cbg 4- Vesicular Transport: Exhaustive Study Guide

Introduction to Vesicular Transport and Global Compartment Movement

  • Conceptual Overview: Protein transport (cargo) is organized into specialized lipid vesicles and transported throughout the cell to specific organelles or the extracellular space.

  • Methods of Protein Movement Between Compartments:     

  • * Gated Transport: Movement between the nucleus and the cytosol.     

  • * Transmembrane Transport: Direct translocation of proteins across a membrane (e.g., into the Mitochondria, Plastids, Peroxisomes, or ER).   

  •   * Vesicular Transport: The movement of proteins via lipid-enclosed vesicles between compartments like the Endoplasmic Reticulum (ER), Golgi apparatus, endosomes, and the plasma membrane.

  • Cargo Path Trajectory:     

  • * Translocation: Soluble and membrane proteins enter the Endoplasmic Reticulum (ER).     * ER to Golgi: Cargo passes through the cis-Golgi, medial-Golgi, and trans-Golgi.    

  •  * TGN (Trans-Golgi Network): Sorting point for proteins directed to the plasma membrane, lysosomes, or secretory vesicles.

Protein Exit and Sorting from the Endoplasmic Reticulum

  • Quality Control via Chaperones:   

  •   * The exit of proteins from the ER is strictly controlled by chaperone proteins.    

  •  * Misfolded Proteins: These are bound by chaperones and retained in the ER to prevent them from entering transport vesicles.     

  • * Properly Folded Proteins: Only correctly folded proteins are packaged into budding transport vesicles for export.

  • Retrograde Transport (Backwards Signaling):     

  • * Certain proteins must return to the ER if they escape to the Golgi.    

  •  * Retrieval Signals:        

  •  * KDEL: Signal peptide for luminal proteins to return to the ER.      

  •  * KKXX: Signal peptide for membrane proteins to return to the ER.

Golgi Apparatus: Structure and Transport Models

  • Structural Organization:    

  •  * Cis Face: The side of the Golgi apparatus situated nearer to the ER.   

  •   * Trans Face: The side situated nearer to the plasma membrane.    

  •  * Compartments: ER $\rightarrow$ CGN (Cis-Golgi Network) $\rightarrow$ Cisternae (Cis, Medial, Trans) $\rightarrow$ TGN (Trans-Golgi Network).

  • Models of Movement Through the Golgi:  

  •    1. Vesicular Transport Model: Stationary cisternae where cargo moves between them via small vesicles.    

  •  2. Cisternal Maturation Model: Cisternae themselves move and mature from cis to trans, carrying cargo within them as they change.     

  • * Conclusion: Real-world movement is likely a mixture of both models.

  • Golgi Destinations: Cargo moving through the Golgi can be directed to the nuclear envelope, lysosomes, late endosomes, early endosomes, the plasma membrane, or secretory vesicles.

Experimental Evidence for Trafficking

  • Historical Pulse-Chase Experiments: Studies using VSVG-GFP (a fluorescently tagged viral protein) visualized the movement of proteins over time.

  • Time-Course Observations:  

  •    * 0 min0\,\text{min}: VSVG-GFP is concentrated entirely in the ER.     * 40 min40\,\text{min}: Cargo has moved markedly into the Golgi apparatus.  

  •    * 180 min180\,\text{min}: Cargo is primarily located at the plasma membrane.

  • Quantitative Data: Graphic analysis show the VSVG-GFP count (×106\times 10^6) peaks in the ER near t=0t=0, peaks in the Golgi roughly between 100100 and 200 min200\,\text{min}, and gradually accumulates at the plasma membrane (PM) over 600 min600\,\text{min}.

The Three Major Types of Vesicle Coats

  • Functional Goal: Coat proteins organize vesicular traffic, concentrate specific cargo into specialized membrane patches, and provide the mechanical force needed to deform the membrane into a bud.

  • Coat Classifications:     

  • 1. COPII: Covers vesicles emanating from the ER destined for the Golgi (Anterograde).     2. COPI: Surrounds vesicles originating from the Golgi, usually moving toward the ER (Retrograde) or between Golgi cisternae.     

  • 3. Clathrin: Surrounds vesicles moving from the plasma membrane to endosomes, or from the Golgi to endosomes/lysosomes.

Molecular Mechanism: COPII (ER to Golgi)

  • Key Molecular Components:     

  • * Sar1 (GTPase): Initiates the process. Sar1-GDP is recruited to the membrane where it is activated to Sar1-GTP by Sec12 (a Guanine Nucleotide Exchange Factor or GEF).     * Inner Coat (Sec23/Sec24): Recruits cargo proteins. Sec24 specifically recognizes cargo sorting motifs.

  •     * Outer Coat (Sec13/Sec31): Provides the structural scaffold to drive membrane curvature.

  • Process Steps:   

  •   1. Budding: Recruitment of Sar1, inner coat, and outer coat proteins.     

  • 2. Movement: The vesicle detaches and moves toward the Golgi.    

  •  3. Uncoating: Triggered by GTP hydrolysis (Sar1-GTP to Sar1-GDP), the coat proteins disassemble so the vesicle can fuse.

