Master’s-Level Cell Biology & Advanced Molecular Biology Notes

1. Introduction

Vesicular transport is the regulated movement of proteins, lipids, and soluble cargo between membrane-bound compartments through transport vesicles.

It is a fundamental mechanism of the endomembrane system, connecting:

Endoplasmic reticulum (ER)
Golgi apparatus
Endosomes
Lysosomes
Plasma membrane
Secretory vesicles
Transport intermediates

The central principle is:

Vesicular transport transfers cargo while preserving membrane-bound compartmentalization and membrane topology.

2. The Endomembrane System

                     ENDOMEMBRANE SYSTEM

                         ER
                         │
                    COPII │
                         ↓
                       Golgi
                    ↙          ↘
                 COPI          Clathrin
                  ↓               ↓
                 ER          Endosome
                                  │
                         ┌────────┴────────┐
                         ↓                 ↓
                     Lysosome         Recycling
                                           │
                                           ↓
                                  Plasma membrane

The major direction of secretory traffic is:

ER → Golgi → plasma membrane/endosome/lysosome

But substantial retrograde transport also occurs.

3. What Is a Transport Vesicle?

A transport vesicle is a small membrane-bound carrier that transfers cargo from one cellular compartment to another.

It generally contains:

Lipid bilayer
Soluble cargo
Membrane proteins
Coat proteins
Adaptor proteins
Targeting machinery

          Transport vesicle

          _____________
       .-'             '-.
      /   soluble cargo   \
     |      ●  ●  ●        |
     |                     |
      \ membrane proteins /
       '.___       ____.'
            \_____/

         Lipid bilayer

4. Why Vesicular Transport Is Necessary

Different organelles maintain distinct:

pH
Lipid composition
Protein composition
Enzyme concentration
Ion concentration

Vesicular transport allows the cell to move selected cargo without mixing the entire contents of different organelles.

5. Three Major Stages

Vesicular transport can be conceptually divided into:

1. Vesicle formation

Cargo selection + membrane budding

2. Vesicle targeting

Movement + recognition of the correct destination

3. Vesicle fusion

Docking + membrane fusion + cargo delivery

FORMATION
    ↓
TARGETING
    ↓
DOCKING
    ↓
FUSION
    ↓
CARGO DELIVERY

6. Vesicle Formation

Formation begins when cargo is concentrated at a particular region of the donor membrane.

The sequence is generally:

Cargo selection → adaptor recruitment → coat assembly → membrane curvature → budding → scission

7. Coat Proteins

Coat proteins perform two major functions:

Cargo selection
Membrane deformation

The three classical coat systems are:

COPII
COPI
Clathrin

8. COPII Vesicles

COPII mediates transport:

ER → Golgi

This is called:

Anterograde transport

A simplified pathway:

ER
 │
 │ Sar1
 ↓
Sec23/Sec24
 ↓
Sec13/Sec31
 ↓
COPII-coated vesicle
 ↓
ER-Golgi intermediate compartment
 ↓
Golgi

9. COPII Assembly

A simplified sequence:

Step 1

Sar1-GDP is converted to Sar1-GTP.

Step 2

Sar1-GTP associates with the ER membrane.

Step 3

Sec23/Sec24 are recruited.

Step 4

Sec13/Sec31 form the outer coat.

Step 5

Cargo becomes concentrated.

Step 6

Membrane buds and undergoes scission.

10. Sar1

Sar1 is a small GTPase central to COPII vesicle formation.

Its cycle:

Sar1-GDP
   ↓
GEF
   ↓
Sar1-GTP
   ↓
ER membrane recruitment
   ↓
COPII assembly
   ↓
GTP hydrolysis
   ↓
Sar1-GDP

Sar1 therefore acts as a molecular switch.

11. Sec23/Sec24

The inner COPII coat contains:

Sec23

Functions as part of the inner coat and contributes to regulation of Sar1 GTP hydrolysis.

Sec24

Important for:

Cargo recognition
Cargo receptor interactions

Thus:

Sec24 is particularly important for selecting cargo for COPII vesicles.

12. Sec13/Sec31

Sec13/Sec31 form the outer COPII coat.

They contribute to:

Coat assembly
Membrane curvature
Vesicle formation

13. COPI Vesicles

COPI mediates primarily:

Golgi → ER

and:

Intra-Golgi retrograde transport

Thus COPI is strongly associated with:

Retrograde transport

Golgi
  │
  │ COPI
  ↓
 ER

14. Why Retrograde Transport Is Necessary

COPI transport helps:

Retrieve ER-resident proteins
Recycle vesicle machinery
Maintain Golgi organization
Rebalance membrane components

The secretory pathway therefore depends on both forward and backward traffic.

