Question

Name and compare the two major protein secretory pathway’s (Protein Secretory Pathway vs Delivery to endosomes) originating from the trans-Golgi Network (TGN)?

Answer 1

Secretory pathway refers to the process by which proteins (lipids) are transported to the cell membrane and are released out to the extracellular environment (exocytosis).

The pathway involves the movement and modification of proteins through specific membrane bound organelles and vesicles: like, from endoplasmic reticulum lumen, through transport vesicles, to cis-Golgi network, followed by trans-Golgi network.

The major two types of protein secretory pathways are:

1. Constitutive secretory pathway	2. Regulated secretory pathway:
It is the default secretory pathway or continuous pathway for protein secretion.	Proteins are destined to storage or secretory vesicles or the pathway terminates at the vesicles. It is not continuous.
The pathway is generally used for restoration of proteins via cell membranes or organelles. The respond to general stimulus for exocytosis.	On receiving specific signals or stimulus, the proteins that are stored in the vesicles, fuse with cell membrane and are released.
Constitutive secretion is generally carried out by all cells and is a general phenomenon, not bound to specific conditions.	Regulated secretion is carried out specifically in biochemical response to stimulations such as cytokines, hormones, neurotransmitters, etc.
Example: Secretion of serum proteins from the hepatocytes.	Example: Pancreatic acinar cells, that secrete precursors of digestive enzymes.
Proteins that are destined for constitutive secretion do not aggregate at Trans-Golgi network.	Proteins that are destined for regulated secretion aggregate at Trans-Golgi network.
Proteins are sorted into transport or secretory vesicles, at trans-Golgi network.	The proteins are sorted Trans-Golgi network, and stored inside storage vesicle (at neutral pH)
Proteins are immediately transported to the plasma membrane, where they fuse and are released	The release is stimulated by stimulators or signal, which causes increase of cytosolic Calcium ions concentration. This triggers the fusion of vesicles and the release of proteins