#endocytosis

Basics of Cell Biology

Which cellular process involves the intake of large molecules via vesicles?

Cells internalize macromolecules too large or polar to cross bilayer via endocytosis involving membrane invagination and vesicle formation consuming energy. Clathrin-mediated endocytosis concentrates receptors such as transferrin receptor, LDL receptor, and EGFR into clathrin-coated pits assembled by adaptor AP2 and accessory proteins EPS15, dynamin GTPase mediates scission producing 100 nm vesicles that shed clathrin via auxilin and Hsc70 and fuse with early endosomes marked by Rab5 and PI3P. Caveolin-dependent uptake forms 60 nm flask-shaped caveolae enriched in cholesterol, regulating lipid homeostasis and signal transduction. Phagocytosis in macrophages uses actin polymerization driven by Rho GTPases to extend pseudopods engulfing bacteria opsonized by IgG. Macropinocytosis non-selectively takes up fluid via ruffling. All require ATP for actin remodeling and dynamin function plus membrane trafficking through endosomal system for sorting, recycling via Rab11 or degradation via Rab7 to lysosomes. Exocytosis is opposite outward fusion, osmosis is water movement, simple diffusion is passive lipid permeation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Endocytosis Mechanisms.

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Membrane Transportation

Which of the following is an example of vesicular transport?

Vesicular transport refers to bulk modality moving macromolecules particles and fluid that cannot cross via single carriers by formation of membrane vesicles requiring cytoskeletal remodeling. Endocytosis exemplifies inward trafficking where plasma membrane invaginates forming coated pits concentrated by adaptor AP2 epsin sorting cargo low-density lipoprotein receptor nutrients antigens pathogens. Clathrin triskelion lattice polymerizes imposing curvature dynamin large GTPase forms helical collar around neck hydrolyzing GTP to pinch vesicle releasing one hundred nanometer vesicle containing sampled extracellular contents. Vesicle uncoats via auxilin Hsc70 ATPase and fuses with early endosome mediated by Rab5-GTP EEA1 tether and SNARE syntaxin13 SNAP25. Unlike simple diffusion where solutes partition directly through lipid down gradient governed by Fick's law without protein or facilitated diffusion using channels uniport carriers enabling passive equilibration or primary ATPases pumping ions vesicular transport translocates large cargo packets energy-dependent requiring ATP for actin and GTP for Rab activation yet classified as bulk transport rather than permeation. Exocytosis exports proteins via Golgi vesicles neurotransmitters via synaptic vesicles. Thus endocytosis represents canonical vesicular mechanism preserving membrane integrity while internalizing environment.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 13: Vesicular Transport – Endocytosis as Example.

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Cell Organelles

Endosomes are formed from:

Endosomal system is central node of vesicular traffic that integrates endocytic and biosynthetic routes to sort cargo after internalization or secretion. Classical early endosomes originate when clathrin-coated pits invaginate with assistance of AP2 adaptor, epsin and dynamin GTPase scission, or when caveolae containing caveolin internalize, delivering extracellular ligands, nutrient receptors such as transferrin receptor, signaling receptors like EGFR, and fluid-phase markers into cell. These primary vesicles rapidly lose coat through auxilin and Hsc70 and fuse homotypically via Rab5-GTP, EEA1 coiled-coil tether and phosphatidylinositol 3-phosphate to generate larger early sorting endosomes. However endosomes also receive substantial and essential input from trans-Golgi network, TGN, where newly synthesized lysosomal hydrolases tagged with mannose-6-phosphate and lysosomal membrane proteins LAMPs are sorted into clathrin and AP1, GGA vesicles that traffic and fuse with late endosomes marked by Rab7. This dual supply merges surface-derived endocytic material with Golgi-derived enzymes, ensuring lysosomes acquire complement of acidic hydrolases while plasma membrane cargo proteins are efficiently recycled via Rab4 and Rab11 routes. Thus endosomes represent convergence of plasma membrane-derived and TGN-derived pathways, with Golgi contributions indispensable for maturation, acquisition of degradative capacity and eventual fusion with lysosomes.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 13: Endosome Formation from Plasma Membrane and TGN.

