Protein Sorting and Transport Mechanisms

Protein Sorting & Mechanism of Transport

• Protein sorting and transport mechanisms are critical for cellular function.
• The endomembrane system plays a vital role in these processes.

The Endomembrane System

• The endomembrane system is a network of interconnected membranes in eukaryotic cells.
• It includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles.
• These interconnected membranes are dynamic and contribute to a cell's functions.

Biosynthetic/Secretory & Endocytic Pathways

• These pathways are key to how the endomembrane system works, unifying the membranes into a interconnected dynamic network.

Pathways

• BIOSYNTHETIC/SECRETORY:
  • CONSTITUTIVE SECRETION: continuous process
  • REGULATED SECRETION: controlled release (e.g., hormones)

Exocytosis and Endocytosis

• Exocytosis: materials move out of the cell.
• Endocytosis: materials move into the cell.
• Both are crucial for transporting substances across cellular boundaries.
• Pathways involved with materials moving in and out of the cell during cell signalling, homeostasis and transport.

Endoplasmic Reticulum (ER)

• It's a network of membranes in the cytoplasm.
• Two types: rough ER (RER) and smooth ER (SER).

Rough Endoplasmic Reticulum (RER)

• Ribosomes attached to RER: synthesis of proteins.
• Synthesizes proteins for extracellular use (e.g., secreted proteins) and membrane bound proteins.
• Major protein processing.
• Involved in:
  • Protein folding.
  • Assembly of multisubunit proteins.
  • Disulfide bond formation.
  • Initial stages of glycosylation.
  • Adding glycolipid anchors to some plasma membrane proteins.
  • Membrane biogenesis
  • Glycosylation (adding carbohydrate chains to proteins)

Smooth Endoplasmic Reticulum (SER)

• Lacking ribosomes: diverse functions.
• Functions:
  • Synthesize lipids (steroids, some hormones).
  • Detoxification (liver): e.g., breakdown of barbiturates and ethanol.
  • Calcium storage: regulates muscle contraction through sarcoplasmic reticulum. (which is a type of SER).

Synthesis of Proteins on Membrane-Bound vs Free Ribosomes

• One-third of mammalian proteins are synthesized on ribosomes attached to the ER.
• This includes secreted proteins, integral membrane proteins and soluble proteins within compartments of the endomembrane system (e.g., ER, Golgi).

Overview of Protein Synthesis

• Proteins synthesized by bound ribosomes enter the ER lumen for modification.
• Proteins synthesized by free ribosomes remain in the cytosol or are targeted to other organelles like mitochondria or chloroplasts.
• Overview diagrams are helpful to understand these differences

Discover the fascinating world of protein sorting and transport mechanisms essential for cellular function. This quiz explores the endomembrane system, including the roles of the endoplasmic reticulum, Golgi apparatus, and the pathways of exocytosis and endocytosis. Test your knowledge on how these processes unify cellular membranes.