Endocytosis and Receptor-Mediated Uptake
How does a cell obtain cholesterol packaged inside a bloodborne lipoprotein particle? This particle is too large to cross the plasma membrane. The cell cannot dissolve the particle in place. Instead, it must swallow a piece of its own surface, particle and all, and pull it inside. This is endocytosis: the invagination and pinching-off of the plasma membrane. This process internalizes extracellular material, membrane lipids, and membrane proteins together. Receptor-mediated endocytosis is the selective version of this process. Specific cargo is captured by transmembrane receptors. It is then concentrated into a forming vesicle before it ever leaves the cell surface. The problem this solves is fundamental to cell biology. Cells must take up nutrients (iron-transferrin, LDL-cholesterol). They must also attenuate or sustain signaling (growth factor receptors). They sample their environment (antigen uptake) and remodel their own surface (turnover of adhesion molecules and transporters). Each of these tasks needs a mechanism that is fast and selective. Its outcome must also be reversible — cargo must sometimes be destroyed and sometimes sent back to the surface intact. The cell solves this with a modular pathway. Coated pits assemble at the plasma membrane and pinch off as vesicles. These vesicles fuse with a sorting compartment called the early endosome. The endosome itself acts as a molecular switchyard. It reads pH, cargo identity, and ubiquitin marks. Using these signals, it decides the fate of everything it received. This article builds directly on Vesicular Transport and Coat Proteins and SNAREs and Vesicle Fusion. It applies those general principles to a specific, extensively studied traffic route. It also revisits ideas from Sorting at the Trans-Golgi Network. The endosome uses many of the same sorting logic tricks the TGN does, just in reverse gear. The story is also one of the best-documented in cell biology. The discovery of the LDL receptor pathway by Michael Brown and Joseph Goldstein turned a genetic disease, familial hypercholesterolemia, into the key that unlocked an entire trafficking system. This work earned a Nobel Prize and reshaped how physicians think about cholesterol and heart disease. By the end of this article you should be able to describe, step by step, how a clathrin-coated pit forms and detaches. You should also understand how the early endosome sorts receptors from ligands using pH. Finally, you should know how the cell decides between recycling a receptor to the surface and shipping it to the lysosome for destruction. These decisions have direct consequences for cholesterol homeostasis, iron uptake, and growth factor signaling duration.
- Receptor-mediated endocytosis concentrates specific cargo into clathrin-coated pits at the plasma membrane, which pinch off as coated vesicles using the GTPase dynamin.
- Adaptor protein complexes (AP-2) link cargo sorting signals in receptor tails to the clathrin lattice, ensuring only tagged receptors are packaged.
- Vesicles shed their clathrin coat, fuse with early endosomes, and encounter a mildly acidic lumen (pH ≈ 6.0–6.5) that dissociates most receptor–ligand complexes.
- Sorting at the endosome is spatially organized: recycling receptors enter tubular extensions back to the surface, while cargo destined for destruction stays in the endosome body as it matures into a late endosome/multivesicular body.
- The LDL receptor pathway, elucidated by Brown and Goldstein using cells from familial hypercholesterolemia patients, was the founding paradigm for the entire field.
- Sorting decisions rely on short peptide motifs (NPXY, YXXΦ, dileucine) and ubiquitination, read out by distinct adaptor and ESCRT machineries — misreading these signals underlies genetic disease and is exploited by viruses and toxins.
Endocytosis captures receptor-bound cargo into clathrin-coated vesicles, and the acidic, ubiquitin- and motif-reading early endosome then sorts each receptor toward recycling, degradation, or transcytosis — turning a single internalization event into three distinct possible fates.