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Cell Organelles

Early endosomes function in:

Endocytic trafficking relies on progressive maturation and sorting of internalized cargo to maintain membrane homeostasis and signaling fidelity. Early endosomes, also called sorting endosomes, receive clathrin-coated and non-clathrin vesicles from the plasma membrane containing receptors such as transferrin receptor, EGF receptor, ligands, low-density lipoprotein, and extracellular fluid. Their pleomorphic tubular-vesicular morphology and mildly acidic pH around 6.0 to 6.2, maintained by vacuolar ATPase delivering protons, facilitate dissociation of many ligands from receptors while preserving receptor structure. Rab5 GTPase in active GTP-bound state and its effectors EEA1 tether, phosphatidylinositol-3-phosphate lipid platform, and SNARE syntaxin 13 coordinate homotypic fusion and recruitment of sorting machineries. Geometrically distinct sorting domains segregate cargo for divergent fates: receptors like transferrin receptor recycle to plasma membrane via Rab4 fast loop and Rab11 slow perinuclear recycling routes, whereas ubiquitinated growth factor receptors are captured by ESCRT-0, I, II, III complexes into intraluminal vesicles directing toward multivesicular bodies for degradation. Lipids, nutrients and signaling molecules are similarly triaged. This organizing function prevents unnecessary degradation, sustains surface receptor homeostasis while initiating downstream MAPK signaling from endosomal platforms.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 13: Intracellular Vesicular Traffic – Early Endosomes and Sorting.

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Cytoskeleton

In receptor-mediated endocytosis, which protein activates the Arp2/3 complex to drive actin assembly?

Endocytosis of cargo such as EGFR transferrin requires actin assistance in mammalian cells against high membrane tension especially under high membrane stiffness. Clathrin coated pit maturation involves transient actin patch assembly lasting 10 to 20 seconds around neck. Key signaling axis Cdc42 GTP recruits N WASP, which in resting state autoinhibited by intramolecular interaction between basic region GBD and VCA. Activation occurs when Cdc42 GTP binds GBD, PIP2 binds basic region, and SH3 proteins TOCA1 syndapin bind proline rich stretch, releasing VCA. Free VCA binds one G actin via WH2 and Arp2/3 via acidic region bringing complex to side of preexisting filament. Branch nucleation at 70 degrees generates pushing force assisting invagination and cooperates with dynamin GTPase for scission. Cells from Wiskott Aldrich syndrome patients lacking functional WASP show defective TCR endocytosis and actin foci. Formins create unbranched filaments elsewhere, myosin I provides tension but not activation, CapZ restricts growth. Thus WASP family as WASp functions as initiator activating Arp2/3 to drive actin assembly during receptor mediated endocytosis and vesicle formation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: WASP Arp2/3 Activation in Endocytosis.

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Protein Sorting and Trafficking

What is the primary function of amphisomes?

Interplay macroautophagy and endocytic system produces hybrid intermediate called amphisome fusion product before lysosomal degradation. Autophagosome double-membrane vesicle size up to 1 micron bearing lipidated LC3-II cargo sequestered including mitochondria protein aggregates bacteria forms upon closure isolation membrane phagophore. Separately endocytic pathway internalizes surface receptors extracellular material into early endosomes marked Rab5 PI3P then matures Rab7 positive late endosomes multivesicular bodies MVBs intraluminal vesicles tetraspanins CD63 LAMP1 LBPA. Amphisome formation occurs when double-membrane autophagosome fuses late endosome driven SNARE syntaxin7 syntaxin8 Vti1b VAMP8 tethering HOPS complex Rab11 effectors ATG14 merging contents creating single hybrid organelle still bounded partially original outer autophagosome membrane plus endosomal membrane containing both LC3-II endosomal markers Rab7 Rab5 CD63. Structure subsequently fuses lysosome autolysosome where hydrolases cathepsins degrade autophagic endocytic cargo efficiently increasing capacity allowing delivery plasma membrane cargo autophagic compartments. ER-Golgi transport via COPII COPI dynamin scission iron transport transferrin recycle distinct trafficking circuits unrelated amphisome formation and degradative merging quality control.

Ref: Nakamura & Yoshimori, Autophagy 2017: Amphisome formation from autophagosome-endosome fusion.