Molecular Mechanism: COPI (Retrograde Transport)

  • Key Molecular Components:   

  •   * Arf1 (GTPase): The initiator for COPI assembly (analogous to Sar1 in COPII).

  •     * GEF: Activates Arf-GDP to Arf-GTP on the Golgi membrane.    

  •  * Coatamer: Preassembled COPI coat subunits recruited by Arf-GTP.    

  •  * Cargo Recognition: COPI subunits recognize cargo sorting motifs directly.

Clathrin-Coated Vesicles and Scission

  • Clathrin Structure: Composed of individual subunits called triskelions.   

  •   * Each triskelion consists of an $\alpha$-zigzag and a $\beta$-propeller structure.     * 3636 triskelions assemble to form a hexagonal clathrin cage.

  • Accessory Proteins:   

  •   * Adaptor Proteins (AP1, AP2, AP3): Connect the clathrin coat to the membrane and select specific integral cargo proteins/receptors.     

  • * Dynamin: A GTPase responsible for "molecular strangulation." It forms a ring around the neck of the budding vesicle and uses GTP hydrolysis to pinch the vesicle off from the membrane.     

  • * Uncoating Proteins: Hsc70 and Auxillin are required to disassemble the clathrin coat after budding, a process that requires ATP.

Vesicle Docking and Fusion: The SNARE Hypothesis

  • Thermodynamics of Fusion: Fusion involves several physical stages:     

  • 1. Stalk Formation: Proximal monolayers of the vesicle and target membrane join.

  •     2. Hemifusion: Distal monolayers remain separate while proximal ones are fused.     3. Fusion Pore: An opening is created (initially  50 A˚~50\,\text{\AA} wide) that eventually expands to merge the two compartments.

  • SNARE Proteins:    

  •  * v-SNARE (Vesicle SNARE): e.g., Synaptobrevin.  

  •    * t-SNARE (Target SNARE): e.g., Syntaxin and SNAP-25.

  •     * Specificity: Specific combinations of v-SNAREs and t-SNAREs ensure vesicles fuse only with the correct target compartment (McNew et al., 2000).

  • Mechanism of Action:     

  • * Trans-SNARE Complex: Formed when v-SNARE and t-SNARE interact while on opposing membranes. This complex exerts an inward force that pulls the membranes together.  

  •    * Cis-SNARE Complex: Formed after fusion occurs, where both SNAREs are now on the same (fused) membrane and exert no force.

  • Recycling SNAREs:    

  •  * NSF (N-ethylmaleimide Sensitive Factor) and $\alpha$-SNAP (Soluble NSF Attachment Protein) are required for recycling.    

  •  * They use ATP hydrolysis to disassemble the stable cis-SNARE complex, freeing the SNAREs for subsequent rounds of transport.

Critical Literature and Attributions

  • Noble, J. et al. (2013): Research on the "Real Deal" of COPII structure (NSMB 20(2):167-173).

  • Brandizzi & Barlowe (2013): Mechanisms of COPI and retrograde transport (Nat Rev Molec Cell Biol 14:382-392).

  • Kirchhausen, T. (2000): Studies on Clathrin structure and triskelions (Nat Rev Molec Cell Biol 1:187-198).

  • McNew et al. (2000): Research on the compartmental specificity of cellular membrane fusion encoded in SNARE proteins (Nature 407: 153–159).

  • Dr. Steve Cook: Contributor of CC-BY-SA-3.0 licensed text and images.

Protein transport is organized into specialized lipid vesicles for movement to specific organelles or the extracellular space. Key methods include: 1. Gated Transport: between the nucleus and cytosol. 2. Transmembrane Transport: across membranes (e.g., mitochondria, ER). 3. Vesicular Transport: movement via lipid vesicles among compartments like the ER, Golgi, and plasma membrane.

Cargo Path: Soluble and membrane proteins enter the ER, then traverse through the Golgi (cis to trans). The Trans-Golgi Network sorts these proteins towards the plasma membrane or lysosomes.

ER Exit and Sorting: Controlled by chaperones that retain misfolded proteins. Retrieval signals (KDEL for luminal, KKXX for membrane proteins) ensure certain proteins return to the ER.

Golgi Structure: The Golgi has a cis face (near ER) and a trans face (near membrane). Two models describe cargo movement: 1. Vesicular Transport (cargo moves via vesicles), 2. Cisternal Maturation (cisternae mature and carry cargo).

Experimental Evidence: Pulse-chase experiments with VSVG-GFP tracked protein movement from the ER to membrane over time.

Vesicle Coats: Three types—1. COPII: from ER to Golgi. 2. COPI: from Golgi to ER. 3. Clathrin: from plasma membrane to endosomes.

Docking and Fusion: Involves SNARE proteins that ensure accurate vesicle-target fusion. NSF and α-SNAP recycle SNAREs post-fusion.

Key Literature: Notable studies by Noble et al., Brandizzi & Barlowe, Kirchhausen, and McNew elucidate vesicular transport mechanisms.