Transmembrane proteins, Golgi and COP part 2

Proteins, upon completion of processing, localize to the trans Golgi network (TGN).
The TGN serves as the final checkpoint before proteins are sorted and packaged into vesicles for their respective destinations.

Proteins from the TGN follow the biosynthetic secretory pathway which includes two major pathways:
- Constitutive Secretory Pathway:
- Proteins are continuously delivered to the plasma membrane or secreted outside the cell without the need for a specific signal.
- Regulated Secretory Pathway:
- Vesicles containing proteins are stored and released only upon receiving a specific trigger signal.
- Example triggers can include hormonal signals or neuronal signals.
Other pathways involve transport to endosomes or lysosomes for further processing, degradation, and recycling of materials.

Exocytosis: The process of sending proteins out to the plasma membrane or releasing them outside the cell.
Endocytosis: The process of bringing materials into the cell for processing or recycling.
These two processes are interrelated, as they represent the movement of materials both into and out of the cellular environment.

Utilizes SNARE proteins for the fusion of vesicles with target membranes.
SNARE Proteins:
- v-SNAREs (vesicle SNARE) are located on the vesicle, and t-SNAREs (target SNARE) are located on the target membrane.
- When vesicles approach the target membrane, they tether via specific recognition proteins, which facilitate membrane fusion.
- Once fused, the cargo is released into the epithelial compartment or extracellular space.

Assemble on the cytosolic surface of the donor membrane to form vesicles.
They interact with cargo receptors that help select specific proteins or lipids for inclusion in the vesicles.
Serve not only to form the vesicle but also to ensure the correct cargo is transported.

COP II:
- Initiates from the ER and travels towards the Golgi.
- Responsible for carrying newly synthesized proteins for processing and sorting through the Golgi.
COP I:
- Operates in a retrograde direction, aiding the return of ER-resident proteins and maintaining enzyme balance within the Golgi.

Key molecules involved in COP II vesicle formation:
- SAR1: Small GTP-binding protein acting as a molecular switch that allows GTP binding leading to its insertion into the ER membrane.
- Sec 23 and Sec 24:
- Sec 23 interacts with SAR1 and regulates GTP hydrolysis; Sec 24 selects and binds the cargo through attachment to the cytosolic tails of cargo receptors.
These interactions facilitate the outward curvature and budding of the membrane, which ultimately pinches off to form a vesicle moving towards the Golgi.

The differences in action between COP I and COP II:
- COP II:
- Moves proteins away from the ER to the Golgi, ensuring a forward, organized transport of cargo.
- COP I:
- Moves in reverse to recycle proteins back to the ER or to other Golgi compartments.

Observed through electron microscopy, COP-coated vesicles show a textured, fuzzy halo due to the protein coat's density and is critical for functionality.
Size range for vesicles is approximately between 60 to 100 nanometers thick.

Goblet cells are specialized secretory cells within epithelial tissues, notable for their primary function of secreting mucin, a glycoprotein critical for mucus formation and lubrication in respiratory and intestinal tracts.
These cells possess a well-developed rough ER and Golgi apparatus to support massive protein synthesis and secretion, constantly releasing mucin into the extracellular environment without needing specific signals (constitutive pathway).
Contrasting regulated secretion includes hormones like insulin, which release vesicles only upon stimuli.

Lysosomes function as key components of the endomembrane system, characterized by their role as digestive organelles containing a variety of hydrolytic enzymes (acid hydrolases) necessary for the breakdown of macromolecules including proteins, lipids, nucleic acids, and carbohydrates.
Size can range from 25 nanometers to 1 micrometer depending on their activity and content.
A key feature is their acidic interior with a pH around 4.6, maintained by a proton pump (H+ ATPase) transporting protons into the lumen; crucial for proper enzyme function.
The lysosomal membrane is glycosylated, forming a protective shield to prevent self-digestion and maintain membrane integrity.

They receive hydrolytic enzymes from the Golgi and fuse with vesicles containing materials to be degraded through mechanisms such as endocytosis, phagocytosis, and autophagy.
The proton pump continuously aids in maintaining the low pH and ensures optimal enzyme activity.
The enzymatic delivery from the Golgi is vital for lysosomal function, necessitating proper receptor targeting and vesicular fusion processes.

Understanding the details of lysosome functions, protein sorting pathways, vesicle dynamics, and the roles of different coat proteins can enhance comprehension and retention of the molecular mechanisms involved in protein trafficking.