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Protein Sorting and Trafficking

During receptor-mediated endocytosis of iron, Fe³⁺ is transported into cells via:

Vertebrates transport ferric iron safely via transferrin family to avoid oxidative Fenton radical damage and precipitation. Hepatocytes secrete apo-transferrin 80 kDa bilobed glycoprotein serum loads two Fe3+ each coordinated synergistically bidentate carbonate anion plus four protein ligands two tyrosines histidine aspartate each lobe producing salmon pink complex termed ferrotransferrin diferric transferrin. Doubly loaded species binds transferrin receptor TfR1 homodimer dissociation constant 1-10 nM neutral pH versus 50-fold lower affinity monoferric negligible apo providing selection iron loaded species for uptake efficiency. After clathrin-mediated endocytosis acidification pH 5.5 protonates carbonate triggering iron release facilitated ferrireductase STEAP3 converting Fe3+ to Fe2+ transported via DMT1 SLC11A2 divalent transporter. Ferritin 24-subunit cytosolic cage storing 4500 atoms ferrihydrite mineral core storage not serum transport. Heme carries Fe2+ within protoporphyrin oxygen transport hemoglobin myoglobin. Lysosome degrades iron proteins not physiological carrier. Ferrotransferrin therefore represents circulating iron import form enabling receptor specificity regulated high-affinity uptake supporting erythropoiesis respiration enzyme activities and DNA synthesis.

Ref: Lodish et al., MCB Figure 14-29: Ferrotransferrin carries Fe3+ for uptake via transferrin receptor.

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Protein Sorting and Trafficking

Which adapter protein interacts with the NPXY motif of LDL receptors?

LDL receptor internalization requires highly specific recognition cytosolic tail by endocytic co-adaptors bridging to clathrin lattice enhancing efficiency. Tail 50 amino acids contains conserved FXNPXY motif positions 802-807 forming tight beta-turn favored aromatic phenylalanine tyrosine. PTB domains autosomal recessive hypercholesterolemia protein ARH LDLRAP1 and Dab2 Disabled-2 liver recognize NPXY tyrosine hydrophobic pocket interacting also basic residues upstream motif. Adaptors contain additional trafficking domains: clathrin box binding heavy chain terminal domain, AP2 beta2 appendage binding DxF sites, N-terminal PIP2-binding FERM-like domain anchoring plasma membrane. Thus co-adaptor physically links receptor tail simultaneously phosphoinositide clathrin AP2 ensuring rapid clustering coated pits half-life two minutes. Direct interaction NPXY to AP2 mu2 very weak; co-adaptor amplifies specificity. AP1 TGN via gamma adaptin, AP3 lysosome-related organelles membranes. Mutation Tyr807Cys causing FH class 4 prevents ARH binding receptors remain diffusely surface normal LDL binding but severely reduced uptake leading hypercholesterolemia reflecting sorting defect not binding defect, highlighting co-adaptor necessity and pathway logic.

Ref: He et al., Biochemistry: NPXY motif of LDLR binds ARH and AP2 adaptor.

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Protein Sorting and Trafficking

Which protein is required for vesicle scission during receptor-mediated endocytosis?

Final separation stage clathrin-coated pit formation yielding free vesicle requires enzymatic fission beyond coat polymerization. Clathrin assembly with adaptor AP2 and BAR proteins epsin, amphiphysin, endophilin generates deeply invaginated bud connected plasma membrane narrow tubule neck diameter around 20 nm bilayer. Separation demands mechanical constriction cutting neck. Specialist protein dynamin large 96 kDa GTPase family founding member N-terminal GTPase domain, middle stalk, PH domain binding PI(4,5)P2 enriched site, GED and PRD binding SH3, polymerizes helical collar around neck. Recruitment mediated curvature-sensing BAR proteins generating curvature. Stoichiometry about 13 dimers per turn aligns G domains efficient hydrolysis. GTP binding tightens helix reducing lumen radius; cooperative hydrolysis drives constriction twisting shear breaking membrane hemi-fission intermediate releasing vesicle. Clathrin provides structural cage not enzymatic fission; Rab GTPases regulate downstream tethering early endosome via EEA1, SNAREs mediate subsequent fusion after uncoating. Temperature-sensitive shibire mutant drosophila dynamin blocks synaptic vesicle recycling restrictive temperature trapping pits long collars demonstrating indispensable scission function receptor-mediated endocytosis essential cellular uptake.

Ref: Kaksonen & Roux, Nat Rev Mol Cell Biol: Dynamin scission in clathrin-mediated endocytosis.