The core idea
Endocytosis is the general process by which cells internalize extracellular fluid, solutes, particles, and plasma membrane components by engulfing them within a membrane-bound vesicle derived from the cell surface. It is topologically the reverse of exocytosis: instead of an intracellular vesicle fusing with the plasma membrane to release contents, a patch of plasma membrane buds inward and pinches off to form an intracellular vesicle. Three broad modes exist — phagocytosis (large particles, largely restricted to specialized cells like macrophages), pinocytosis or fluid-phase endocytosis (bulk, non-selective uptake of extracellular fluid), and receptor-mediated endocytosis, the selective uptake of specific macromolecules bound to cell-surface receptors. This last mode is the focus here because it illustrates, with unusual clarity, the general coat-and-cargo logic already introduced for COPI, COPII, and clathrin/AP-1 traffic at the Golgi. The central molecular machine is the clathrin-coated pit, a specialized region of plasma membrane where clathrin triskelia and adaptor protein complexes assemble on the cytosolic face, deform the membrane into a pit, and concentrate specific transmembrane receptors — and their bound ligands — into the forming vesicle. The adaptor protein complex AP-2 is the key selectivity filter here, reading short linear sorting motifs in the cytoplasmic tails of receptors (such as NPXY and YXXΦ, where Φ is a bulky hydrophobic residue) and linking them physically to the clathrin lattice. Vesicle scission is catalyzed by the large GTPase dynamin, which oligomerizes into a helical collar around the neck of the budding pit and constricts it, in a GTP-hydrolysis-dependent mechanochemical action, until the membrane fissions. Once internalized, the vesicle rapidly loses its clathrin coat and fuses with the early endosome, a pleomorphic, mildly acidic (pH ≈ 6.0–6.5) sorting compartment maintained by vacuolar H⁺-ATPases (V-ATPases) that pump protons into its lumen. This acidification is not incidental — it is the mechanistic trigger for sorting. Many receptor–ligand pairs, most famously the LDL receptor and LDL particle, have much lower affinity for each other at pH 6 than at neutral pH 7.4, so acidification causes dissociation inside the endosome. The endosome then physically separates freed receptors, which cluster into narrow recycling tubules that bud off and return to the plasma membrane (or to the trans-Golgi network), from ligands and receptors marked for destruction, which remain in the vacuolar body of the endosome as it matures into a late endosome / multivesicular body (MVB) and ultimately fuses with a lysosome. This three-way fork — recycle, degrade, or (in polarized cells) transcytose to the opposite membrane domain — is the single most important functional idea in this article.
Mechanism
Step 1 — Cargo capture and coated pit nucleation. Transmembrane receptors with the appropriate cytoplasmic sorting signals diffuse in the plane of the plasma membrane and become concentrated at sites where clathrin assembly is beginning. Clathrin itself does not touch the membrane or the receptor directly; it forms a three-legged triskelion (three heavy chains, three light chains) that polymerizes into a curved, geodesic lattice on the cytosolic face. The physical link between this lattice and the membrane/cargo is the heterotetrameric adaptor AP-2 (subunits α, β2, μ2, σ2), which binds phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) in the inner leaflet, binds clathrin, and binds sorting motifs in receptor tails (μ2 subunit reads YXXΦ motifs; the α/σ2 hemicomplex reads dileucine-type motifs). Additional accessory proteins (epsin, AP180, Eps15, FCHO1/2) help nucleate curvature and recruit AP-2 to the correct membrane patches.
Step 2 — Invagination and constriction. As more clathrin and AP-2 assemble, the flat lattice reorganizes into a curved basket, bending the underlying membrane into a budding pit roughly 100–150 nm in diameter. BAR-domain proteins and the intrinsic curvature-generating property of the clathrin lattice itself drive this shape change. The neck of the pit narrows until it is a thin membrane tubule connecting the bud to the plasma membrane.
Step 3 — Scission by dynamin. The GTPase dynamin self-assembles into a helical collar around this neck. GTP hydrolysis drives a conformational "power stroke" — the mechanistic details (constriction vs. a twisting/pinching motion) are still actively debated — that ultimately severs the membrane neck, releasing a free clathrin-coated vesicle into the cytoplasm. This step can be blocked pharmacologically (dynasore, a small-molecule dynamin GTPase inhibitor) or genetically (the temperature-sensitive shibire mutant in Drosophila, discussed below), both of which trap deeply invaginated, unreleased pits — direct evidence that dynamin's job is specifically fission, not budding.
