Actin Cytoskeleton I

Vocabulary practice flashcards covering cytoskeletal filaments, actin filament structure, polymerization kinetics, polarity, treadmilling, and regulatory actin-binding proteins from the Actin Cytoskeleton I lecture.

Cytoskeleton (Regional Definition)

The detergent-insoluble portion of the cell remaining after detergent dissolves the membrane, membrane organelles, and allows soluble cytoplasmic components to escape.

Cytoskeleton (Structural Definition)

A collection of intracellular protein fibrils together with their associated proteins.

Fibroblast Cytoskeleton Filaments

The three main classes of cytoplasmic fibrils: microtubules (thickest, $\sim 25\text{ nm}$, red), intermediate filaments (intermediate thickness, $\sim 10\text{ nm}$, blue), and actin filaments (thinnest and most abundant, $\sim 7\text{ nm}$, yellow).

Microtubules

The thickest cytoskeletal fibers ($\sim 25\text{ nm}$ in diameter) composed of hollow cylinders of the protein tubulin.

Intermediate Filaments

Cytoskeletal fibers of intermediate thickness ($\sim 10\text{ nm}$ in diameter) made of linear subunit assemblies.

Multi-Strand Architecture

A structural design of cytoskeletal polymers where middle subunits form multiple bonds with neighbors to maintain structural strength, while end subunits have fewer neighbors to allow rapid association and dissociation for dynamic assembly.

G-actin (Globular Actin)

A $\sim 43\text{ kDa}$ asymmetric, flattened square-shaped monomeric protein with four subdomains and a deep central cleft that binds a nucleotide ($\text{ATP}$ or $\text{ADP}$).

F-actin (Filamentous Actin)

A two-stranded helical polymer of actin subunits exhibiting a crossover period of 13 subunits ($\sim 37\text{ nm}$), an axial rise of $28\text{ \AA}$, and a rotation twist of $-166.7^\circ$.

Actin Polymerization Rate Equation

The mathematical relationship $R = k_{on}[\text{G-actin}] - k_{off}$, where $k_{on}$ is the association rate constant ($\text{M}^{-1}\text{s}^{-1}$) and $k_{off}$ is the dissociation rate constant ($\text{s}^{-1}$).

Critical Concentration ($C_c$)

The free monomer concentration ($[\text{G-actin}] = k_{off} / k_{on} = K_D$) at which polymerization and depolymerization rates are equal ($R = 0$); filaments grow only when monomer concentration exceeds this value.

Nucleation Phase (Lag Phase)

The initial, rate-limiting step of actin polymerization where unstable monomer dimers slowly recruit a third monomer to assemble a stable, trimeric actin nucleus.

Myosin Decoration (Arrowhead Complex)

An experimental technique saturating actin filaments with myosin motor domains in the absence of $\text{ATP}$, producing an arrowhead pattern that defines structural polarity into a barbed (plus) end and a pointed (minus) end.

Barbed End ((+) End)

The fast-growing end of an actin filament where terminal subunits are already in a flattened conformation, subdomains 1 and 3 are exposed, and only incoming monomers must undergo conformational flattening.

Pointed End ((-) End)

The slow-growing end of an actin filament with subdomains 2 and 4 exposed, requiring both the terminal subunit and the incoming monomer to change conformation upon addition.

Actin ATPase Activity

The enzymatic hydrolysis of bound $\text{ATP}$ to $\text{ADP} + \text{P}_i$ triggered when an actin monomer incorporates and flattens into the filament, followed by the subsequent release of inorganic phosphate.

Actin Treadmilling

A steady-state translocational process occurring at monomer concentrations between the critical concentrations of both ends ($C_c^{\text{barbed}} < [\text{G-actin}] < C_c^{\text{pointed}}$), characterized by net subunit addition at the plus end and net dissociation from the minus end.

Steady-State Treadmilling

The specific state where the rate of polymerization at the barbed (+) end exactly equals the rate of depolymerization at the pointed (-) end, maintaining a constant filament length.

Retrograde Flow

The rearward movement of the actin network occurring when barbed-end polymerization encounters an immovable physical barrier (such as the plasma membrane) without forward cell advancement.

Lamellipodium

A broad, sheet-like cellular protrusion composed of a dense, branched network of actin filaments whose forward-facing barbed ends generate pushing force during locomotion.

Filopodia

Slender, finger-like protrusions containing parallel bundles of long actin filaments with barbed ends oriented toward the plasma membrane, acting as sensory antennas.

Actin-Binding Proteins (ABPs)

A diverse class of approximately 300 proteins that regulate actin filament dynamics (nucleation, elongation, severing), cellular organization (cross-linking, bundling), and motility (motors).

Profilin

A small actin-binding protein that binds the barbed-end face of monomeric actin, sterically suppressing spontaneous nucleation while directing monomer addition exclusively to filament barbed ends.

Adenine Nucleotide Exchange Factor (AEF)

A functional role of profilin wherein it opens the nucleotide-binding cleft of $\text{ADP}$-bound actin monomers to accelerate the exchange of $\text{ADP}$ for $\text{ATP}$.

ADF / Cofilin

A $15\text{--}20\text{ kDa}$ actin-binding protein that preferentially binds cooperatively to $\text{ADP}$-F-actin, alters helical twist (shortening the repeat to $< 37\text{ nm}$), severs filaments at boundary interfaces, and accelerates minus-end depolymerization.

Cooperative Binding (ADF/Cofilin)

The mechanism by which the binding of a single cofilin molecule alters the local filament twist, facilitating and promoting the adjacent binding of subsequent cofilin molecules.

Synergistic Treadmilling

The coupled action of profilin and ADF/cofilin that recycles actin subunits and accelerates physiological actin treadmilling rates by more than 100-fold.

Fascin

An actin-bundling cross-linker that tightly packs parallel actin filaments together to prevent buckling under plasma membrane load in filopodia.

ERM Proteins

Cross-linking proteins that physically tether the sides of bundled actin filaments to the plasma membrane to maintain the narrow, finger-like architecture of filopodia.

Formins

Dimeric actin polymerases that bind processively to the barbed end of actin filaments via an FH2 ring while recruiting profilin-actin complexes through flexible FH1 arms to accelerate elongation.

Keratocyte

A fish epidermal cell specialized for rapid wound repair that migrates unusually fast and persistently in a directional trajectory without erratic turning.

Melanophore

A specialized pigment cell from fish that coordinates pigment dispersal across the cytoplasm in the dark or aggregates pigment granules in the cell center against light backgrounds.