15. Clathrin-Coated Vesicles

Clathrin participates in several transport pathways, particularly:

Plasma membrane → endosome
Trans-Golgi network → endosome
Some endosomal trafficking pathways

Clathrin itself is mainly a coat/scaffold, while adaptor proteins provide much of the cargo specificity.

16. Clathrin-Mediated Endocytosis

Plasma membrane
────────────────────────

       Cargo
         ↓
      adaptor
         ↓
    clathrin coat
         ↓
      membrane
       bending
         ↓
      bud
         ↓
     dynamin
         ↓
      fission
         ↓
  Endocytic vesicle

17. Adaptors

Adaptor proteins connect:

Cargo ↔ coat

Examples include:

AP complexes
GGA proteins
Other cargo-specific adaptor systems

They help determine what enters the vesicle.

18. Dynamin

Dynamin is a large GTPase involved in scission of many clathrin-coated vesicles.

It assembles around the neck of a budding vesicle.

      Vesicle
        ○
       / \
      /   \
─────╯     ╰─────
       ███
       ███
      Dynamin

GTP-dependent conformational changes promote constriction and scission.

19. Vesicle Uncoating

Coats must usually be removed after vesicle formation.

This is essential because the vesicle needs to interact with:

Tethering proteins
Target membrane
SNAREs

A generalized pathway is:

Budding → scission → uncoating → transport → tethering

20. Rab GTPases

Rab proteins are central regulators of vesicle targeting.

They help establish:

Vesicle identity
Target specificity
Tether recruitment
Membrane docking

Different cellular compartments contain characteristic Rab proteins.

21. Rab GTPase Cycle

        GDP
         ↓
       Rab
         ↓
       GEF
         ↓
      Rab-GTP
         ↓
 Membrane-associated
         ↓
 Effector recruitment
         ↓
        GAP
         ↓
      Rab-GDP

GEF

Guanine nucleotide exchange factor

GAP

GTPase-activating protein

22. Vesicle Transport

After formation, vesicles may travel through the cytoplasm.

The cytoskeleton provides major transport tracks.

Microtubules

Long-range transport

Actin filaments

Short-range transport and cortical movement

23. Molecular Motors

Major motor proteins include:

Kinesins

Usually move cargo toward the microtubule plus end.

Dyneins

Usually move cargo toward the microtubule minus end.

Myosins

Move along actin filaments.

Microtubule
══════════════════════════>
          Kinesin →

<══════════════════════════
          ← Dynein

The exact direction depends on the specific motor and cellular context.

24. Vesicle Targeting

A vesicle must identify the correct target membrane.

This requires multiple layers of specificity:

Vesicle identity
      ↓
Rab GTPase
      ↓
Tethering factor
      ↓
SNARE pairing
      ↓
Membrane fusion

This provides high fidelity in intracellular trafficking.

25. Tethering

Tethering is the initial physical capture of a vesicle by its target membrane.

Tethering factors include:

Long coiled-coil proteins
Multisubunit tethering complexes

They act before tight SNARE-mediated docking/fusion.

26. Docking

Docking is a more stable interaction between the vesicle and target membrane.

At this stage:

Rab proteins
Tethering proteins
SNAREs
Regulatory proteins

work together to prepare the membranes for fusion.

27. SNARE Proteins

SNAREs provide a major molecular mechanism for membrane fusion.

They form a tight complex between:

Vesicle SNARE
Target-membrane SNAREs

Historically:

v-SNARE + t-SNARE

Modern classification:

R-SNARE + Qa/Qb/Qc SNAREs

28. SNARE Zippering

Vesicle membrane
════════════════
       │
       │ R-SNARE
       │
       ╲
        ╲
         ╲
         ╱
        ╱
       ╱
       │
Target membrane
════════════════

Progressive SNARE assembly brings the two membranes into extremely close proximity.

This promotes:

Hemifusion → fusion pore → complete fusion

29. Membrane Fusion

Fusion ultimately delivers vesicle cargo to the target compartment.

Vesicle
   ○
   │
   ↓
Tethering
   ↓
Docking
   ↓
SNARE zippering
   ↓
Fusion
   ↓
Target compartment

30. NSF and SNARE Recycling

After fusion, SNARE proteins are present in a cis-SNARE complex.