Step 4 — Uncoating. Almost immediately after release, the clathrin coat is shed. The chaperone Hsc70, together with its cofactor auxilin, uses ATP hydrolysis to actively disassemble the clathrin lattice, freeing clathrin triskelia for reuse and exposing a naked vesicle membrane that can now fuse with target compartments — a step governed by Rab GTPases and SNAREs as in general SNAREs and Vesicle Fusion machinery (notably Rab5 and its effectors on the forming early endosome).
Step 5 — Fusion with the early endosome and acidification. The uncoated vesicle fuses homotypically/heterotypically with the early endosome, a Rab5-positive compartment. V-ATPases pump H⁺ into the lumen, dropping pH to about 6.0–6.5 (versus pH 7.4 outside and in the cytosol). This acidic environment is the decisive sorting trigger for many receptor systems.
Step 6 — Sorting: three exits from one compartment.
- Recycling: Receptors whose ligands dissociate at low pH (e.g., LDL receptor, transferrin receptor after iron release) diffuse into narrow, tubular extensions of the endosome that bud off as recycling vesicles, returning receptors to the plasma membrane, often via a Rab11-positive recycling endosome, within minutes.
- Degradation: Receptors and ligands tagged with ubiquitin (added by cytosolic E3 ubiquitin ligases, e.g. Cbl acting on activated EGF receptor) are recognized by ESCRT complexes (ESCRT-0, -I, -II, -III), which cluster ubiquitinated cargo and drive inward budding of the limiting endosomal membrane, generating intraluminal vesicles and converting the early endosome into a multivesicular body (MVB). The MVB matures into a late endosome and fuses with a lysosome, where acid hydrolases degrade both cargo and the intraluminal vesicle membranes.
- Transcytosis: In polarized epithelial cells, some receptor–ligand complexes (e.g., the polymeric immunoglobulin receptor carrying IgA) are routed across the cell to the opposite plasma membrane domain, delivering intact cargo to a different extracellular face entirely.
Step 7 — Receptor reuse and its consequences. Because sorting, not destruction, is the default outcome for many receptors, a single LDL receptor can complete this entire round trip — surface, coated pit, endosome, recycling — in about 10–15 minutes and be reused hundreds of times over its ~20-hour lifetime. This recycling efficiency is precisely what makes LDL receptor deficiency states so damaging: losing the receptor doesn't just block one round of uptake, it disables a highly efficient repeating cycle.
The evidence
The definitive experimental foundation for receptor-mediated endocytosis comes from Michael Brown and Joseph Goldstein's work on the LDL receptor pathway, beginning in the early 1970s at the University of Texas Southwestern Medical School. They started from a human genetic disease, familial hypercholesterolemia (FH), in which patients — especially those homozygous for the mutation — develop massive elevations in plasma LDL-cholesterol and severe, early atherosclerosis, sometimes suffering heart attacks in childhood. Brown and Goldstein cultured skin fibroblasts from normal individuals and from FH patients and measured how each cell type handled radiolabeled LDL (labeled with ¹²⁵I). Normal fibroblasts bound LDL with high affinity at the cell surface, internalized it, degraded the protein component, and released the cholesterol to suppress the cell's own HMG-CoA reductase (the rate-limiting enzyme of cholesterol synthesis) — a beautiful feedback loop. Fibroblasts from homozygous FH patients, in stark contrast, showed little or no high-affinity binding of LDL, failed to internalize or degrade it efficiently, and consequently continued to synthesize cholesterol unchecked, because the suppressive feedback signal never arrived. This was the first clue that a discrete, saturable, high-affinity cell-surface receptor was responsible for LDL uptake, and that this receptor was missing or defective in FH. Using electron microscopy with ferritin-conjugated or gold-conjugated LDL as an electron-dense tracer, Brown, Goldstein, and their collaborator Richard Anderson showed directly, at the ultrastructural level, that LDL particles bound not diffusely across the cell surface but were concentrated in specific, morphologically distinct indentations of the plasma membrane — regions independently