Golgi apparatus (GA), entering its lumen.
As the proteins and lipids pass through the GA, they undergo modifications such as the addition of short chains of sugar molecules, making them glycoproteins, and sometimes tagging with phosphate groups. These modifications are important for the sorting mechanism that directs these molecules to their final destinations. The completed vesicles finally pinch off from the GA's trans face and travel to the cell membrane or other organelles, where they merge and integrate their contents.
Integral membrane proteins that are made in the RER can sometimes be modified there before they reach the Golgi apparatus. In the GA, they undergo further modifications like glycosylation, which can be crucial for their function in the plasma membrane once released from the GA's trans face and combined with the cell membrane.