Actin Cytoskeleton I

Vocabulary practice flashcards covering cytoskeletal architecture, actin biochemistry, polymerization kinetics, polar filament assembly, treadmilling mechanics, and actin-binding proteins.

Cytoskeleton

A cellular collection of protein fibrils together with their associated proteins that provides structural integrity and generates mechanical forces for cellular functions.

Cytoskeleton (Operational Definition)

The detergent-insoluble structural framework remaining after detergent dissolves the lipid membrane, releasing soluble cytoplasmic components and membrane-bound organelles.

Cytoskeletal Fibril Classes

The three distinct structural polymers that comprise the cytoskeleton: microtubules (thickest, approximately $25\,\text{nm}$), actin filaments (thinnest and most abundant, approximately $7\,\text{nm}$), and intermediate filaments (intermediate thickness, approximately $10\,\text{nm}$).

Multi-Stranded Cytoskeletal Polymer Architecture

A polymer design where internal subunits form multiple non-covalent contacts with adjacent neighbors to provide mechanical strength, while terminal subunits have fewer bonds to enable rapid assembly and disassembly.

Keratocyte

A rapidly migrating epithelial cell from fish skin involved in wound healing that moves directionally and persistently without frequent changes in direction.

Melanophore

A specialized pigment cell from fish that concentrates black pigment granules in the cell center on a light background (appearing pale) and disperses them throughout the cytoplasm on a dark background.

Fibroblast Motility

Cell locomotion characterized by erratic migration in multiple directions as the cell responds and turns toward varying external growth factor signals.

G-Actin (Globular Actin)

A monomeric, approximately $43\,\text{kD}$ protein with a flattened square shape containing four subdomains and a deep central cleft that binds and hydrolyzes ATP or ADP.

F-Actin (Filamentous Actin)

A polar, two-stranded helical microfilament formed by the head-to-tail assembly of G-actin subunits with a rise of $28\,\text{\AA}$, a twist of $-166.7^\circ$, and a crossover repeat every $13\text{ subunits}$ ($\sim 37\,\text{nm}$).

Rate of Actin Polymerization ($R$)

The net velocity of filament assembly given by $R = k_{\text{on}}[\text{G-actin}] - k_{\text{off}}$, where the forward rate depends on free monomer concentration and the reverse rate depends solely on the number of available filament ends.

Critical Concentration ($C_c$)

The threshold monomer concentration ($C_c = K_D = \frac{k_{\text{off}}}{k_{\text{on}}}$) at which the rate of subunit addition equals the rate of loss ($R = 0$), enabling filament assembly above it and disassembly below it.

Phases of Actin Polymerization

The three stages of in vitro actin filament formation: nucleation (lag phase forming unstable dimers and stable trimers), elongation (growth phase of linear subunit addition), and steady state (equilibrium phase where assembly balances disassembly).

Actin Nucleation

The initial, rate-limiting step of actin assembly where monomers collide to form an unstable dimer that must bind a third monomer to yield a stable trimer (actin nucleus).

Barbed End (Plus End)

The fast-growing, dynamic end of an actin filament that exposes subdomains 2 and 4, exhibiting higher association ($k_{\text{on}}$) and dissociation ($k_{\text{off}}$) rate constants than the opposite end.

Pointed End (Minus End)

The slow-growing end of an actin filament that exposes subdomains 1 and 3, requiring both the incoming monomer and the terminal subunit to undergo conformational remodeling during addition.

Myosin S1 Decoration Assay

An electron microscopy method in which isolated myosin motor heads saturate an actin filament in the absence of ATP, creating an arrowhead pattern whose pointed tips point toward the minus (pointed) end.

Conformational Asymmetry of Actin Ends

The structural difference where terminal subunits at the barbed end already maintain a flat F-actin-like state, whereas pointed end subunits remain in a twisted G-actin-like conformation requiring energy to flatten.

ATP-Actin Critical Concentrations

The distinct critical concentrations at opposing filament ends arising from ATP hydrolysis: $C_c = 0.12\,\mu\text{M}$ at the barbed end (enriched in $\text{ADP-P}_i\text{-actin}$) and $C_c = 0.6\,\mu\text{M}$ at the pointed end (enriched in $\text{ADP-actin}$).

Actin Treadmilling

A steady-state dynamic process occurring when monomer concentration lies between the two critical concentrations ($0.12\,\mu\text{M} < [\text{G-actin}] < 0.6\,\mu\text{M}$), resulting in simultaneous assembly at the barbed end and disassembly at the pointed end.

Retrograde Flow

The rearward movement of the actin meshwork away from the leading edge that occurs when barbed-end polymerization pushes against an immovable physical barrier such as the plasma membrane.

Lamellipodium

A broad, sheet-like cellular protrusion composed of a dense, branched network of actin filaments whose barbed ends orient toward the leading plasma membrane to generate pushing force.

Filopodium

A thin, finger-like sensory extension containing a tight, parallel bundle of actin filaments with barbed ends oriented toward the tip, functioning as an environmental antenna.

Actin-Binding Proteins (ABPs)

A group of roughly 300 specialized proteins that regulate actin filament dynamics, higher-order cytoskeletal organization, and motor-driven cellular movements.

Profilin

A small monomer-binding protein that blocks spontaneous nucleation, acts as an adenine nucleotide exchange factor, and selectively promotes subunit addition onto filament barbed ends.

Adenine Nucleotide Exchange Factor (AEF)

A regulatory protein (such as profilin) that opens the nucleotide-binding cleft of actin monomers to promote the release of ADP and rapid binding of ATP.

ADF/Cofilin

An actin-depolymerizing protein ($15\text{--}20\,\text{kD}$) that cooperatively binds ADP-actin filaments, shortens the helical repeat below $37\,\text{nm}$, severs filaments at boundary zones, and accelerates pointed-end disassembly.

Profilin-Cofilin Synergy

A cooperative regulatory cycle between profilin and ADF/cofilin that accelerates physiological actin treadmilling rates by more than 100-fold.

Fascin

An actin-crosslinking protein that packs parallel actin filaments into rigid bundles within filopodia to resist buckling forces exerted by the plasma membrane.

ERM Proteins

A family of linker proteins (Ezrin, Radixin, Moesin) that anchor actin filament bundles laterally to the plasma membrane to preserve the finger-like cylindrical morphology of filopodia.

Formins

Dimeric actin nucleators and elongators that processively track growing barbed ends via an FH2 ring while using flexible FH1 domains to recruit profilin-actin complexes.

FH2 Domain

A doughnut-shaped dimeric domain of formin that encircles the barbed end of an actin filament, processively stepping forward to protect the end from capping without blocking monomer addition.

FH1 Domain

The flexible, proline-rich arm of formin that binds profilin-actin complexes to locally enrich monomers and accelerate barbed-end polymerization.