They must be recycled.

Important components:

NSF
α-SNAP

NSF uses ATP to facilitate SNARE complex disassembly.

31. Cargo Types

Vesicular transport can carry:

Soluble proteins

Examples:

Hormones
Digestive enzymes
Neurotransmitter-containing cargo

Membrane proteins

Examples:

Receptors
Transporters
Channels

Lipids

Membrane lipids themselves are transferred as part of vesicle membranes.

32. Soluble vs Membrane Cargo

            VESICLE
       ┌───────────────┐
       │ ● ● ● ●       │ ← soluble cargo
       │               │
       │ █ █ █ █       │ ← membrane proteins
       └───────────────┘

Soluble cargo resides in the lumen.

Membrane proteins remain embedded in the vesicle membrane.

33. Topological Principle

A critical concept:

Vesicular transport preserves membrane topology.

The cytosolic face remains cytosolic.

The luminal face remains luminal.

ER lumen
   ↓
Golgi lumen
   ↓
Vesicle lumen
   ↓
Extracellular space

This is why the ER/Golgi lumen is topologically equivalent to the extracellular environment.

34. Secretory Pathway

The classical secretory pathway is:

Ribosome
   ↓
Rough ER
   ↓
COPII vesicle
   ↓
Golgi
   ↓
Trans-Golgi network
   ↓
 ┌───────┼────────┐
 ↓       ↓        ↓
PM    Endosome   Other

Proteins entering the ER can subsequently reach multiple destinations.

35. Constitutive Secretion

Most cells continuously secrete proteins through:

Constitutive secretion

No special external stimulus is required.

Examples include secretion of:

Extracellular matrix proteins
Plasma membrane components
Some enzymes

36. Regulated Secretion

Specialized cells store cargo in secretory vesicles until stimulation.

Examples:

Neurotransmitter release
Insulin secretion
Digestive enzyme secretion

General sequence:

Synthesis → storage → stimulus → Ca²⁺ signal → fusion → secretion

37. Calcium in Regulated Exocytosis

At synapses:

Action potential
      ↓
Voltage-gated Ca²⁺ channel
      ↓
Ca²⁺ influx
      ↓
Synaptotagmin
      ↓
SNARE activation
      ↓
Vesicle fusion
      ↓
Neurotransmitter release

Calcium provides extremely rapid temporal control.

38. Endocytosis

Endocytosis transfers material:

Plasma membrane → intracellular compartments

Major types include:

Clathrin-mediated endocytosis
Caveolar pathways
Macropinocytosis
Phagocytosis

39. Receptor-Mediated Endocytosis

A ligand binds a receptor.

Ligand
  ↓
Receptor
  ↓
Adaptor
  ↓
Clathrin
  ↓
Endocytic vesicle
  ↓
Early endosome

This allows selective uptake of extracellular molecules.

40. Early Endosome

The early endosome acts as a major sorting station.

Cargo may be directed toward:

             EARLY ENDOSOME
                  │
       ┌──────────┼──────────┐
       ↓          ↓          ↓
   Recycling    Degradation  Golgi
       │          │
       ↓          ↓
 Plasma membrane Lysosome

41. Recycling Pathway

Some receptors return to the plasma membrane.

Example:

Transferrin receptor

Plasma membrane
      ↓
Early endosome
      ↓
Recycling endosome
      ↓
Plasma membrane

This allows repeated use of receptors.

42. Lysosomal Delivery

Some cargo is transported toward lysosomes.

Early endosome
      ↓
Late endosome
      ↓
Lysosome
      ↓
Degradation

This pathway is important for receptor downregulation and macromolecule degradation.

43. Mannose-6-Phosphate Sorting

Lysosomal hydrolases are synthesized in the ER and processed through the Golgi.

Many are tagged with:

Mannose-6-phosphate (M6P)

M6P receptors recognize these enzymes and help deliver them toward endosomal/lysosomal compartments.

ER
 ↓
Golgi
 ↓
M6P tagging
 ↓
M6P receptor
 ↓
Transport vesicle
 ↓
Endosome
 ↓
Lysosome

44. Retrieval of ER Proteins

Some proteins that accidentally leave the ER must be returned.

An important signal is:

KDEL

KDEL-containing soluble ER proteins can be retrieved from the Golgi.

A simplified model:

ER → Golgi
      ↓
   recognition
      ↓
   COPI vesicle
      ↓
      ER

For many ER membrane proteins, different cytosolic retrieval signals are used.