known from earlier work by Roth and Porter and by Barbara Pearse (who first isolated and named clathrin in 1976) to be coated with a bristly cytoplasmic protein lattice. These coated pits, comprising only about 2% of the fibroblast cell surface, were shown to capture the great majority of bound LDL, which was then followed by time-lapse EM into coated vesicles, then into larger smooth-walled vesicles (endosomes), and finally into lysosomes where the ferritin/gold label and the LDL protein disappeared as digestion occurred. Crucially, in a specific class of FH patients (later shown to carry mutations in the receptor's cytoplasmic tail, disrupting its internalization signal), LDL bound normally at the cell surface but was not internalized into coated pits — it remained stranded on the surface. This genetic dissociation of binding from internalization proved that receptor binding and endocytic internalization are separable, sequential steps, each requiring distinct molecular features of the receptor. Brown and Goldstein were awarded the 1985 Nobel Prize in Physiology or Medicine for this work, which established the general paradigm — receptor binding, clustering into coated pits, internalization, endosomal delivery, lysosomal degradation, feedback regulation of metabolism — that was subsequently shown to apply, with variations, to transferrin, EGF, insulin, and dozens of other receptor systems. A second, complementary line of evidence nailed down the machinery of vesicle scission itself. In the 1970s, Jean-Maurice Poodry and Loyal Edelson, and later extensively characterized by others, studied the Drosophila temperature-sensitive paralytic mutant shibire</strong> (Swahili for "paralyzed"). At the restrictive temperature, shibire flies rapidly and reversibly become paralyzed because synaptic vesicle recycling fails at nerve terminals. Electron microscopy of shibire nerve terminals at the restrictive temperature revealed a strikingly specific defect: the plasma membrane became studded with hundreds of deeply invaginated, coated pits connected to the surface by long, thin necks — pits that had done everything except pinch off. The shibire gene was subsequently cloned and shown to encode the Drosophila homolog of dynamin, a GTPase. This was decisive genetic proof that dynamin's specific job is the fission step, since its loss stalls vesicles at exactly the invaginated-but-connected stage, rather than blocking coat assembly or cargo selection. Subsequent biochemical and structural work (dynamin self-assembly into helical collars around lipid tubules in vitro, GTPase-dependent constriction observed by cryo-EM) confirmed the mechanical model directly.
Worked examples
Example 1 — Genetics of familial hypercholesterolemia (a dominant, gene-dosage disease). The LDL receptor gene is autosomal, and FH-causing mutations behave as incompletely dominant at the level of plasma cholesterol, because receptor number directly sets uptake capacity — there is no full molecular compensation from the single remaining normal allele. Consider a cross (conceptually, since this is a human pedigree analysis, not an experimental cross) between two heterozygous FH carriers (Rr × Rr, where R = normal allele, r = defective LDL receptor allele):
| Genotype | Ratio | Phenotype | Approx. surface LDL receptors | Plasma LDL-C |
|---|---|---|---|---|
| RR | 1/4 | unaffected | ~100% | normal (~130 mg/dL) |
| Rr | 1/2 | heterozygous FH | ~50% | ~2× elevated (~300–400 mg/dL) |
| rr | 1/4 | homozygous FH | ~0% | ~6–10× elevated (>600–1000 mg/dL), MI often before age 20 |
The mechanistic reason the heterozygote is intermediate, not silent, is that receptor number is rate-limiting for the flux of LDL through coated pits: with half the receptors, the cell captures LDL at roughly half the maximal rate, HMG-CoA reductase is only partially suppressed, and hepatic LDL clearance from plasma is correspondingly reduced — producing a real, measurable, dose-dependent phenotype rather than a Mendelian "carriers are normal" outcome. This is precisely the pattern Brown and Goldstein exploited: they could titrate receptor gene dosage against measured ¹²⁵I-LDL binding and degradation rates in cultured fibroblasts and find quantitative agreement, which is strong causal evidence, not merely correlation, that receptor number determines uptake rate.
