Endocytosis, Exocytosis, and Vesicular Trafficking (2A) - MCAT Biological and Biochemical Foundations of Living Systems

Primary sorting station for internalized cargo and receptors. Early endosomes receive and sort endocytosed materials, directing receptors for recycling and ligands for degradation based on pH-dependent dissociation.

Link cargo receptors to clathrin by binding sorting motifs in cytosolic tails. Adaptins recognize specific motifs on receptor tails, bridging them to clathrin to ensure selective cargo incorporation into forming vesicles.

v-SNAREs are on vesicles; t-SNAREs are on target membranes. This classification distinguishes vesicle-associated SNAREs (v) from target membrane SNAREs (t), ensuring directional and specific fusion.

Dynamin. Dynamin assembles into a collar at the vesicle neck, and its GTP hydrolysis constricts and severs the membrane to release the vesicle.

Clathrin. Clathrin forms a polyhedral lattice around pits, driving membrane curvature and vesicle formation for selective cargo uptake.

Selective uptake of ligands via receptors concentrated in coated pits. It facilitates efficient, specific internalization of essential molecules by clustering ligand-bound receptors in pits that form coated vesicles.

Dynein. Dyneins harness ATP to move cargo along microtubules toward minus ends, supporting retrograde transport to central cellular regions.

Continuous, unregulated secretion and membrane delivery to the cell surface. It maintains steady-state secretion of proteins and lipids, supporting cell growth and membrane turnover without external stimuli.

Signal-triggered vesicle fusion, often $Ca^{2+}$-dependent (e.g., neurotransmitters). It allows rapid, controlled release of stored contents in response to signals, crucial for processes like synaptic transmission.

Recycling back to the plasma membrane. Receptor recycling conserves cellular resources by returning unbound receptors to the surface for further ligand binding and uptake.

Acidic organelle containing hydrolytic enzymes for macromolecule degradation. Lysosomes maintain an acidic environment optimal for enzyme activity, breaking down internalized materials into reusable components.

Engulfment of large particles into phagosomes, typically by immune cells. This mechanism enables specialized cells to ingest large particles like pathogens by extending pseudopods and forming phagosomes for degradation.

Nonselective endocytosis of extracellular fluid and dissolved solutes. It allows cells to non-specifically sample and uptake surrounding fluid and solutes through small vesicle formation for nutrient acquisition.

Secretion by vesicle fusion with the plasma membrane. This mechanism releases cellular contents extracellularly or incorporates membrane proteins by merging vesicle membranes with the plasma membrane.

Uptake of extracellular material via membrane invagination and vesicles. This process enables cells to internalize molecules and particles that cannot diffuse across the plasma membrane by forming intracellular vesicles.

Trafficking to late endosomes and then lysosomes. Cargo marked for breakdown progresses through endosomal maturation to lysosomes, ensuring degradation and nutrient recycling.

Ubiquitination. Ubiquitin tags signal sorting into multivesicular bodies via ESCRT complexes, directing proteins to lysosomes for proteolysis.

COPII. COPII coats assemble on ER membranes to form vesicles that transport newly synthesized proteins forward to the Golgi apparatus.

COPI. COPI coats facilitate vesicle formation for retrieving escaped ER proteins and recycling components back from the Golgi.

Mediate specific vesicle docking and membrane fusion. SNAREs form trans-complexes that zipper membranes together, providing energy and specificity for fusion events in trafficking pathways.

Provide targeting specificity by recruiting tethering and docking factors. Rabs cycle between GTP- and GDP-bound states to recruit effectors that guide vesicles to correct destinations and promote docking.

Microtubules. Microtubules provide polarized tracks for efficient, directed transport of vesicles over long distances within the cell.

Kinesin. Kinesins use ATP hydrolysis to walk along microtubules toward plus ends, facilitating anterograde transport to peripheral sites.