A researcher mutates the cytoplasmic tail of the LDL receptor, eliminating its YXXΦ internalization signal. What is the most direct consequence for LDL receptor internalization?
AClathrin can no longer assemble into a lattice at the membrane near the LDL receptor
BDynamin cannot be recruited to sever the forming coated pit
CAP2 cannot recognize the LDL receptor tail, so it cannot bridge the receptor to clathrin, blocking internalization
DThe LDL receptor is degraded in the endoplasmic reticulum before reaching the cell surface
The YXXΦ motif is a sorting signal recognized by the AP2 adaptor complex — not by clathrin directly. AP2 acts as a molecular bridge: one face binds the receptor's cytoplasmic tail (via its cargo-recognition domain), the other face recruits clathrin. Without the YXXΦ signal, AP2 cannot bind the receptor, so clathrin is not recruited to that location, no coated pit forms around the LDL receptor, and it cannot be internalized. This is the basis of familial hypercholesterolemia — mutations in the LDL receptor internalization signal block cholesterol uptake despite a normal receptor and normal clathrin machinery.
Question 2 Multiple Choice
What is the role of dynamin in clathrin-mediated endocytosis?
AIt recruits clathrin triskelions to form the initial coated pit lattice
BIt recognizes cargo sorting signals on receptor cytoplasmic tails
CIt polymerizes into a helical collar around the vesicle neck and uses GTP hydrolysis to drive membrane scission
DIt uncoats the clathrin lattice from the vesicle after scission using ATP hydrolysis
Dynamin is the GTPase responsible for the final pinching-off step. It polymerizes into a helix around the narrow neck connecting the deepened coated pit to the plasma membrane. GTP hydrolysis drives a conformational change in the dynamin helix that constricts and severs the neck, releasing the coated vesicle into the cytoplasm. Dynamin is not involved in recognizing cargo (that is AP2), recruiting clathrin (that is AP2 and other adaptors), or removing the coat (that is Hsc70 and auxilin). Dominant-negative dynamin mutants block endocytosis specifically at the scission step, trapping coated pits with elongated necks.
Question 3 True / False
The clathrin coat must be removed from a newly released vesicle before it can fuse with early endosomes.
TTrue
FFalse
Answer: True
Coat removal is not optional — it is a prerequisite for membrane fusion. The clathrin lattice physically obstructs the fusion machinery (SNAREs and other membrane proteins) from accessing the vesicle surface. Immediately after scission, Hsc70 (a constitutive ATPase) and its cofactor auxilin disassemble the triskelion lattice, exposing the vesicle membrane. Only after uncoating can the vesicle be recognized by Rab5 and EEA1, the tethering factors that guide it to early endosomes. This is a general principle of vesicular transport: the coat drives cargo capture and membrane curvature, but must be shed before delivery.
Question 4 True / False
Clathrin triskelions directly recognize and bind to internalization signals on the cytoplasmic tails of transmembrane cargo receptors.
TTrue
FFalse
Answer: False
This is the key misconception explicitly flagged in this topic. Clathrin never directly contacts cargo or the membrane's cytoplasmic face in a cargo-specific way. Clathrin's role is structural: it self-assembles into a polyhedral lattice that imposes curvature on the membrane. All cargo specificity resides in the adaptor proteins — primarily the AP2 complex — which bind both the cargo receptor's sorting signal and the clathrin triskelion. This two-step logic (adaptors recognize cargo, clathrin provides mechanical force) allows the same clathrin machinery to internalize many different cargo types by swapping or combining different adaptor proteins.
Question 5 Short Answer
Why must the cell rapidly shed the clathrin coat immediately after vesicle scission, and what molecular machinery accomplishes this?
Think about your answer, then reveal below.
Model answer: The clathrin coat must be shed because it physically obstructs the fusion machinery required for the vesicle to merge with its target compartment (the early endosome). The coat also sterically blocks tethering factors and SNARE proteins from accessing the vesicle membrane. Coat removal is carried out by the ATPase Hsc70, a constitutive heat shock cognate protein, working with its cofactor auxilin. Auxilin binds to the clathrin lattice and recruits Hsc70; ATP hydrolysis by Hsc70 provides the mechanical energy to pry clathrin triskelions off the vesicle surface. The free triskelions are recycled into the cytoplasmic pool for the next round of endocytosis.
Understanding why coat removal is required — not just when it happens — reveals the logic of vesicular transport. Coats serve two purposes: capturing cargo (via adaptors) and deforming the membrane (via the rigid lattice). Once the vesicle is released, the coat's job is done, and its continued presence becomes an obstacle. This same logic applies to other coated transport vesicles (COPI, COPII): every coat must be shed before the vesicle can fuse with its destination compartment.