Cell Biology: Formation of Transport Vesicles, Endocytosis, Exocytosis, Transcytosis & Lysosomes

Comprehensive vocabulary flashcards covering vesicle formation, endocytic mechanisms (pinocytosis, phagocytosis, macropinocytosis), exocytosis, lysosome biology, autophagy, and associated clinical pathologies.

Vesicle-Mediated Transport

The cellular process of moving macromolecules and particulate material between membrane-bound compartments or across the plasma membrane using transport vesicles.

COPII-Coated Vesicles

Transport vesicles assembled via the monomeric GTPase Sar1-GTP that mediate anterograde transport from the endoplasmic reticulum (ER) to the cis-Golgi network.

COPI-Coated Vesicles

Coatomer-coated vesicles that mediate retrograde transport from the Golgi apparatus back to the rough endoplasmic reticulum (RER) and between Golgi cisternae from advanced to earlier cisternae.

Clathrin

A coat protein assembled into a triskelion lattice that drives vesicle formation in receptor-mediated endocytosis at the plasma membrane, transport from the trans-Golgi network to endosomes/lysosomes, and synaptic vesicle recycling.

Auxilin and Hsc70

A cofactor and chaperone pair responsible for uncoating clathrin-coated vesicles; auxilin binds the clathrin coat and recruits Hsc70, which hydrolyzes $\text{ATP}$ to $\text{ADP}$ to disassemble the clathrin lattice for recycling.

Dynamin

A large GTPase that forms a helical collar around the neck of invaginated coated pits and hydrolyzes GTP to mediate membrane scission, releasing vesicles into the cytoplasm.

SNARE Proteins (v-SNARE and t-SNARE)

Transmembrane pairing proteins that mediate vesicle targeting and membrane fusion; v-SNAREs on the transport vesicle bind specifically to complementary t-SNAREs on the target organelle membrane.

NSF and SNAPs

Proteins (N-ethylmaleimide-sensitive factor and soluble NSF attachment proteins) that utilize energy from $\text{ATP}$ hydrolysis to disassemble the stable four-helix trans-SNARE complex after membrane fusion has occurred.

Rab Proteins

A family of monomeric GTPases that associate with specific transport vesicles and target membranes to regulate vesicle budding, motility along cytoskeletal tracks, tethering, and docking in the endolysosomal system.

Rab5

A membrane-associated GTPase localized to early endosomes that catalyzes homotypic vesicle fusion and coordinates early endocytic trafficking.

Rab7

A GTPase marker of late endosomes and phagolysosomes that governs endosomal maturation, retrograde transport, and fusion with lysosomes.

Rab9

A late endosomal GTPase that mediates the retrograde recycling of mannose-6-phosphate receptors from late endosomes back to the trans-Golgi network (TGN).

Pinocytosis

The non-specific or receptor-mediated uptake of extracellular fluids and dissolved solutes into small vesicles ($< 200\,\text{nm}$), occurring continuously in virtually all eukaryotic cells for nutrition, metabolic regulation, and membrane recycling.

Phagocytosis

The actin-dependent engulfment of large, solid particles ($> 250\,\text{nm}$), such as microorganisms and cell debris, by specialized professional phagocytes (macrophages, neutrophils, and dendritic cells) primarily for defense and tissue homeostasis.

Caveolae

Flask-shaped plasma membrane invaginations of approximately $65\,\text{nm}$ in diameter enriched in cholesterol, sphingolipids, and caveolin proteins that function in transcytosis, lipid regulation, and signal transduction.

Caveolins

Integral membrane proteins that insert into the inner leaflet of the plasma membrane to form caveolae; includes Caveolin-1 and Caveolin-2 (widespread) and Caveolin-3 (specific to striated muscle).

Cavin Complex

A multi-protein complex composed of Cavin-1 (PTRF; caveolae formation), Cavin-2 (SDPR; caveolar membrane elongation), Cavin-3 (SRBC; budding and microtubule transport), and muscle-specific Cavin-4 (MURC).

Macropinocytosis

A dynamin-independent endocytic process driven by actin-mediated plasma membrane protrusions (lamellipodia, circular ruffles, or blebs) that collapse to enclose large droplets of extracellular fluid into macropinosomes.

Early Endosome

A sorting organelle located in the peripheral cytoplasm with a slightly acidic lumen ($\text{pH } 5.9 - 6.2$) maintained by proton pumps, which directs endocytosed ligands and receptors toward recycling, transcytosis, or degradation.

Multivesicular Body (MVB)

An endosomal intermediate containing internal vesicles formed by the inward budding of the limiting membrane, sequestering membrane proteins earmarked for degradation in the lysosome via ESCRT complexes.

Late Endosome

A perinuclear compartment with an acidic internal environment ($\text{pH } 5.0 - 5.5$) containing Rab7 and mannose-6-phosphate receptors ($\text{M6PR}^+$), functioning as the receiving station for acid hydrolases transported from the trans-Golgi network.

Mannose-6-Phosphate (M6P) Tag

A specialized carbohydrate modification added in the cis-Golgi to lysosomal enzyme precursors, directing their recognition by M6P receptors in the trans-Golgi network for delivery to the endolysosomal pathway.

Familial Hypercholesterolemia (FH)

An inherited metabolic disorder characterized by defective or absent LDL receptors, preventing clathrin-dependent endocytosis of LDL particles and resulting in severe cholesterol accumulation in blood.

TMPRSS2

A host cell surface serine protease that cleaves and primes the SARS-CoV-2 spike (S) protein at the S1/S2 and S2' sites, promoting viral entry via ACE2 receptor binding followed by endocytosis or direct plasma membrane fusion.

