Overview: Cytoplasmic Membrane Systems and Vesicle Transport
Endomembrane system proteins can be targeted to and reside in any region of the system; sorting begins in the rough ER and final sorting occurs in the trans-Golgi network (TGN).
Protein targeting requires specific “tags” on proteins directing them to the appropriate transport vesicle; lysosomal targeting is the most established example.
Protein residence is maintained by two mechanisms:
Retention: resident molecules are excluded from transport vesicles.
Retrieval: “tags” can be used to return escaped proteins to their proper location.
Sorting signals must fulfill key requirements:
If mutated, sorting is lost.
If transplanted to an unsorted protein, that protein becomes sorted.
Protein Targeting and Residence
Proteins synthesized in the rough ER must be directed to various destinations (e.g., ER, ER-Golgi intermediate compartment, Golgi, plasma membrane, lysosomes).
Each protein carries a targeting tag that determines which transport vesicle will carry it to its destination.
Tags can be:
Amino acid sequence
Hydrophobic domain
Oligosaccharide side chain
Other features (context-dependent cues)
Retention vs retrieval mechanisms:
Retention keeps soluble or membrane proteins within a compartment.
Retrieval sends escaped proteins back to their proper location via vesicles.
Sorting Signals and Tags
Tags are essential for proper cargo selection into vesicles.
Sorting signals must be functional to maintain correct localization; mutation disrupts sorting, transplantation can confer sorting to other proteins.
Subcellular localization depends on the combination of sorting signals and the coat/adaptor machinery.
Vesicle Transport: Core Concepts
Vesicles bud off from a donor compartment, target another compartment, and fuse with it.
Cargo is carried from the lumen and membrane of the donor to the lumen and membrane of the target.
Coat proteins on vesicles serve two roles:
1) Curve the membrane to deform it into a vesicle.
2) Select cargo to be carried in the vesicle (cargo specificity).
Vesicle Coat Architecture
The vesicle coat has two protein layers:
Outer layer: scaffolding/cage that shapes the vesicle.
Inner layer: adaptor between the outer coat and the lipid bilayer; cargo selection occurs here via affinity interactions with cytoplasmic domains of transmembrane proteins.
Adaptors (inner layer) recognize cargo and link them to the coat, helping to determine vesicle composition.
Three Main Types of Coated Vesicles
COPII: ER to cis-Golgi (anterograde transport toward the Golgi/ERGIC).
COPI: Golgi to ER and intra-Golgi retrograde transport (retrograde from later compartments back to earlier ones).
Clathrin-coated vesicles: from the plasma membrane to endosomes/lysosomes and from the trans-Golgi network (TGN) to endosomes/lysosomes; also involved in endocytic and post-Golgi trafficking.
Budding is initiated by recruitment of the small GTPase SAR1 to a patch of donor membrane.
A vesicle coat forms to drive budding; cargo selection occurs via coat interactions.
Inner coat (Sec23/Sec24) selects cargo; Sec24 acts as adaptor binding ER export signals.
Outer coat (Sec13/Sec31) forms a lattice that shapes the vesicle.
v-SNAREs (integral membrane proteins) are included in the budding vesicle and are crucial for later fusion.
Assembly details:
SAR1-GDP is recruited to the ER membrane; Sec12 (a GEF) exchanges GDP for GTP, producing SAR1-GTP.
The SAR1-GTP amphipathic N-terminal helix inserts into the membrane, driving coat assembly.
SAR1-GTP recruits Sec23/Sec24 (inner coat); Sec24 recognizes cargo via ER export signals and recruits cargo proteins.
Sec13/Sec31 form the outer coat lattice around the vesicle.
Vesicle maturation and uncoating:
After budding, the coat must disassemble to release coat components into the cytosol.
GTP hydrolysis to SAR1-GDP triggers coat disassembly and exposes v-SNAREs for subsequent fusion; SAR1-GDP dissociates from the membrane and the coat falls apart.
Vesicle formation is initiated by Arf1 (ADP-ribosylation factor 1) GTPase.
Gea (Gea1/Gea2) acts as the GEF to promote Arf1 GDP→GTP exchange on Golgi membranes.
Arf1-GTP associates with the membrane via a myristoylated amphipathic helix.
The COPI coatomer complex (β/β′/γ/δ/ζ-COP, among others; seven subunits) assembles with Arf1 to form the coat.
The COPI outer coat (α/β′/ε-COP) polymerizes to generate the vesicle.
Cargo and retrieval: COPI vesicles mediate retrograde transport from ERGIC/Golgi to ER, or trafficking from trans-Golgi to cis-Golgi; this recycling helps maintain compartment-specific residents and cargo distribution.
Key concept: Coat assembly is driven by Arf1-GTP and coat proteins; hydrolysis of GTP on Arf1 is required for coat disassembly after vesicle scission, enabling fusion at the appropriate target.
The COPI coat is referred to as the coatamer complex.
Clathrin-Coated Vesicles (PM and TGN)
Roles:
Clathrin-coated vesicles move materials from the plasma membrane back to endosomes/lysosomes and also from the TGN to endosomes/lysosomes.