Example 2 — Kinetics of receptor recycling: how many rounds can one receptor make? Suppose a hepatocyte has 20,000 LDL receptors on its surface at steady state, and each completes one full internalization–recycling cycle (surface → coated pit → endosome → recycle back to surface) in an average of 12 minutes, spending only about 3 minutes of that cycle actually inside the cell. Assume the receptor's total functional lifetime before being degraded is about 20 hours (a simplification; some receptors degrade earlier via different pathways, but this approximates the physiological turnover number reported in classic kinetic studies). Number of cycles per receptor lifetime: 20 hours = 1200 minutes 1200 minutes ÷ 12 minutes/cycle = 100 cycles If each cycle delivers, on average, one bound LDL particle (each carrying roughly 1,500 cholesterol ester molecules) into the cell, then over its lifetime one receptor mediates uptake of ~100 LDL particles, i.e., roughly 150,000 cholesterol molecules — a striking amplification that explains why loss of even a modest fraction of receptors (as in heterozygous FH) produces a large cumulative deficit in cholesterol clearance over a lifetime, and why receptor recycling (rather than one-shot, single-use receptors) is essential: without recycling, the cell would need 100-fold more receptor synthesis to achieve the same lifetime uptake, an enormous biosynthetic burden. This calculation also explains clinically why statins (which upregulate LDL receptor expression by inhibiting HMG-CoA reductase and relieving feedback repression of the receptor gene) are so effective — a modest increase in receptor number, multiplied over ~100 cycles each, produces a large increase in total LDL clearance.
Common misconceptions & traps
| Misconception | Why it's wrong |
|---|---|
| "Endocytosis and phagocytosis are the same process" | Phagocytosis is actin-driven engulfment of large particles by specialized cells (macrophages, neutrophils); clathrin-mediated endocytosis handles small (~100 nm) vesicles and uses a completely different coat and motor machinery. |
| "Receptors are destroyed every time they're used" | Most receptors (LDL receptor, transferrin receptor) are efficiently recycled and reused hundreds of times; degradation is the fate for a specific, ubiquitin-tagged subset, not the default. |
| "Low endosomal pH destroys the ligand" | Acidification typically just weakens receptor–ligand binding affinity so they dissociate; actual proteolytic degradation requires delivery to the lysosome and its acid hydrolases, a later and separate step. |
| "Clathrin coats the vesicle all the way to the lysosome" | Clathrin is shed by Hsc70/auxilin within seconds of vesicle release, before fusion with the early endosome; the endosome and its onward carriers are clathrin-free. |
| "All internalized receptors go to the same place" | The early endosome actively sorts cargo into at least three distinct fates (recycling, degradation, transcytosis) based on molecular signals, not a single default pathway. |
| "Dynamin builds the coated pit" | Dynamin's role is scission at the neck of an already-formed, deeply invaginated pit; clathrin and AP-2 build the pit itself. |
The scientist behind it
Michael S. Brown and Joseph L. Goldstein met as young physicians-in-training at the National Institutes of Health in the late 1960s and formed one of the most productive partnerships in modern biomedical science, working side by side for over five decades at UT Southwestern in Dallas. Their starting point was clinical, not biochemical: they were struck by the extraordinary severity of atherosclerosis in children with homozygous familial hypercholesterolemia, some of whom had heart attacks before age five, and they reasoned that understanding this rare genetic extreme would illuminate the general biology of cholesterol regulation in everyone. Rather than starting from a purified protein or an established pathway, they built a simple, quantitative cell-culture assay — measuring ¹²⁵I-LDL binding, internalization, and degradation in skin fibroblasts — and let the comparison between normal and FH cells reveal the receptor's existence, its two separable functions (binding and internalization), and its role in feedback control of cholesterol synthesis. Their collaboration with electron microscopist Richard Anderson supplied the crucial morphological correlate: coated pits as the actual physical structures responsible for the biochemically inferred selective uptake.