Opsonization

The immune coating of pathogens or foreign particles with molecules such as IgG antibodies or complement fragments, which bind to specific surface receptors (e.g., $\text{Fc}\gamma\text{RI}$) on phagocytes to stimulate engulfment.

Pseudopodia

Actin-rich, transient projections of the leukocyte plasma membrane formed through the coordinated activation of Src, Cdc42, Rac1, WASP, and the Arp2/3 complex to engulf target particles during phagocytosis.

Heterophagy

The pathway in which exogenous materials captured outside the cell through endocytosis or phagocytosis are digested by lysosomal acid hydrolases within phagolysosomes.

Phagolysosome

A digestive vacuole formed by the fusion of a phagosome with one or more primary or secondary lysosomes, exposing engulfed microbes or cellular debris to lethal acid hydrolases.

Autophagy

A conserved intracellular degradation mechanism in which cytoplasmic components, damaged organelles, or misfolded proteins are sequestered in double-membrane vesicles and delivered to lysosomes for degradation and nutrient recycling.

Macroautophagy

The form of autophagy where a crescent-shaped isolation membrane (phagophore) nucleated by the Atg12-Atg5-Atg16 complex elongates and closes into a double-membrane autophagosome that subsequently fuses with a lysosome.

LC3-II

The lipid-conjugated (phosphatidylethanolamine-linked) form of cytosolic LC3-I that integrates into the autophagosomal membrane, serving as a definitive molecular marker of autophagosome formation.

Mitophagy

The selective degradation of damaged or depolarized mitochondria initiated by the accumulation of PINK1 on the outer mitochondrial membrane, which recruits the E3 ubiquitin ligase Parkin to ubiquitinate surface proteins for autophagic clearance.

Chaperone-Mediated Autophagy (CMA)

A selective autophagic pathway where cytosolic proteins containing a specific recognition motif are bound by Hsc70 and co-chaperones, recognized by the lysosomal receptor Lamp-2A, unfolded, and directly translocated across the lysosomal membrane.

Microautophagy

The direct sequestration of cytosolic portions or small organelles via invagination and scission of the lysosomal or vacuolar limiting membrane.

Lipofuscin

An undegradable 'wear-and-tear' pigment consisting of oxidized, cross-linked lipid and protein residues that accumulates as golden-brown granules near the nucleus in long-lived or post-mitotic cells such as neurons and cardiomyocytes.

Primary Lysosome

A virgin lysosome budded from the Golgi apparatus that contains newly synthesized acid hydrolases but has not yet participated in intracellular catabolic digestion ($\text{pH } 6.5 - 7.0$).

Secondary Lysosome

An actively digesting organelle formed by the fusion of a primary lysosome with an endosome, phagosome, or autophagosome, operating at an optimal degradative $\text{pH } 4.5 - 5.0$.

Tertiary Lysosome (Residual Body)

The final stage of lysosomal digestion containing indigestible residues (e.g., concentric pseudomyelin whorls, lipofuscin) that persist permanently in the cytoplasm or are eliminated via exocytosis.

LAMPs (Lysosome-Associated Membrane Proteins)

Heavily glycosylated transmembrane proteins (such as LAMP1 and LAMP2) whose carbohydrate coats project into the lysosomal lumen to protect the limiting membrane from enzymatic self-destruction by acid hydrolases.

V-Class $\text{H}^+$ ATPase

An ATP-dependent proton pump located in the membrane of endosomes and lysosomes that actively pumps protons from the cytosol into the organelle lumen to establish and maintain an acidic interior.

Tay-Sachs Disease

A fatal autosomal recessive lysosomal storage disease caused by a deficiency of the enzyme hexosaminidase A, preventing the breakdown of ganglioside $GM_2$ to $GM_3$ and leading to toxic neuronal lipid accumulation characterized by laminated pseudomyelin structures.

Gaucher Disease

A lysosomal storage disease caused by a genetic deficiency in glucocerebrosidase, leading to the accumulation of glucocerebroside in macrophages and producing liver, spleen, bone, and neurological defects.

Acrosome Reaction

A specialized exocytic event wherein the acrosomal granule of a sperm releases lysosomal enzymes, such as hyaluronidase, to digest the extracellular jelly coat and vitelline layer of the egg during fertilization.

Osteoclast Bone Resorption

The process by which osteoclasts create an extracellular low-pH microenvironment sealed by a clear zone at their ruffled border, secreting protons and the lysosomal enzyme Cathepsin K to dissolve mineralized bone matrix.

Constitutive Exocytosis

A continuous, unregulated, $\text{Ca}^{2+}$-independent secretory pathway present in all cells that supplies newly synthesized lipids and proteins to the plasma membrane and continually releases extracellular matrix components or serum proteins.

Regulated Exocytosis

A stimulus-dependent secretory pathway in which cell products (e.g., hormones, neurotransmitters, digestive enzymes) are concentrated and stored in secretory vesicles, fusing with the plasma membrane only in response to a rise in cytosolic $\text{Ca}^{2+}$.

"Kiss-and-Run" Exocytosis

A rapid mode of secretory vesicle release wherein the vesicle connects transiently with the plasma membrane via a narrow fusion pore to discharge its contents, rapidly closing and detaching without fully collapsing into the membrane.

Porosome

A specialized, cup-shaped lipoprotein supramolecular structure at the presynaptic plasma membrane where secretory vesicles dock to establish t-/v-SNARE channels for the regulated expulsion of intravesicular contents.

Transcytosis

The vesicular transport of macromolecules across the cell interior from one extracellular surface to the opposite surface of a polarized cell (e.g., IgA transport via the poly-Ig receptor or maternal IgG across neonatal intestinal epithelium).