Questions: Receptor-Mediated Endocytosis and Clathrin-Coated Vesicles

Question 1

A cell's LDL receptors are internalized via receptor-mediated endocytosis. In the endosome, most receptors dissociate from LDL (due to acidic pH) and are recycled back to the plasma membrane. What is the functional consequence of this recycling?

• A. Cholesterol uptake is permanently terminated because LDL is now trapped inside the cell
• B. The cell loses sensitivity to future LDL signals because receptors accumulate in endosomes
• C. The cell maintains its capacity to bind and internalize LDL in subsequent rounds of endocytosis
• D. Clathrin accumulates inside the cell over time because it cannot re-associate with the membrane

Answer: (C)

Receptor recycling returns functional receptors to the cell surface, replenishing the supply available for future ligand capture. This is how the cell maintains sustained uptake capacity — each receptor can make multiple rounds of endocytosis. The alternative fate (degradation in lysosomes) would permanently reduce receptor number and dampen the cell's responsiveness. The LDL receptor undergoes hundreds of cycles over its lifetime. Option D is wrong because clathrin is actively uncoated by Hsp70/auxilin immediately after vesicle formation and the free triskelions return to the cytoplasmic pool for reuse.

Question 2

Which component is directly responsible for physically deforming the plasma membrane into a curved pit during receptor-mediated endocytosis?

• A. Dynamin, which wraps around the membrane and forces it to curve inward
• B. AP2 adaptor proteins, which bridge receptor tails to the membrane and pull it inward
• C. Clathrin triskelions, which polymerize into a lattice on the cytoplasmic face and impose curvature on the membrane
• D. Hsp70 chaperones, which unfold membrane proteins to create flexibility for bending

Answer: (C)

Clathrin's triskelion structure spontaneously assembles into a polyhedral cage-like lattice. As clathrin polymerizes on the inner leaflet of the plasma membrane, the geometric constraints of the lattice force the membrane to curve inward, forming the clathrin-coated pit. Dynamin (option A) acts later — it wraps around the *neck* of the nearly complete pit and uses GTP hydrolysis to pinch off the vesicle. AP2 (option B) links receptors to clathrin but does not itself deform the membrane. The curvature is a direct mechanical consequence of clathrin lattice geometry.

Question 3

Receptor-mediated endocytosis internalizes a sample of extracellular fluid and its contents whenever membrane invagination occurs, similar to macropinocytosis.

• T. True
• F. False

Answer: False

Receptor-mediated endocytosis is inherently selective: it specifically captures molecules that are bound to cell-surface receptors. The ligand must first bind its cognate receptor; this binding event recruits AP2 and triggers clathrin assembly only at that site. Molecules in the extracellular fluid that lack a receptor partner are not efficiently captured. This selectivity is what distinguishes receptor-mediated endocytosis from macropinocytosis (bulk fluid uptake) and constitutive endocytosis, and it is why the cell can concentrate specific cargo hundreds of times relative to the extracellular fluid.

Question 4

The mildly acidic pH (around 6.0) inside early endosomes serves a functional purpose in receptor-mediated endocytosis by promoting ligand-receptor dissociation, which enables receptor recycling.

• T. True
• F. False

Answer: True

Many receptor-ligand complexes have pH-sensitive binding affinities — they bind tightly at neutral extracellular pH (~7.4) but release at the mildly acidic endosomal pH (~6.0). This acid-triggered dissociation is not incidental; it is essential for sorting. Once the ligand is released in the endosome, the free receptor can be packaged into recycling vesicles and returned to the cell surface, while the ligand (e.g., LDL) is routed to late endosomes and lysosomes for processing. The pH gradient thus directly enables the cell to decouple receptor fate from cargo fate.

Question 5

After a signaling receptor like EGFR is internalized by receptor-mediated endocytosis, what determines whether the receptor is recycled or degraded, and why does this choice matter for cell signaling?

Model answer: The sorting decision occurs at the early endosome and is governed by ubiquitination of the receptor's cytoplasmic tail. Heavily ubiquitinated receptors are recognized by the ESCRT machinery, sorted into multivesicular bodies, and ultimately delivered to lysosomes for degradation — permanently reducing receptor number and dampening future signaling. Minimally ubiquitinated or unmodified receptors are packaged into recycling tubules and returned to the plasma membrane. The functional consequence is profound: recycling maintains or restores the cell's sensitivity to the ligand, while degradation constitutes signal termination and downregulation of the pathway. Cells use this sorting decision to tune how long and how strongly they respond to growth factors and other extracellular signals.

This receptor-fate decision is a major mechanism of signal regulation. Overactivated growth factor signaling (e.g., in cancer) is often linked to mutations that impair receptor ubiquitination and degradation, causing continuous recycling and sustained signaling. Viruses exploit the same pathway in reverse — hijacking endocytosis for cell entry but then escaping lysosomal degradation by fusing with the endosomal membrane.