Overview of Secretory Vesicles

Eukaryotic cells secrete proteins both as a continuous process and in response to specific stimuli.

The continuous release, also called constitutive secretion, occurs without any specific stimulus. It is the default pathway that transports molecules to the plasma membrane, including extracellular matrix proteins, interleukins, and certain growth factors.

Constitutively secreted proteins are continuously transported to the plasma membrane and are sorted into unregulated secretory vesicles. In contrast, secretory proteins, such as insulin and neurotransmitters, are released upon stimulation and are transported in regulated secretory vesicles.

Newly synthesized secretory proteins are first loosely aggregated in immature secretory vesicles that bud off from the trans-Golgi network. During maturation, the vesicles fuse with each other.

As the vesicles fuse, the concentration of V-type ATPases present on the vesicular membrane also increases. The V-type ATPases pump protons into the vesicle, leading to a drop in the intraluminal pH of the vesicles.

This process forms mature secretory vesicles with concentrated protein aggregates. The concentration ensures the delivery of large amounts of protein to the cell surface.