BIOL 3030 - LEC 12 - Vocabulary

Vocabulary flashcards covering major proteins, processes, and regulatory molecules involved in actin microfilament structure, dynamics, and function as presented in BIOL 3030 Lecture 12.

Microfilament

Thin, flexible cytoskeletal filament made of actin that supports cell shape, motility, and contractility.

Actin

Highly conserved protein that polymerizes to form microfilaments; exists as G-actin monomers and F-actin filaments.

G-actin

Globular actin monomer that binds ATP/ADP and Mg²⁺; polymerizes to form F-actin.

F-actin

Filamentous actin polymer with intrinsic polarity (barbed + end, pointed – end).

Critical Concentration (Cc)

Minimum G-actin concentration required for filament assembly; differs at + and – ends.

Treadmilling

Steady-state where actin monomers add at the + end and dissociate at the – end, maintaining filament length.

Profilin

Actin-binding protein that exchanges ADP for ATP on G-actin, promoting polymerization at the + end.

Cofilin

Protein that binds ADP-F-actin, severs filaments, and accelerates depolymerization at the – end.

Thymosin-β4

G-actin sequestration protein that stores ATP-actin monomers, preventing spontaneous polymerization.

CapZ

Capping protein that binds the + end of actin filaments to halt elongation.

Tropomodulin

Protein that caps the – end of actin filaments in muscle, stabilizing filament length.

Formin

Dimeric actin nucleator that remains at the + end to generate long, unbranched filaments.

Arp2/3 Complex

Seven-subunit complex that binds existing F-actin to nucleate 70° branches at the – end.

Listeria monocytogenes

Pathogenic bacterium that hijacks the host’s Arp2/3-mediated actin polymerization for intracellular motility.

Endocytic Actin Comet Tail

Branched actin network driving movement of endocytic vesicles inside cells.

Spectrin

Flexible actin-binding protein that forms meshwork under the plasma membrane for mechanical support.

Dystrophin

Cytoskeletal linker that connects actin cortex to membrane in muscle cells; mutation causes Duchenne muscular dystrophy.

Myosin

Actin-based motor protein superfamily that converts ATP hydrolysis into mechanical force.

Myosin II

Conventional two-headed myosin forming thick filaments; drives muscle contraction and cytokinesis.

Myosin V

Processive cargo-transport motor with long neck and 36 nm step size along actin.

Sarcomere

Contractile unit of striated muscle composed of interdigitating actin (thin) and myosin (thick) filaments.

Sliding Filament Model

Mechanism of muscle contraction where myosin heads slide actin filaments past thick filaments, shortening the sarcomere.

Lamellipodium

Broad, sheet-like actin network at a cell’s leading edge that pushes the membrane forward during migration.

Filopodium

Thin, actin-bundled protrusion extending from cells to sense environment and guide movement.

Stress Fiber

Contractile actin–myosin bundle in non-muscle cells linked to focal adhesions, generating tension.

Focal Adhesion

Cell-ECM contact site where integrins connect extracellular matrix to intracellular actin cytoskeleton.

Rho Family GTPases

Molecular switches (Rho, Rac, Cdc42) that regulate actin organization in response to signaling.

Cdc42

Rho family GTPase that activates WASP and Arp2/3 to form filopodia and establish cell polarity.

Rac

GTPase that stimulates Arp2/3 via WAVE complex to promote lamellipodia formation at the leading edge.

Rho

GTPase that activates formin-mediated actin assembly and myosin II contractility, generating stress fibers.

WASP

Adaptor protein activated by Cdc42 and PIP₂ that stimulates Arp2/3-dependent actin branching.

Formin Autoinhibition

Intramolecular interaction in formin relieved by Rho-GTP binding, enabling actin nucleation.

Chemotaxis

Directed cell migration along a chemical gradient, relying on polarized actin polymerization.

CapZ vs. Tropomodulin

CapZ caps + ends in muscle Z-disks; Tropomodulin caps – ends, together fixing thin-filament length.