The most likely pathway taken by a newly synthesized protein that will be secreted by a cell is the conventional secretory pathway, which involves several key steps:
- Synthesis and translocation into the rough endoplasmic reticulum (ER): The protein is synthesized by ribosomes bound to the rough ER. A signal sequence on the nascent protein directs the ribosome to the ER membrane, where the protein is co-translationally translocated into the ER lumen
- Transport from the ER to the Golgi apparatus: The protein is packaged into transport vesicles that bud from the ER and fuse with the Golgi apparatus. This step involves vesicle formation and budding, followed by vesicle fusion with the Golgi
- Processing and sorting in the Golgi apparatus: Within the Golgi, the protein undergoes further modifications and is sorted for its final destination. The Golgi directs the protein into secretory vesicles destined for the plasma membrane
- Vesicle transport to and fusion with the plasma membrane: Secretory vesicles carrying the protein move to the plasma membrane, dock, and fuse with it, releasing the protein outside the cell (secretion)
In summary, the pathway is: Rough ER → Golgi apparatus → Secretory vesicles → Plasma membrane → Extracellular space This pathway is well characterized and is the primary route for proteins destined for secretion in eukaryotic cells