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Protein Sorting and Trafficking

The acidic pH in the late endosome causes:

Endosomal maturation involves Rab conversion progressive luminal acidification orchestrated by vacuolar V-type ATPase multisubunit rotary pump hydrolyzing ATP pumping protons generating pH early endosome 6.0-6.5 late 5.5-6.0 lysosome 4.5-5.0. Gradient critical sorting function beyond hydrolysis activation many ligand-receptor complexes exhibit pH dependent affinity due histidine protonation altering charge complementarity. Examples: LDL separates from LDL receptor pH below 6 releasing particle for lysosomal degradation while receptor recycles via retromer, mannose-6-phosphate receptors release lysosomal enzymes upon protonation binding pocket, transferrin releases Fe3+ upon protonation carbonate while apotransferrin remains bound retained high affinity acidic pH. Dissociation allows physical separation receptors segregate narrow recycling tubules via SNX while liberated ligands continue lysosome catabolism. Without acidification bafilomycin or weak base ammonium chloride pH rise sorting fails LDL remains bound preventing recycling and signaling. Clathrin assembly independent acidity, proteasomal degradation cytosolic, mitochondria fusion unrelated pH-triggered endosomal dissociation ensuring efficient cargo segregation and receptor reuse essential homeostasis.

Ref: Maxfield & McGraw, Nat Rev Mol Cell Biol: Endosomal acidification dissociates receptor-ligand complexes.

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Protein Sorting and Trafficking

Which type of endocytosis involves the uptake of small extracellular fluid droplets?

Cells internalize extracellular material by mechanistically distinct endocytic routes distinguished by particle size, actin requirement, coat dependence. Phagocytosis mediates uptake large particulates above 0.5 micron such as bacteria, yeast, dead cells via extension actin-driven pseudopods triggered clustering Fc gamma receptors, complement receptors CR3, scavenger receptors signaling via Syk, PI3K, Rho GTPases prominent professional phagocytes macrophages neutrophils forming phagosome destined lysosome. Pinocytosis literally cell drinking from Greek pinein describes constitutive uptake small droplets extracellular fluid and dissolved solutes without particle binding via multiple clathrin-dependent and independent mechanisms including macropinocytosis involving Rac1 mediated ruffling producing 0.2-5 micron vacuoles and micropinocytosis via caveolae and CLIC GEEC pathways 60-80 nm vesicles. Fluid-phase markers horseradish peroxidase, fluorescent dextran accumulate linearly in pinosomes. Receptor-mediated endocytosis concentrates specific ligands LDL, transferrin using AP2 clathrin coats with high efficiency saturable. Autophagy delivers internal cytoplasmic components organelles to lysosome for self-digestion opposite direction. Therefore pinocytosis uniquely denotes non-concentrative fluid-phase small droplet uptake distinct from phagocytosis and receptor mediated.

Ref: Alberts et al., MBC Chapter 13: Pinocytosis for fluid-phase uptake.

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Protein Sorting and Trafficking

The process of vesicle scission during clathrin-mediated endocytosis requires:

Formation of endocytic vesicle demands energetically unfavorable fission of neck connecting bud to parent membrane against line tension and bending resistance. In clathrin-mediated pathway adaptor AP2 and accessory proteins FCHo, epsin together with clathrin lattice driving polymerization stabilize deeply invaginated bud with omega shape. Constriction of neck performed by dynamin large 96 kDa GTPase assembling into helical oligomer about two turns around neck 50 nm diameter PH domain binding PI(4,5)P2. Polymerization aligns G domains enhancing GTPase 100 fold. Upon concerted GTP binding and cooperative hydrolysis helix compacts from 20 nm to 4 nm radius and twist extends producing high curvature shear stress breaking membrane continuum executing hemi-fission then full fission releasing coated vesicle. Actin polymerization via Arp2/3 and cortactin assists under high tension at apical membranes. SNARE interactions drive downstream fusion with early endosome after uncoating, NSF ATPase disassembles cis-SNAREs post-fusion, Rab5 GTP hydrolysis coordinates early tethering but not scission. Dominant negative dynamin K44A unable to bind GTP arrests pits as elongated tubules. Therefore vesicle scission uniquely depends dynamin-mediated GTP hydrolysis providing mechanoenzyme activity essential for endocytosis.

Ref: Schmid & Frolov, Annu Rev Cell Dev Biol: Dynamin GTP hydrolysis drives membrane scission.

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