Source: PCB3023L Assigned Reading, University of Florida
Tags: EGFR, vesicular transport, endocytosis, clathrin, coat proteins, COPI, COPII, cargo selection, membrane trafficking, cell biology
Difficulty: Intermediate Prerequisites: Basic understanding of cell membrane structure, protein structure, and receptor-ligand binding. Familiarity with EGFR from earlier weeks of PCB3023L is helpful.
This material sits at the intersection of cell signalling and intracellular logistics. Every cell needs to move proteins and lipids between compartments, and the machinery that does this (vesicular transport) is also the machinery that controls how long a signal lasts. EGFR is the running example throughout: when EGF binds EGFR, the receptor is pulled inside the cell via vesicles, and the cell then decides whether to recycle the receptor (keeping the signal going) or destroy it (shutting the signal down). If you understand how vesicles form, select their cargo, and deliver it, you understand one of the central regulatory switches in cell biology, and why its failure is linked to cancer.
Cells use vesicles coated with specific proteins (clathrin, COPI, COPII) to shuttle cargo between compartments. EGFR enters the cell mainly through clathrin-mediated endocytosis and then reaches early endosomes, where the cell decides to recycle it or send it for degradation. The whole process is tightly regulated by adaptor proteins, small GTPases, and ubiquitin tags.
Vesicular transport
The movement of proteins, lipids, and other macromolecules between membrane-bound compartments via small membrane-enclosed sacs (vesicles). Think of it as the cell's internal courier service: parcels of cargo budding off one compartment and fusing with another.
Coat proteins
Specialised proteins that shape a donor membrane into a vesicle and help select the cargo inside. In simple terms, they are the scaffolding that moulds and pinches off the vesicle.
Clathrin
A coat protein that forms a lattice around vesicles budding from the plasma membrane. It is the primary coat involved in receptor-mediated endocytosis, including EGFR internalisation. Think of it as a cage that wraps around the membrane pocket as it forms.
COPI (coat protein complex I)
A coat protein complex that mediates retrograde transport, moving cargo from the Golgi apparatus back to the endoplasmic reticulum (ER). In simple terms, COPI runs the return route from the Golgi to the ER.
COPII (coat protein complex II)
A coat protein complex that mediates anterograde transport from the ER to the Golgi apparatus. Think of it as the outbound shuttle from the ER.
Cargo receptors
Membrane-embedded proteins in the donor compartment that recognise and bind specific molecules destined for transport, ensuring only the right cargo gets packaged.
Endocytosis
The process by which cells internalise extracellular molecules, membrane proteins, and lipids by engulfing them in vesicles formed from the plasma membrane. Think of it as the cell swallowing material from outside.
Clathrin-mediated endocytosis (CME)
The major endocytic pathway for EGFR. Adaptor proteins (e.g. AP2) link EGFR to clathrin-coated pits on the plasma membrane; the pits invaginate and pinch off as vesicles. This is the primary route for pulling activated EGFR into the cell.
Adaptor proteins (e.g. AP2)
Proteins that bridge cargo (like EGFR) to the clathrin coat during vesicle formation. In simple terms, they are the connectors that match a specific receptor to the clathrin machinery.
Caveolae
Flask-shaped invaginations of the plasma membrane, rich in cholesterol and caveolin proteins, involved in clathrin-independent endocytosis.
Epidermal growth factor receptor (EGFR)
A receptor tyrosine kinase on the cell surface. When its ligand (EGF) binds, EGFR dimerises, autophosphorylates, and triggers downstream signalling cascades that control cell growth and division. Its trafficking after activation determines how long the signal persists.
Vesicle formation (budding)
The process starts when coat proteins assemble on a donor membrane and curve it outward to form a bud.
Coat proteins serve two roles simultaneously: they physically deform the membrane into a sphere, and they recruit the correct cargo via cargo receptors.
Three major coat protein systems to know:
Clathrin: operates at the plasma membrane for endocytosis (this is the EGFR-relevant one). Also works at the trans-Golgi network.
COPII: carries newly synthesised proteins from the ER to the Golgi (anterograde, i.e. forward direction).
COPI: returns escaped ER-resident proteins from the Golgi back to the ER (retrograde, i.e. reverse direction).
Cargo selection
Cargo receptors sit in the donor membrane and recognise sorting signals on the proteins to be transported.
Only molecules bound by a cargo receptor are concentrated into the forming vesicle. Everything else is excluded.
This selectivity is what keeps the right proteins in the right compartment.
Coat assembly and disassembly
Once the vesicle pinches off, the coat is rapidly stripped away (uncoating).
Uncoating is essential because the coat physically covers the surface proteins the vesicle needs for docking and fusion with its target membrane.
After uncoating, SNARE proteins and Rab GTPases on the vesicle surface guide it to the correct destination.
Clathrin-mediated endocytosis (CME)
The primary route for internalising EGFR.