General mechanism (coats and adaptors):
Clathrin triskelion lattice forms the outer coat; adaptor proteins link clathrin to cargo receptors in the membrane.
The clathrin coat cargo selection involves adaptor proteins that recognize cargo and link to clathrin.
Vesicle Movement and Targeting to the Right Membrane
Movement through the cytoplasm often involves microtubules and motor proteins, effectively pulling cargo along tracks (a “locomotive” analogy).
Transport vesicles travel long distances to reach their destinations.
Transit along microtubules is important for efficient organelle-to-organelle transport.
Targeting and Tethering at the Destination
Initial contact between a vesicle and the target membrane is mediated by tethering proteins.
Two groups of tethers:
Rod-shaped fibrous tethers that form long bridges between membranes.
Multi-protein tethering complexes that bring membranes into close apposition.
Rab GTPases provide docking specificity:
Rab proteins are small GTPases with isoprenylation that associate with membranes in the GTP-bound form.
There are >60 Rab family members; different Rabs mark different membrane compartments.
Rab-GTP recruits tethering factors to the membrane surface, enabling docking.
After docking, SNARE-mediated fusion occurs (see SNARE section for details).
SNARE-Mediated Membrane Fusion
SNAREs come in two families:
v-SNAREs: located on vesicles.
t-SNAREs: located on target membranes.
The SNARE hypothesis: Complementary v- and t-SNAREs recognize each other, facilitating vesicle targeting and fusion.
Mechanism:
v-SNAREs and t-SNAREs form a tight helical complex that brings membranes into close proximity, driving fusion.
In vitro the SNARE complex is strong enough to fuse membranes; in vivo, Ca^{2+} rise triggers fusion.
Fusion and post-fusion disassembly:
After fusion, the v- and t-SNAREs remain associated in the same membrane.
NSF (N-ethylmaleimide-sensitive factor) and SNAPs (soluble NSF attachment proteins) use ATP hydrolysis to pry apart SNAREs and recycle them for another round of trafficking.
Notes:
SNAREs represent a central specificity determinant in vesicle fusion.
There are many SNARE family members across membranes; specificity is achieved by the complementarity of v- and t-SNAREs.
ER Retention and Golgi Retrieval Mechanisms
The ER maintains its soluble and membrane protein complement through retention and retrieval signals:
Soluble ER-resident proteins commonly carry the KDEL retrieval signal at the C-terminus: extLys−Glu−Asp−Leu (KDEL).
ER-resident transmembrane proteins carry cytosolic retention/retrieval signals, the most common being KKXX (Lys-Lys-any-any).
Retrieval cycle for soluble ER residents:
If KDEL-tagged proteins escape to the Golgi, they bind to the KDEL receptor under the relatively low pH of the Golgi.
The receptor–KDEL complex is packaged into COPI-coated vesicles and retrieved back to the ER.
In the ER (higher pH), the KDEL receptor releases the cargo, and the receptor is recycled.
The pH dependence of KDEL receptor binding illustrates how pH gradients regulate retrieval efficiency:
Golgi (lower pH) favors binding of KDEL-tagged proteins to the receptor.
ER (higher pH) favors release of KDEL-tagged proteins in the ER.
Practical implication: If a soluble ER resident protein (e.g., PDI) loses its KDEL sequence, it is no longer efficiently retrieved and will be mislocalized, potentially being secreted or accumulating elsewhere.
Retrograde transport via COPI vesicles supports retrieval of ER residents and ensures proper organelle composition.
Summary of Key Concepts and Implications
Endomembrane transport relies on coordinated budding, cargo selection, targeting/docking, and fusion steps governed by specialized proteins and signals.
The three main coat systems (COPII, COPI, Clathrin) mediate forward, retrograde, and post-Golgi trafficking; each uses distinct GTPases and coat components.
Cargo selection is driven by adaptor proteins (inner coat) and cargo signals; retention vs retrieval ensures steady-state localization.
Rab GTPases and tethering factors provide compartment-specific docking; SNAREs drive membrane fusion with regulation by Ca^{2+} and recycling by NSF/SNAPs.
Sorting signals are not only about destination but also maintenance of organelle identity through retrieval pathways (e.g., KDEL/KKXX).
Structural and mechanistic details (e.g., Sec23/24, Sec13/31, Gea1/2, Sec12, SAR1, Arf1) illustrate the modular design of vesicle trafficking, attachment, and fusion.
Notable Terms and Molecules to Remember
COPII components: SAR1-GTP, Sec23, Sec24, Sec13, Sec31, Sec12 (GEF)
Vesicle trafficking underpins protein sorting, organelle biogenesis, and overall cellular compartmentalization.
Defects in any of these steps can lead to mislocalization of enzymes, impaired lysosome biogenesis, and various diseases related to protein mis-sorting.
The endomembrane system exemplifies how physical processes (membrane curvature, coat assembly) integrate with molecular specificity (sorting signals, Rab/Tether/SNARE specificity) to achieve precise intracellular logistics.