Cell Biology Lecture 09 – Actin Cytoskeleton & Motility

Vocabulary flashcards covering key proteins, processes, and structures involved in actin cytoskeleton dynamics and cell motility, as presented in Lecture 09.

Cytoskeleton

Network of actin filaments, microtubules, and intermediate filaments that shapes cells and enables movement.

Cell Motility

Ability of cells or their components to move, essential for fertilization, embryogenesis, immunity, and transport.

Actin Filament (F-actin)

Polar helical polymer of actin subunits that resists deformation, supports membranes, and serves as tracks for myosin.

Microvilli

Finger-like plasma-membrane protrusions containing tightly packed actin bundles that increase absorptive surface.

Stress Fiber

Contractile bundle of actin filaments and myosin II found in non-muscle cells, providing tensile strength.

Contractile Ring

Actin–myosin structure that pinches dividing cells during cytokinesis to form two daughter cells.

Barbed End (+ End)

Fast-growing end of an actin filament where ATP-actin adds rapidly.

Pointed End (– End)

Slow-growing end of an actin filament where subunits dissociate more readily.

Treadmilling

Steady-state where actin subunits add at the barbed end and leave at the pointed end, keeping filament length constant.

Actin Polymerization

Process of nucleation, elongation, and ATP hydrolysis that builds actin filaments from monomers.

Nucleation

Formation of a stable actin trimer ‘seed’; the rate-limiting step of filament assembly.

Actin-Related Proteins (Arps)

Family of proteins sharing actin’s fold but <60 % identity; includes Arp1, Arp2/3, and chromatin regulators.

Arp2/3 Complex

Seven-subunit complex that nucleates 70° branched actin filaments when activated by NPFs.

Nucleation-Promoting Factor (NPF)

Proteins such as WASP/N-WASP, Scar/WAVE, WASH, WHAMM that activate Arp2/3 to form new branches.

WASP

NPF autoinhibited until Rho-GTPases, PIP2, or SH3 proteins expose its VCA domain to trigger Arp2/3.

Formin

Barbed-end polymerase that nucleates and elongates unbranched actin; activated by Rho GTPases via FH2/FH1 domains.

VASP

Tetrameric actin polymerase that binds barbed ends, recruits profilin-actin, and opposes capping in filopodia and focal adhesions.

Profilin

Monomer-binding protein that accelerates ADP/ATP exchange on actin and delivers ATP-actin to barbed ends.

Thymosin-β4

Small peptide that sequesters ATP-actin monomers, preventing spontaneous nucleation.

ADF/Cofilin

Protein that binds ADP-actin filaments, induces twist, severs filaments, and accelerates depolymerization.

Gelsolin

Ca²⁺-activated protein that severs actin and caps barbed ends, blocking further assembly.

Capping Protein (CapZ)

Heterodimer that binds barbed ends to terminate elongation and stabilize filaments.

Tropomodulin

Pointed-end capping protein that, with tropomyosin, stabilizes long-lived filaments in muscle and RBCs.

Tropomyosin

Elongated dimer that binds along actin, strengthens filaments, and regulates myosin interaction in a Ca²⁺-dependent manner.

Nebulin

Giant protein in skeletal muscle that determines thin-filament length by spanning actin filaments.

Caldesmon

Long protein that, with tropomyosin and Ca²⁺–calmodulin, controls actin–myosin interaction in smooth muscle.

α-Actinin

Dimeric crosslinker that connects actin filaments in stress fibers, cortex, and Z-disks; regulated by Ca²⁺.

Fimbrin

Small actin-bundling protein that organizes tightly packed parallel bundles in microvilli.

Villin

Actin-bundling and severing protein specialized for microvillar core bundles.

Filamin

V-shaped dimer that crosslinks actin into orthogonal networks and links cortex to integrins.

Spectrin

Flexible tetramer that connects actin to membrane proteins in the erythrocyte skeleton.

Dystrophin

Cytolinker that attaches cortical actin to the dystroglycan complex; its loss causes muscular dystrophy.

Unpolymerized Actin Pool

Reservoir of ATP-actin monomers (~50 % of total) buffered by profilin and thymosin-β4 for rapid filament assembly.

Severing

Cutting of actin filaments to create new ends, executed by cofilin, gelsolin, severin, fragmin, or some formins.

Crosslinking

Connecting two actin filaments via proteins with dual actin-binding sites to form bundles or networks.

Rho GTPases

Molecular switches (Rho, Rac, Cdc42) that regulate actin nucleators; induce stress fibers, lamellipodia, or filopodia.

Filopodia

Thin, spike-like protrusions supported by parallel actin bundles driven by Cdc42 and formins/VASP.

Lamellipodium

Sheet-like leading edge with Arp2/3-generated branched actin network controlled by Rac.

Stress Fibers

Actomyosin cables induced by Rho that generate contractile tension and anchor at focal adhesions.

Viscoelastic Cytoplasm

Mechanical property where dynamic actin crosslinks make cytoplasm act solid at fast strain, fluid over time.

Cytochalasin

Drug that caps barbed ends, blocking new filament growth and revealing actin retrograde flow.

Latrunculin

Marine toxin that sequesters actin monomers, causing filament disassembly in live-cell studies.