Protein Sorting & Transport Mechanisms

Protein Sorting & Transport Mechanisms

• Protein sorting and transport mechanisms are crucial for cellular function.
• The endomembrane system plays a vital role in these processes.
• This system includes interconnected organelles such as the endoplasmic reticulum (ER), Golgi apparatus, and vesicles.

Endomembrane System Overview

• The endomembrane system is a dynamic network that unites endomembranes.
• It facilitates the biosynthetic/secretory and endocytic pathways.

Pathways

• Biosynthetic/Secretory Pathways: These pathways have two types:
  • Constitutive secretion: A continuous process where proteins are secreted to the cell exterior.
  • Regulated secretion: A controlled process where proteins are stored in secretory vesicles and released upon specific signals. This involves a process of packaging and storing until stimulated.

Endocytosis

• Endocytosis involves bringing substances into the cell.
• The pathways involved in endocytosis include early endosome and late endosome/ lysosome pathway.

Endoplasmic Reticulum (ER)

• The ER is a network of membranes and the largest organelle in most eukaryotic cells.
• Its membrane accounts for about half of all cell membranes.
• The lumen (cisternal space) represents about 10% of total cell volume.
• It exists in two forms: rough ER and smooth ER.

Rough ER (RER)

• The RER synthesizes proteins, carbohydrates, and phospholipids.
• It also participates in protein processing (folding, assembly of multisubunit proteins, and disulfide bond formation).
• RER is critical for membrane biosynthesis and the initial stages of glycosylation.
• It also has a role in attaching glycolipids to plasma membrane proteins.

Smooth ER (SER)

• Functions include:
  • Synthesizing steroid hormones in endocrine cells of the gonads and adrenal glands.
  • Detoxification of a wide array of organic substances (like barbiturates, ethanol) in the liver. Liver cells may proliferate the SER in response to chronic use of these substances.
  • Sequestering calcium ions for release in muscle cells; the calcium is stored within the SER forming the sarcoplasmic reticulum which triggers muscle contraction

Ribosomes

• A significant proportion (one-third) of mammalian proteins synthesized on ribosomes.
• Free ribosomes and membrane-bound ribosomes synthesize different proteins.
• Membrane-bound ribosomes are found on the surface of the RER and synthesize secreted proteins, integral membrane proteins, and soluble proteins.

Protein Synthesis

• Proteins' synthesis, processing, and transport are essential cellular processes.
• Palade's research highlighted the ER's role in protein processing and sorting.

Explore the intricate processes of protein sorting and transport within the endomembrane system. This quiz covers essential pathways including biosynthetic, secretory, and endocytic mechanisms vital for cellular functionality. Test your knowledge on how organelles like the ER and Golgi apparatus contribute to these processes.