Endocytosis and Intracellular Trafficking

Vocabulary flashcards generated from lecture notes on endocytosis mechanisms, structural proteins, scission, and intracellular trafficking regulators.

Endocytosis

The cellular process by which material from outside the cell is internalized through the plasma membrane into intracellular compartments.

Phagocytosis

A specialized form of endocytosis involved in the uptake of large particles ($> 500\,\text{nm}$) such as bacteria, dead cells, or cellular debris by specialized immune cells like macrophages.

Trigger mechanism

A mechanism of bacterial-induced phagocytosis where a bacterium directly contacts the host cell and injects/delivers effectors to stimulate host cytoskeletal machinery.

Zipper mechanism

A mechanism of bacterial-induced phagocytosis where bacterial surface molecules bind to multiple host cell surface receptors, causing receptor clustering and progressive membrane wrapping around the pathogen.

Macropinocytosis

An actin-dependent endocytic process allowing cells to internalize large volumes of extracellular fluid ("cell drinking") through membrane ruffles.

Clathrin

The main structural coat protein in clathrin-dependent endocytosis, composed of heavy and light chains organized into triskelions that assemble around budding vesicles.

Triskelion

A three-legged protein complex formed by three clathrin heavy chains and three clathrin light chains that acts as the basic structural unit of the clathrin coat.

Clathrin-coated vesicle

A membrane vesicle formed during clathrin-dependent endocytosis with a coat of clathrin triskelions and an approximate diameter of $60\,\text{to}\,63\,\text{nm}$.

Adaptor complex

A protein complex required in clathrin-dependent endocytosis to directly link membrane-bound cargo receptors to the outer clathrin coat.

Dynamin

A GTPase protein that forms a helical structure around the neck of a budding vesicle and uses GTP hydrolysis ($\text{GTP} \rightarrow \text{GDP} + \text{P}_i$) to execute vesicle scission/pinching-off.

Caveolin

A structural membrane protein that directly interacts with membrane lipids and cholesterol to form flask-like caveolae or tubular endocytic structures.

Caveolae

Small flask-like plasma membrane invaginations composed of caveolin proteins and rich in cholesterol that can participate in endocytosis or serve as membrane reserves.

CLIC-GEEC

A clathrin-independent and dynamin-independent endocytic pathway that forms tubular carriers driven by actin and small GTPases to internalize GPI-anchored proteins (GPI-APs).

Shiga toxin subunit B (STxB)

A toxin subunit that binds to Gb3 glycolipids in the plasma membrane, inducing negative membrane curvature and lipid-mediated endocytosis.

Small GTPase

A regulatory protein that functions as a molecular switch, existing in an active GTP-bound state and an inactive GDP-bound state.

GEF (Guanine nucleotide exchange factor)

A regulatory protein that promotes the release of GDP and loading of GTP, converting a small GTPase into its active state.

GAP (GTPase-activating protein)

A regulatory protein that accelerates GTP hydrolysis by a small GTPase, converting it into its inactive GDP-bound state.

GDI (GDP dissociation inhibitor)

A regulatory factor that inhibits GDP dissociation and regulates the membrane association of small GTPases.

Rab proteins

The largest subfamily of small GTPases (over 60 members in humans) that regulate intracellular trafficking and organelle targeting.

Arf proteins

A family of small GTPases involved in regulating membrane trafficking and coat recruitment.

Rho proteins

A family of small GTPases that primarily regulate actin cytoskeleton dynamics and membrane remodeling.

Ras proteins

A family of small GTPase proteins predominantly involved in cellular signaling pathways.

Clathrin-dependent vs Caveolin-dependent endocytosis

Comparison of main characteristics, structural proteins, membrane association, and components between clathrin-dependent and caveolin-dependent pathways.

Endocytic Pathways Summary

Classification and comparison of major endocytic pathways based on particle volume, characteristics, and key components.

Small GTPase Families Overview

Summary table listing the major families of small GTPases (Ras, Rho, Rab, Arf, Ral/Rap) and their primary cellular functions.