Sequence of events:
EGF binds EGFR on the cell surface, triggering receptor dimerisation and autophosphorylation.
Adaptor protein AP2 recognises the activated EGFR tail and links it to the clathrin lattice forming at that patch of membrane.
The clathrin-coated pit deepens (invaginates).
Dynamin, a GTPase, wraps around the neck of the pit and pinches it off, releasing a clathrin-coated vesicle into the cytoplasm.
The clathrin coat is removed (uncoating), and the vesicle fuses with an early endosome.
Caveolae-mediated endocytosis
Uses flask-shaped membrane invaginations called caveolae.
Caveolae are rich in cholesterol and lined with caveolin proteins.
Important for internalising certain lipids, signalling molecules, and some pathogens, but is not the main route for EGFR.
Clathrin-independent pathways
A catch-all for endocytic routes that do not rely on clathrin or caveolae.
Often involve lipid rafts or other membrane microdomains.
Relevant for some cargo types, but for this reading the focus stays on CME for EGFR.
Cancer therapies that target EGFR (e.g. cetuximab, erlotinib) work in part because EGFR trafficking is disrupted in many tumours. When the degradation pathway fails, EGFR keeps recycling back to the surface and signalling non-stop, driving uncontrolled cell growth. Understanding the normal trafficking route is what makes it possible to design drugs that restore proper receptor turnover.
Students often think clathrin itself recognises and grabs the cargo. It does not. Adaptor proteins (like AP2) are the bridge between the receptor and the clathrin coat.
Students sometimes confuse the direction of COPI and COPII. COPII goes forward (ER to Golgi). COPI goes backward (Golgi to ER). A useful mnemonic: COPII has two I's, pointing forward.
Endocytosis is sometimes treated as though it always leads to degradation. It does not. Many internalised receptors are recycled back to the surface. The cell makes a sorting decision at the early endosome.
Coat removal is sometimes overlooked. Without uncoating, the vesicle cannot dock or fuse with its target membrane, because the coat covers the docking machinery.
Know the three coat protein systems (clathrin, COPI, COPII) and which route each one serves.
Be able to outline the steps of clathrin-mediated endocytosis from ligand binding through to early endosome delivery.
Understand why uncoating must happen before vesicle fusion.
Expect questions on the difference between cargo receptors (select cargo) and coat proteins (shape the vesicle).
True or false: COPII mediates transport from the Golgi back to the ER. (False. COPII is ER-to-Golgi. COPI is the return route.)
Fill in the blank: The adaptor protein ______ links EGFR to clathrin-coated pits during endocytosis. (AP2)
True or false: Coat proteins must be removed before a vesicle can fuse with its target membrane. (True.)
Fill in the blank: The three major coat protein systems are ______, ______, and ______. (Clathrin, COPI, COPII)
True or false: All endocytosis is clathrin-mediated. (False. Caveolae and clathrin-independent pathways also exist.)
Q: Name the three major coat protein complexes and state the transport route each one mediates.
A: Clathrin mediates endocytosis at the plasma membrane. COPII mediates anterograde transport from the ER to the Golgi. COPI mediates retrograde transport from the Golgi back to the ER.
Q: Describe the sequence of events in clathrin-mediated endocytosis of EGFR, starting from ligand binding.
A: EGF binds EGFR, causing dimerisation and autophosphorylation. AP2 recognises the activated receptor and recruits clathrin to form a coated pit. The pit invaginates, dynamin pinches off the vesicle, the clathrin coat is removed, and the uncoated vesicle fuses with an early endosome.
Q: Why is coat disassembly (uncoating) a necessary step after vesicle budding?
A: The coat physically covers the vesicle's surface proteins (e.g. SNAREs) that are needed for docking and fusion with the target membrane. Without uncoating, the vesicle cannot recognise or fuse with its destination.
Q: What is the role of cargo receptors in vesicular transport?
A: Cargo receptors are membrane-embedded proteins in the donor compartment that recognise sorting signals on specific cargo molecules. They ensure that only the correct proteins and lipids are concentrated into the forming vesicle, maintaining the fidelity of intracellular trafficking.
This material connects directly to EGFR signalling (earlier weeks of PCB3023L): the trafficking machinery described here is what determines whether an activated EGFR signal persists or is terminated. It also connects to the secretory pathway (ER to Golgi transport via COPII) and to lysosomal biology (the degradative endpoint of the trafficking route covered in Part 2).
Vesicular transport, vesicle budding, coat proteins, clathrin, clathrin-coated pit, COPI, COPII, cargo receptor, cargo selection, adaptor protein, AP2, endocytosis, clathrin-mediated endocytosis, CME, caveolae, caveolin, clathrin-independent endocytosis, lipid rafts, EGFR, EGF, receptor tyrosine kinase, membrane trafficking, intracellular transport, uncoating, dynamin, vesicle fusion, SNARE proteins