Final answer:
Proteins for regulated secretion are sorted and packaged in the Golgi apparatus, where they undergo modification and are placed into secretory vesicles for extracellular release or other destinations within the cell.
Step-by-step explanation:
Proteins that are destined for regulated secretion are sorted and packaged within the Golgi apparatus. Initially synthesized in the rough endoplasmic reticulum (RER), they undergo a series of modifications, such as the addition of carbohydrate groups to become glycoproteins. These modifications commence in the RER and continue as the proteins move through the Golgi complex. In the Golgi, further processing occurs, and proteins are sorted according to their final destinations. Specifically, proteins bound for secretion outside the cell are packaged into secretory vesicles at the trans face of the Golgi. These vesicles bud off and eventually fuse with the plasma membrane, releasing their contents extracellularly.
Integral membrane proteins that serve functions in the cell membrane are also synthesized in the RER and follow a similar path through the Golgi, where they receive their final modifications before getting embedded in vesicles that fuse with the plasma membrane.
Packaged proteins can also be sorted to various organelles within the cell, such as lysosomes and peroxisomes, each with its unique targeting signals for proper delivery and function.