Origins · 1 minMichael S. Brown and Joseph L. Goldstein and how this was discovered
Where this matters today
Cardiovascular medicine: The entire rationale for statin therapy, the most widely prescribed class of drugs in cardiology, rests directly on this pathway — statins inhibit HMG-CoA reductase, which relieves feedback repression of LDL receptor gene transcription, increases hepatic surface receptor number, and accelerates clathrin-mediated clearance of LDL from plasma. The newer biologic class PCSK9 inhibitors (evolocumab, alirocumab) work by blocking PCSK9, a protein that normally binds the LDL receptor and redirects it to lysosomal degradation instead of recycling — inhibiting PCSK9 shifts receptor fate toward recycling, prolonging receptor lifetime and further lowering LDL-cholesterol, a direct clinical exploitation of the recycle-versus-degrade fork described above.
Iron metabolism and anemia: The transferrin receptor cycle — iron-loaded transferrin binds the receptor at neutral pH, is endocytosed, releases iron in the acidic endosome, and the receptor plus apo-transferrin recycle back to the surface — is a textbook parallel system used diagnostically; soluble transferrin receptor levels help distinguish iron-deficiency anemia from anemia of chronic disease.
Cancer biology: Growth factor receptors such as EGFR are regulated by ubiquitination-dependent endosomal sorting; mutations that impair EGFR degradation (escaping the ESCRT/MVB pathway) prolong signaling and contribute to oncogenic transformation, and several targeted cancer therapies aim to force sustained receptor internalization and degradation.
Genetic disease diagnosis: FH remains one of the most common autosomal dominant genetic disorders (heterozygote frequency roughly 1 in 200–250 in most populations, higher in certain founder populations), and genetic testing for LDL receptor, PCSK9, and APOB mutations now directly guides cascade screening of family members and early statin initiation.
Virology and toxinology: Many viruses (influenza, adenovirus) and toxins (diphtheria toxin) hijack clathrin-mediated endocytosis and endosomal acidification as their entry route into cells, using the low pH as the trigger for their own membrane-penetration machinery — understanding this pathway directly informs antiviral drug design targeting entry steps.
Neuroscience: Synaptic vesicle recycling at nerve terminals depends on the same clathrin/dynamin/Hsc70 machinery operating at very high speed and frequency, and drugs or mutations affecting dynamin function (as first revealed by shibire) directly impair neurotransmission — a foreshadowing link to how specialized membrane domains and adhesion, covered next in Cell Junctions Overview, must coexist with this constant local membrane remodeling at synapses and epithelial surfaces.
Ask the tutor: How does the endosome distinguish which receptors get ubiquitinated for degradation versus which are left alone to recycle?
Why does the early endosome need to be acidic, rather than neutral like the cytosol?
Acidification (pH ≈ 6.0–6.5, maintained by V-ATPase proton pumps) is the mechanistic trigger that separates receptor from ligand for many pathways. The LDL receptor, for instance, binds LDL tightly at the neutral pH of blood plasma but loses affinity at endosomal pH, so acidification causes dissociation without requiring any enzymatic cleavage. This lets the cell recycle the receptor intact while retaining the ligand for further processing. Different receptor systems tune their pH-sensitivity differently — the transferrin receptor actually binds its apo-transferrin product more tightly at low pH, which is why the empty transferrin stays receptor-bound during recycling and is only released back into plasma at the neutral pH of the extracellular space.
How is a coated vesicle different from an early endosome, and how do you distinguish them experimentally?
