Chapter 12: The Endomembrane System and Protein Sorting – Structure, Function, and Transport Mechanisms

Endomembrane System

Overview of the Endomembrane System

The endomembrane system is a network of membranous organelles within eukaryotic cells that coordinates the synthesis, modification, sorting, and transport of proteins and lipids. Major components include the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, endosomes, and the plasma membrane.

• Function: Facilitates complex cellular transport processes and compartmentalization.

• Key Organelles: ER, Golgi complex, vesicles, lysosomes, endosomes.

• Transport: Involves vesicular trafficking between organelles.

• Example: Secretory pathway for protein export.

Golgi Complex: Structure and Function

Organization of the Golgi Apparatus

The Golgi complex is a stack of flattened, membrane-bound cisternae with distinct polarity: the cis face (entry side) and trans face (exit side). It serves as a central hub for protein and lipid modification and sorting.

• Cis face: Receives vesicles from the ER.

• Trans face: Dispatches processed molecules to their destinations.

• Transport vesicles: Shuttle materials between Golgi cisternae and other organelles.

Modification and Sorting in the Golgi

Proteins and lipids undergo extensive modification in the Golgi, including glycosylation and phosphorylation, which are critical for proper sorting and function.

• Removal of Mannose: Mannose residues are removed from oligosaccharide chains.

• O-linked Glycosylation: Addition of sugars to the hydroxyl group of amino acids serine and threonine.

• Phosphorylation of Sugars: Addition of phosphate groups, such as the formation of mannose-6-phosphate tags for lysosomal targeting.

• Sorting: Modified proteins are sorted for delivery to lysosomes, plasma membrane, or secretion.

• Example: Lysosomal enzymes receive mannose-6-phosphate tags for recognition by lysosome-targeting receptors.

Protein Targeting and Sorting in the Endomembrane System

Signals for Protein Targeting

Proteins are directed to specific organelles by signal sequences and post-translational modifications such as glycosylation and phosphorylation.

• Amino Acid Sequences: Short stretches, often at the N- or C-terminus, direct proteins to their correct location.

• Examples of Targeting Signals: Sequences rich in aspartic acid, serine, leucine are common in lysosomal targeting signals.

• Mannose-6-phosphate: A key tag for lysosomal enzymes, added in the Golgi.

• Soluble vs. Membrane Proteins: Soluble proteins are released into organelle lumens; membrane proteins are inserted into organelle membranes.

Vesicular Transport and Endocytic Pathways

Vesicle Formation and Types

Transport between organelles is mediated by vesicles that bud from donor membranes and fuse with target membranes. Vesicles are classified by their coat proteins, which determine their origin and destination.

• COPII Vesicles: Mediate anterograde transport (ER to Golgi).

• COPI Vesicles: Mediate retrograde transport (Golgi to ER).

• Clathrin-coated Vesicles: Involved in transport from the Golgi and in endocytosis at the plasma membrane.

• Caveolin-coated Vesicles: Specialized for certain types of endocytosis.

Table: Major Coated Vesicle Types in Eukaryotic Cells

Vesicle Type	Coat Protein	Main Function
COPII	Sar1, Sec23/24, Sec13/31	Anterograde transport (ER to Golgi)
COPI	Coatomer complex	Retrograde transport (Golgi to ER)
Clathrin	Clathrin, AP1/AP2 adaptors	Transport from Golgi to endosomes; endocytosis
Caveolin	Caveolin	Specialized endocytosis

Vesicle Fusion and Cargo Delivery

Vesicles deliver their cargo by fusing with target organelles or the plasma membrane, a process regulated by SNARE proteins and other factors.

• Fusion: Ensures specific delivery of proteins and lipids.

• Endocytosis: Uptake of extracellular materials via vesicle formation.

• Exocytosis: Release of vesicle contents to the cell exterior.

• Sorting in Endosomes: Internalized materials are sorted for recycling, degradation, or transport to other compartments.

Endocytic Pathway and Lysosomal Targeting

Endocytosis and Vesicle Types

Endocytosis is the process by which cells internalize extracellular substances. It can be classified based on vesicle size and mechanism.

• Phagocytosis: Uptake of large particles (>0.5 μm diameter).

• Pinocytosis: Uptake of fluids and small molecules.

• Receptor-mediated Endocytosis: Highly specific uptake via clathrin-coated vesicles.

Clathrin-mediated Endocytosis

Clathrin-coated vesicles form at the plasma membrane to internalize specific cargo, guided by adaptor proteins (e.g., AP2) and dynamin.

• Clathrin: Forms a polyhedral lattice around vesicles.

• Adaptor Proteins (AP2): Link clathrin to cargo receptors.

• Dynamin: GTPase required for vesicle scission.

Lysosomal Targeting and Sorting

Proteins destined for lysosomes are tagged with mannose-6-phosphate in the Golgi and recognized by specific receptors for delivery to lysosomes via endosomes.

• Mannose-6-phosphate: Added to lysosomal enzymes in the Golgi.

• Early Endosome: Receives internalized material and sorts it for recycling or degradation.

• Late Endosome: Matures and fuses with lysosomes for degradation of cargo.

• Recycling: Some components are returned to the plasma membrane or Golgi.

Summary of Key Equations and Concepts

• Glycosylation Reaction: $\text{Protein} + \text{Sugar} \rightarrow \text{Glycoprotein}$

• Phosphorylation of Mannose: $\text{Mannose} + \text{ATP} \rightarrow \text{Mannose-6-phosphate} + \text{ADP}$

Additional info: The notes have been expanded to include definitions, examples, and a reconstructed table for clarity and completeness.