Trace the pathway of a secretory protein from its synthesis in the RER to its exocytosis from the cell.

A secretory protein's journey begins with its synthesis on ribosomes attached to the Rough Endoplasmic Reticulum (RER). As it's synthesized, it enters the RER lumen, where it undergoes folding and modification. From the RER, the protein is packaged into transport vesicles that bud off and travel to the Golgi apparatus. Within the Golgi, the protein moves through the cis, medial, and trans cisternae, undergoing further processing, sorting, and packaging. Finally, at the trans-Golgi network, the protein is enclosed in secretory vesicles. These vesicles then move to the cell membrane, fuse with it, and release their contents outside the cell via exocytosis.