45. COPI vs COPII vs Clathrin

Feature	COPII	COPI	Clathrin
Main direction	ER → Golgi	Golgi → ER/intra-Golgi	PM/endosome/TGN pathways
Major GTPase	Sar1	Arf	Arf in many pathways; dynamin for scission in endocytosis
Major coat	Sec23/24 + Sec13/31	Coatomer	Clathrin + adaptors
Major role	Anterograde ER export	Retrograde/recycling	Selective trafficking/endocytosis

46. Arf GTPases

ADP-ribosylation factor (Arf) GTPases regulate several coat-dependent trafficking pathways, including COPI and many clathrin-associated pathways.

Arf-GTP promotes recruitment of:

Coat proteins
Adaptors
Membrane-remodeling machinery

47. Vesicle Identity

Vesicle identity is determined by combinations of:

Rab proteins
Arf/Sar proteins
Coat proteins
SNAREs
Lipids
Cargo receptors

Thus no single protein determines the entire identity of a transport vesicle.

48. Cargo Receptors

Some soluble proteins cannot directly interact efficiently with coat proteins.

They use:

Cargo receptors

The receptor binds the cargo lumenally and exposes a cytosolic region that interacts with coat machinery.

Lumen
Cargo ●
      │
Cargo receptor
      │
──────────── membrane
      │
Adaptor/coat

49. Selective vs Bulk Transport

Selective transport

Specific cargo is concentrated.

Examples:

Receptor-mediated endocytosis
Lysosomal enzyme sorting

Bulk transport

Large amounts of extracellular fluid or membrane are internalized.

Examples:

Macropinocytosis
Some forms of constitutive secretion

50. Vesicular Transport and Membrane Homeostasis

Cells must maintain approximately appropriate membrane balance.

Exocytosis adds membrane to the plasma membrane.

Endocytosis removes membrane.

Therefore:

        EXOCYTOSIS
            ↓
      + membrane
            ↕
      MEMBRANE POOL
            ↕
      - membrane
            ↑
        ENDOCYTOSIS

The balance helps maintain cell-surface area.

51. Vesicular Transport and Organelle Identity

Organelles maintain their identities through:

Selective protein retention
Retrieval pathways
Controlled vesicle fusion
Lipid composition
Resident enzymes
Rab GTPases

Without continuous sorting and retrieval, organelle identity would gradually deteriorate.

52. Vesicular Transport and Protein Quality Control

Misfolded proteins can be removed from the ER through:

ER-associated degradation (ERAD)

Although ERAD ultimately relies heavily on cytosolic ubiquitin-proteasome machinery, membrane trafficking pathways also participate in broader protein quality-control systems.

53. Vesicular Transport and Autophagy

Autophagy involves extensive membrane trafficking.

A simplified sequence:

Isolation membrane
       ↓
Phagophore
       ↓
Autophagosome
       ↓
Lysosome fusion
       ↓
Autolysosome
       ↓
Cargo degradation

Thus autophagy is intimately connected with vesicle fusion and membrane remodeling.

54. Vesicular Transport and Cell Polarity

Polarized cells must deliver different proteins to different membrane domains.

For epithelial cells:

             Apical
        ───────────────
        ↑ specific cargo

        Epithelial cell

        ↓ specific cargo
        ───────────────
            Basolateral

Vesicular trafficking therefore contributes directly to:

Epithelial polarity
Neuronal polarity
Tissue organization

55. Neuronal Vesicular Transport

Neurons are highly dependent on vesicle trafficking.

Important processes include:

Axonal transport
Synaptic vesicle recycling
Neurotransmitter release
Endosomal trafficking

Long-distance transport occurs primarily along microtubules.

56. Vesicular Transport in Neurons

Cell body
   │
   │ microtubule transport
   ↓
Axon
   │
   ↓
Synaptic terminal
   │
   ↓
Vesicle docking
   ↓
Ca²⁺-dependent fusion
   ↓
Neurotransmitter release

57. Molecular Motors and Vesicle Directionality

A vesicle can be transported:

Toward microtubule plus end

Often kinesin-mediated.

Toward microtubule minus end

Often dynein-mediated.

This allows bidirectional movement.

58. Vesicle Fusion Specificity

A major question in cell biology is:

How does a vesicle find the correct membrane?

The answer involves multiple recognition layers:

Rab → tether → SNARE → fusion

This multilayered mechanism provides high specificity.

