Final answer:
Secretory proteins are synthesized on ribosomes attached to the RER, modified within the RER, tagged and sorted in the Golgi apparatus, and then distributed in vesicles either to other parts of the cell or secreted outside the cell.
Step-by-step explanation:
Synthesis and Targeting of Secretory Proteins to the ER
The synthesis of secretory proteins begins with the formation of a translation initiation complex on a ribosome. The ribosome typically attaches to the rough endoplasmic reticulum (RER) once the N-terminal signal sequence of the nascent polypeptide emerges. This sequence guides the ribosome to the RER where the growing polypeptide chain elongates and is threaded into the RER lumen.
In the RER, the polypeptides undergo various modifications such as proper folding and the addition of side chains like carbohydrates. Following these modifications in the RER, the proteins are packaged into vesicles that bud from the RER's membrane and are transported to the Golgi apparatus.
Within the Golgi apparatus, proteins are further modified, tagged, and sorted for their final destination. They are packaged into secretory vesicles which bud off the trans face of the Golgi. These vesicles either fuse with other parts of the cell to deliver their content or fuse with the plasma membrane to secrete their proteins outside the cell.
In summary, the process involves the synthesis of the protein on the ribosome, modification in the endoplasmic reticulum, tagging in the Golgi apparatus, and distribution via vesicles.