Copy of Chapter 13 Part 1 B (second half) lectures 5-9.pptx

1. Introduction to Intracellular Membrane Traffic

  • Focus on vesicle transport mechanisms in cellular biology.

2. Vesicle Targeting and Fusion Mechanisms

2.1 Role of Rab Proteins

  • Rab Proteins: Monomeric GTPases essential for directing vesicles to their specific target membranes.

    • 60 different types used to identify various membrane types.

    • Each Rab protein associates with specific organelles in the biosynthetic secretory pathway.

2.2 Activation and Regulation

  • Rab proteins can be GTP-bound (active) or GDP-bound (inactive).

    • GAPs (GTPase-activating proteins): Inactivate Rab by promoting GTP hydrolysis.

    • GEFs (guanine nucleotide exchange factors): Activate Rab by promoting GTP binding.

2.3 Interaction with SNARE Proteins

  • Rab effectors interact with Rab-GTP, facilitating the docking and fusion of membranes.

  • SNARE Proteins: v-SNAREs (vesicle) and t-SNAREs (target) mediate membrane fusion by pairing.

3. Membrane Fusion Process

3.1 SNARE Complex Formation

  • SNARE proteins wrap around each other to catalyze the merging of lipid bilayers.

  • Tight pairing of SNAREs removes water between membranes, facilitating fusion.

3.2 Role of NSF

  • NSF (N-ethylmaleimide-sensitive factor) separates SNARE complexes post-fusion, requiring ATP hydrolysis for energy.

4. Cargo Transport and Retrieval

4.1 Cargo Selection and Vesicle Budding

  • COPII-Coated Vesicles: Transport proteins from the ER to the Golgi apparatus.

    • Sar1-GTP recruits COPII for selective cargo packaging.

    • Exit signals on proteins facilitate proper sorting.

4.2 Retrieval Pathways

  • Misfolded proteins cannot enter vesicles and must be sent back for degradation.

  • Retrieval signals (KDEL sequence and KKXX motif) guide the return of resident ER proteins.

5. Golgi Apparatus Functionality

5.1 Structural Organization

  • The Golgi consists of cisternae (flattened compartments) where proteins are modified.

5.2 Protein Modifications

  • Proteins undergo specific modifications (e.g., glycosylation) as they transit from cis to trans Golgi.

5.3 Sorting to Final Destinations

  • Post-trans modification, proteins can move to different cellular compartments (e.g., secretory vesicles, lysosomes).

6. Glycosylation: Importance and Mechanisms

6.1 Types of Oligosaccharides

  • High-Mannose Oligosaccharides: Trimming without adding new sugars.

  • Complex Oligosaccharides: Involve further modification by adding sugars.

6.2 Biological Significance

  • Promotes protein folding, protects proteins from degradation, and aids in cell-cell recognition.

7. Summary of Key Processes

  • Rab proteins target vesicles.

  • SNARE proteins mediate fusion.

  • Golgi apparatus modifies and sorts proteins for various pathways.

  • Glycosylation plays a critical role in protein functionality and cellular interactions.