59. Vesicle Formation vs Targeting

These processes should be distinguished.

Formation

Determines:

What cargo enters
Which membrane buds
Which coat is assembled

Targeting

Determines:

Where the vesicle goes
Which membrane it recognizes
Whether fusion occurs

60. Integrated Vesicular Transport Model

             DONOR MEMBRANE
                    │
                    ↓
             Cargo selection
                    │
                    ↓
             Coat assembly
                    │
                    ↓
              Membrane bend
                    │
                    ↓
                  Bud
                    │
                    ↓
                 Fission
                    │
                    ↓
               Uncoating
                    │
                    ↓
              Cytoskeletal
                 transport
                    │
                    ↓
              Rab recognition
                    │
                    ↓
                Tethering
                    │
                    ↓
                 Docking
                    │
                    ↓
              SNARE pairing
                    │
                    ↓
                 Fusion
                    │
                    ↓
             CARGO DELIVERY

61. Advanced Concept: Vesicular Transport Is a Molecular Logistics System

A useful way to understand the entire system is:

Molecular component	Functional equivalent
Cargo	Package
Cargo receptor	Label
Coat	Packaging machinery
Rab GTPase	Addressing system
Tether	Initial receiver
SNARE	Fusion machinery
Motor protein	Transport vehicle
Cytoskeleton	Road/rail network
Lysosome	Recycling/destruction center

This analogy is useful, but remember that cellular transport is regulated by biochemical interactions rather than a simple one-way delivery system.

62. Advanced Concept: Transport Is Bidirectional

The secretory pathway is not simply:

ER → Golgi → plasma membrane

It is a dynamic network containing:

Anterograde traffic
Retrograde traffic
Recycling
Degradation
Inter-organelle exchange

ER ⇄ Golgi ⇄ Endosome
       ↓       ↕
      TGN    Lysosome
       ↓
      PM
       ↕
    Endosome

63. Advanced Concept: Organelle Identity

Organelle identity is maintained by a combination of:

[
\text{Identity} =
\text{Rab proteins}
+
\text{lipids}
+
\text{resident proteins}
+
\text{SNAREs}
+
\text{retrieval pathways}
]

Therefore, vesicle trafficking is not merely cargo movement—it actively maintains cellular organization.

64. Advanced Concept: Vesicular Transport and Membrane Topology

During vesicular trafficking:

Cytosolic domains remain cytosolic.

Luminal domains remain luminal.

Therefore, a protein glycosylated in the ER lumen will ultimately display those carbohydrate groups on the extracellular surface if it reaches the plasma membrane.

This is one of the most important conceptual connections between:

ER biology → Golgi biology → membrane topology → cell surface biology.

65. Major Vesicular Pathways

Pathway	Main direction	Major machinery
COPII	ER → Golgi	Sar1, Sec23/24, Sec13/31
COPI	Golgi → ER	Arf, coatomer
Clathrin	PM/TGN/endosome pathways	Clathrin, adaptors
Recycling	Endosome → PM	Rab/SNARE systems
Lysosomal delivery	Endosome → lysosome	Rab/SNARE systems
Regulated secretion	Secretory vesicle → PM	Rab, SNARE, synaptotagmin in Ca²⁺-triggered pathways

66. Important GTPases

GTPase	Major function
Sar1	COPII vesicle formation
Arf	COPI and many clathrin-associated pathways
Rab	Vesicle identity, targeting and fusion
Dynamin	Fission
DRP1	Mitochondrial fission
Atlastin	ER membrane fusion

67. Important ATP-Dependent Factors

Protein	Function
NSF	SNARE disassembly
VPS4	ESCRT remodeling/disassembly
Kinesin/Dynein	Motor-driven transport
Myosin	Actin-based transport

68. Clinical Relevance

Defects in vesicular transport can cause:

Neurodegenerative disorders
Lysosomal storage disorders
Immunodeficiency
Diabetes-related secretion defects
Cancer
Intracellular trafficking disorders
Neurotransmission abnormalities

Examples of disease mechanisms can involve defective:

Rab proteins
SNARE machinery
Lysosomal trafficking
Endosomal sorting
Motor proteins

69. Vesicular Transport and Cancer

Cancer cells can alter:

Receptor internalization
Recycling
Exocytosis
Integrin trafficking
Growth-factor signaling

For example, altered receptor recycling can increase the duration or intensity of signaling pathways that promote proliferation and migration.

