Final answer:
Secretory proteins are concentrated in vesicles through binding to cargo receptors and budding off from the trans Golgi network, which is then followed by vesicle fusion with the plasma membrane and exocytosis.
Step-by-step explanation:
Secretory proteins are packed into secretory vesicles at high concentrations due to the machinery involved in vesicle formation and protein sorting at the Golgi apparatus. When proteins are synthesized in the rough endoplasmic reticulum (RER), they are then transported to the Golgi where they are modified and sorted. Cargo receptor proteins bind to specific cargo proteins, resulting in highly concentrated protein packages. Coat proteins and clathrin then assist these cargo receptor proteins to bud off from the trans Golgi network, forming the secretory vesicles. These vesicles contain V-SNARE proteins that interact with target membranes having complementary T-SNARE proteins, enabling the vesicles to fuse with the plasma membrane and release their contents via exocytosis.