Cytoskeleton Structure, Organization & Behavior

Vocabulary flashcards covering the structure, organization, behavior, and accessory proteins of the cytoskeleton based on Biochemistry lecture notes.

Actin Filaments

Two-stranded helical polymers of the protein actin that appear as flexible structures with a diameter of $5\text{--}9\,\text{nm}$, responsible for cell surface shape, locomotion, endocytosis, and secretion.

Microtubules

Long, hollow cylinders made of tubulin subunits with an outer diameter of $25\,\text{nm}$ that determine organelle positions, direct intracellular transport, and form the mitotic spindle.

Intermediate Filaments

Ropelike fibers with a diameter of around $10\,\text{nm}$ made of a large, heterogeneous family of elongated, fibrous proteins that extend across the cytoplasm to provide mechanical strength.

Hereditary Spherocytosis (HS)

A disease caused by a defective red blood cell cytoskeleton, resulting in spherical, fragile red blood cells that burst easily and have a lifespan reduced from $120\,\text{days}$ to $12\,\text{hours}$.

Protofilament

A long linear string of protein subunits joined end to end, which associates laterally with other protofilaments to form a thermally stable cytoskeletal filament.

Centrosome

A single microtubule-organizing center (MTOC) to which one end of a microtubule is typically attached in a cell.

Nucleation

The initial rate-limiting process in cytoskeletal filament formation wherein individual subunits assemble into an initial aggregate or nucleus.

Treadmilling

A dynamic process in filaments, predominantly actin, where subunit addition at the plus end occurs at the same rate as subunit removal at the minus end, maintaining a constant filament length.

Dynamic Instability

A property predominant in microtubules involving rapid conversion between growing and shrinking states at the same end of the filament, regulated by GTP hydrolysis.

Catastrophe

The transition in microtubule dynamic instability from a growing state to rapid shrinkage following the accidental loss of its GTP cap.

Rescue

The transition in microtubule dynamic instability from rapid shrinkage back to a growing state upon regaining a GTP cap.

ARP Complex (Arp2/3)

An actin accessory complex comprising Arp2 and Arp3 that nucleates assembly from the minus end to form web-like, highly branched actin networks.

Formin

A dimeric actin accessory protein that nucleates the growth of straight, unbranched actin filaments and remains associated with the rapidly growing plus end as it elongates.

Thymosin

An actin accessory protein that binds actin monomers to keep them in a locked, soluble state, preventing their assembly into filaments.

Profilin

An actin accessory protein that binds actin monomers and recruits them to the plus end of an actin filament to speed up elongation.

Tropomodulin

An actin accessory protein that binds to the minus end of actin filaments to prevent assembly and disassembly, stabilizing long-lived filaments such as those in muscle.

Tropomyosin

An elongated actin accessory protein that binds along actin filaments to stabilize them and prevent interaction with other proteins.

Capping Protein

An actin accessory protein that binds to the plus end of actin filaments, preventing both assembly and disassembly to reduce polymerization and depolymerization rates.

Cofilin

An actin accessory protein (actin depolymerizing factor) that binds ADP-actin filaments, forcing a tighter twist that weakens subunit contacts and accelerates disassembly.

Gelsolin

An actin accessory protein that severs actin filaments and binds to the newly exposed plus end, aiding in rapid assembly or disassembly depending on cellular conditions.

$\alpha$-actinin

An actin-bundling protein that cross-links actin filaments into loose contractile bundles, allowing myosin-II to enter.

Fimbrin

An actin-bundling protein that cross-links actin filaments into tight parallel bundles, preventing myosin-II from entering.

Filamin

An actin accessory protein dimer that clamps two actin filaments together roughly at right angles to form a three-dimensional, gel-like network.

Spectrin

An actin accessory protein that attaches the actin cytoskeleton to the cell membrane, helping maintain red blood cell shape and mechanical strength.

ERM Family Proteins

A family of proteins (Ezrin, Radixin, Moesin) that attach the actin cytoskeleton to the plasma membrane by cross-linking transmembrane proteins to actin filaments.

ActA

A surface protein produced by Listeria monocytogenes that acts as an activating factor for the Arp2/3 complex, driving local actin nucleation to form comet tails for motility.