70. Experimental Approaches

Vesicular transport can be studied using:

Live-cell fluorescence microscopy

Tracks vesicle movement.

FRAP

Measures molecular mobility.

TIRF microscopy

Useful for studying events near the plasma membrane.

Electron microscopy

Provides ultrastructural information.

Cryo-electron microscopy

Can reveal structures of molecular complexes.

Fluorescent cargo assays

Track trafficking pathways.

Rab/SNARE perturbation

Can test the roles of specific trafficking proteins.

Proteomics

Identifies cargo and trafficking complexes.

71. High-Yield Examination Table

Question	Answer
ER → Golgi	COPII
Golgi → ER	COPI
Major endocytic coat	Clathrin
COPII GTPase	Sar1
COPI-associated GTPase	Arf
Vesicle targeting	Rab
Membrane fusion	SNAREs
Fission in many endocytic pathways	Dynamin
SNARE disassembly	NSF + α-SNAP
Lysosomal enzyme targeting	Mannose-6-phosphate system
Long-distance vesicle movement	Microtubules
Plus-end motor	Kinesin, generally
Minus-end motor	Dynein, generally

72. Examination Short Note

Vesicular Transport

Vesicular transport is the regulated movement of membrane and cargo between compartments of the endomembrane system. It involves three major stages: vesicle formation, targeting and fusion. Vesicle formation involves cargo selection, coat assembly, membrane curvature and scission. COPII mediates anterograde transport from ER to Golgi, whereas COPI participates mainly in retrograde Golgi-to-ER and intra-Golgi transport. Clathrin-coated vesicles mediate important pathways involving the plasma membrane, endosomes and trans-Golgi network. Small GTPases such as Sar1 and Arf regulate coat recruitment, while Rab GTPases contribute to vesicle identity and targeting. Vesicles are transported along microtubules and actin using molecular motors such as kinesins, dyneins and myosins. Tethering factors initially capture vesicles at target membranes, followed by SNARE-mediated docking and fusion. NSF and α-SNAP recycle SNAREs after fusion. Vesicular transport maintains organelle identity, protein distribution, membrane composition, secretion, endocytosis, lysosomal degradation and cellular polarity.

73. Viva Questions

Q1. What is vesicular transport?

Movement of cargo and membrane between cellular compartments through membrane-bound vesicles.

Q2. What are the three major coat systems?

COPI, COPII and clathrin.

Q3. Which coat mediates ER-to-Golgi transport?

COPII.

Q4. Which coat is primarily involved in Golgi-to-ER retrieval?

COPI.

Q5. What is the major coat involved in receptor-mediated endocytosis?

Clathrin.

Q6. What GTPase is associated with COPII formation?

Sar1.

Q7. What is the principal role of Rab proteins?

Vesicle identity, targeting, tether recruitment and trafficking regulation.

Q8. What proteins mediate membrane fusion?

SNARE proteins.

Q9. What is the function of dynamin?

It promotes membrane scission in several fission pathways.

Q10. What is the role of NSF?

ATP-dependent disassembly and recycling of SNARE complexes.

Q11. What is the role of mannose-6-phosphate?

It helps target many lysosomal hydrolases toward endosomal/lysosomal compartments.

Q12. Why are microtubules important?

They provide tracks for long-range intracellular vesicle transport.

74. One-Minute Revision

                 VESICULAR TRANSPORT
                         │
        ┌────────────────┼────────────────┐
        ↓                ↓                ↓
     FORMATION        TARGETING         FUSION
        │                │                │
   Coat proteins        Rab             SNAREs
        │                │                │
 Cargo selection      Tethers          Docking
        │                │                │
 Membrane bending      Motors           Fusion pore
        │                │                │
      Fission       Cytoskeleton      Cargo delivery

Core pathways

ER ──COPII──→ Golgi
ER ←─COPI──── Golgi

PM ──Clathrin──→ Endosome
TGN ─Clathrin──→ Endosome

Endosome ──→ Lysosome
Endosome ──→ PM

Secretory vesicle ──SNARE──→ PM

Final Take-Home Message

Vesicular transport is a highly coordinated molecular logistics system in which coat proteins select and package cargo, small GTPases establish vesicle identity and targeting, cytoskeletal motors provide movement, tethering factors recognize target membranes, and SNARE proteins drive membrane fusion. The system is bidirectional and continuously maintains organelle identity, membrane composition, protein distribution, secretion, endocytosis and cellular homeostasis.