A coated vesicle is a small (~100 nm), clathrin-and-AP-2-coated, cytosol-facing bristly vesicle formed directly by pinching off from the plasma membrane; it exists only transiently, seconds to a couple of minutes, before uncoating. The early endosome is a larger, pleomorphic, tubular-vacuolar, clathrin-free compartment marked by Rab5 and EEA1, that receives multiple uncoated vesicles and actively sorts their cargo. Experimentally, they are distinguished by electron microscopy (coat morphology, presence/absence of the bristly clathrin lattice), by biochemical markers (clathrin heavy chain versus Rab5/EEA1), and by kinetics — pulse-chase experiments show ligand moving from a coated-vesicle-enriched fraction into an EEA1-positive fraction within roughly 1–5 minutes.
Do all internalized receptors eventually reach a lysosome?
No, and this is one of the most consequential facts in the pathway. Only receptors and ligands marked appropriately — typically by ubiquitination recognized by ESCRT machinery, or by intrinsically low pH-independent binding to their ligand — are retained in the endosome body as it matures into a multivesicular body destined for the lysosome. Most receptors, including LDL receptor and transferrin receptor, are efficiently sorted away from this fate into recycling tubules and returned to the plasma membrane within roughly 10–15 minutes, allowing them to be reused for many further rounds of uptake, which is far more biosynthetically economical than making a new receptor for every ligand internalized.
How does receptor-mediated endocytosis relate to signal transduction, beyond nutrient uptake?
Many cell-surface receptors that transduce signals — receptor tyrosine kinases like the EGF receptor, and G-protein-coupled receptors — are themselves internalized by clathrin-mediated endocytosis following ligand binding and activation. This internalization can terminate signaling (if the receptor is ubiquitinated and routed to lysosomal degradation, effectively downregulating the pathway) or, in some well-documented cases, can actually sustain or reshape signaling from endosomal membranes, which have a different local composition of adaptor and scaffold proteins than the plasma membrane. This makes endocytosis not merely a housekeeping uptake mechanism but an integral, tunable part of signal transduction dynamics, with direct relevance to why some cancer mutations that impair receptor downregulation are oncogenic.
What would happen to a cell if dynamin were completely nonfunctional?
Clathrin-coated pits would still form, cluster the correct receptors via AP-2, and invaginate deeply, because none of these steps require dynamin. However, the vesicles would never separate from the plasma membrane — exactly what is observed in shibire flies and in cells treated with the dynamin inhibitor dynasore, where electron microscopy shows plasma membranes studded with deeply invaginated, coated pits still connected to the surface by thin necks. The functional consequence is a severe block in receptor-mediated endocytosis (and, at nerve terminals, in synaptic vesicle recycling), because cargo capture without scission delivers nothing to the cell interior.
Why is receptor recycling so important for cellular economy, and what happens if recycling fails?
Recycling lets a fixed pool of receptors mediate uptake repeatedly — a receptor completing roughly 100 cycles over its functional lifetime achieves the equivalent uptake of 100 single-use receptors, at a fraction of the biosynthetic cost of continually synthesizing new receptor protein. If recycling machinery fails, or if mutations redirect receptors constitutively to degradation (as happens when PCSK9 activity is excessive, or in some LDL receptor mutations affecting recycling-relevant tail sequences), the effective surface receptor number falls even though the gene itself may be normal, producing an acquired or genetic phenotype that mimics reduced receptor synthesis — elevated plasma LDL-cholesterol and increased cardiovascular risk, in the LDL receptor case specifically.
How does receptor-mediated endocytosis differ from bulk-phase (fluid-phase) pinocytosis?
Fluid-phase pinocytosis internalizes whatever soluble material happens to be present in the small volume of extracellular fluid trapped inside a forming vesicle, with no molecular selectivity — concentration in the vesicle simply equals concentration outside. Receptor-mediated endocytosis actively concentrates specific ligands, sometimes several hundred-fold above their free extracellular concentration, because high-affinity receptors capture and cluster them into coated pits before internalization. This is why receptor-mediated pathways can achieve efficient uptake of scarce molecules (like LDL or iron-transferrin, present at low nanomolar-to-micromolar concentrations) that fluid-phase uptake alone could never capture fast enough to meet cellular needs.