Becker's World of the Cell - Chapter 13: Cytoskeletal Systems

Vocabulary practice flashcards covering cytoskeletal systems, including microtubules, microfilaments, intermediate filaments, accessory proteins, regulatory GTPases, and associated cellular pathologies based on Chapter 13 of Becker's World of the Cell.

Cytoskeleton

A dynamic and changeable network of interconnected filaments and tubules extending through the cytosol that mediates cell movement, structural stability, and division.

Microtubules (MTs)

Hollow cylindrical cytoskeletal elements with an outer diameter of approximately $25\,\text{nm}$ composed of longitudinal arrays of tubulin heterodimers, functioning in cell shape, chromosome movements, and intracellular transport.

Microfilaments (MFs)

Cytoskeletal filaments measuring $7\,\text{nm}$ in diameter formed by two intertwined chains of polymerized actin, functioning in muscle contraction, cell locomotion, cytoplasmic streaming, and cytokinesis.

Intermediate Filaments (IFs)

Stable, tension-bearing cytoskeletal fibers measuring $8\text{--}12\,\text{nm}$ in diameter that lack known polarity, assembled from six distinct classes of tissue-specific fibrous proteins.

Cytoplasmic Microtubules

Singlet microtubules pervading the cytosol that maintain cell shape and polarity, support nerve axon extensions, direct vesicle trafficking, and form mitotic and meiotic spindles.

Axonemal Microtubules

Highly organized, stable doublets or triplets of microtubules present in cilia, flagella, and their anchoring basal bodies.

Axoneme

The central core shaft of a cilium or flagellum composed of a highly ordered bundle of axonemal microtubules.

Protofilament

A linear polymer formed by end-to-end association of alternating $\alpha$-tubulin and $\beta$-tubulin heterodimers; thirteen protofilaments typically align side-by-side to form the hollow cylindrical wall of a microtubule.

Doublet and Triplet Microtubules

Specialized microtubule structures featuring one complete 13-protofilament cylinder (A tubule) fused with one or two additional incomplete rings (B and C tubules) composed of 10 or 11 protofilaments each.

Nucleation (Microtubules)

The initial lag phase of microtubule formation during which $\alpha\beta$-tubulin heterodimers aggregate into small oligomers that act as seeds for further filament elongation.

Critical Concentration

The free tubulin dimer concentration at which the rate of subunit assembly exactly equals the rate of disassembly; microtubules grow when tubulin concentration exceeds this threshold.

Microtubule Treadmilling

A steady-state process occurring when free tubulin concentration is above the critical concentration of the plus end but below that of the minus end, causing simultaneous subunit addition at the plus end and loss at the minus end.

Colchicine

An antimitotic alkaloid derived from the autumn crocus (Colchicum autumnale) that binds the $\beta$-subunit of tubulin heterodimers, inhibiting assembly and triggering microtubule destabilization.

Paclitaxel (Taxol)

An antimitotic drug produced by symbiotic fungi in the bark of the Pacific yew tree (Taxus brevifolia) that binds tightly to microtubules, stabilizing them and blocking mitosis by depleting the free tubulin subunit pool.

Nocadazole

A synthetic antimitotic drug that binds $\beta$-tubulin to inhibit microtubule polymerization.

Vinblastine and Vincristine

Antimitotic anticancer compounds derived from the periwinkle plant (Vinca rosea) that inhibit mitotic spindle assembly by causing tubulin heterodimers to aggregate.

Dynamic Instability

A property of microtubules in which individual filaments undergo alternating phases of slow growth via subunit addition and rapid disassembly (catastrophe) at their plus ends.

GTP Cap

A protective cluster of GTP-bound tubulin subunits at the plus end of a growing microtubule that prevents depolymerization; cap depletion via GTP hydrolysis leads directly to microtubule catastrophe.

Centrosome

The primary microtubule-organizing center (MTOC) in animal cells located near the nucleus, composed of two centrioles oriented at right angles surrounded by pericentriolar material.

$\gamma$-Tubulin Ring Complex ($\gamma$-TuRC)

Large, ring-shaped protein complexes localized within centrosomes containing $\gamma$-tubulin that nucleate new microtubules away from the centrosome and anchor their minus ends.

Microtubule-Associated Proteins (MAPs)

Proteins that bind at regular intervals along microtubules to stabilize filament structure and mediate interactions with other cellular elements.

Tau

A neuronal microtubule-associated protein (MAP) that stabilizes microtubules and promotes their organization into tightly packed parallel bundles within axons.

$+\text{TIP}$ Proteins

Plus-end tubulin interacting proteins, such as EB1 (end-binding protein 1), that capture and protect growing microtubule plus ends to prevent catastrophic disassembly.

Catastrophins

Microtubule-destabilizing proteins, such as MCAK, that bind to the ends of microtubules and stimulate the peeling and rapid loss of tubulin subunits.

G-Actin

Globular, monomeric actin that binds ATP or ADP and polymerizes reversibly to form microfilaments.

F-Actin

Filamentous actin consisting of two intertwined helical strands formed by the polymerization of G-actin monomers.

Myosin Subfragment 1 (S1) Decoration

A structural assay using cleaved myosin heads that bind actin filaments to generate an arrowhead appearance, establishing polarity with a barbed (plus) end and a pointed (minus) end.

Stress Fibers

Contractile, antiparallel bundles of actin microfilaments present in non-motile or tightly adhering cells that anchor to focal adhesions.

Cell Cortex

A high-density actin meshwork crosslinked into a gel directly beneath the plasma membrane, providing mechanical reinforcement and maintaining cell shape.

Lamellipodium

A broad, flattened sheetlike protrusion at the leading edge of a migrating cell containing a branched meshwork of actin filaments.

Filopodium

A slender, fingerlike protrusion extending from the leading edge of a crawling cell, composed of tight, parallel actin bundles with plus ends directed toward the tip.

Thymosin $\beta 4$

An actin-binding protein that binds and sequesters free ATP-bound G-actin monomers, preventing their spontaneous polymerization into microfilaments.

Profilin

An actin-regulatory protein that competes with thymosin $\beta 4$ to bind G-actin and catalyzes the addition of actin monomers onto the plus ends of growing filaments.

CapZ

A capping protein that binds tightly to the barbed (plus) ends of microfilaments to prevent addition or loss of actin monomers.

Filamin

A long, flexible actin-crosslinking protein that links intersecting actin microfilaments into 3D crisscross networks and gels.

Microvillus

A mucosal projection on intestinal epithelial cells containing an internal core of bundled actin microfilaments crosslinked by fimbrin and villin, linked to the membrane by myosin I and calmodulin.

Terminal Web

A dense structural network at the base of microvilli composed primarily of myosin II and spectrin that crosslinks actin filament bundles together and anchors them to the plasma membrane and intermediate filaments.

Arp2/3 Complex

A complex of actin-related proteins activated by WASP or WAVE/Scar that nucleates the formation of new actin branches emerging at a $70^\circ$ angle from existing microfilaments.

Formin

A dimeric actin-polymerizing protein that tracks the barbed (plus) end of microfilaments and uses flexible whisker domains to recruit profilin-actin complexes, promoting unbranched filament elongation.

Rho Family GTPases

Monomeric G proteins—specifically Rho, Rac, and Cdc42—that act as molecular switches regulating actin assembly; Rho promotes stress fibers, Rac induces lamellipodia, and Cdc42 stimulates filopodia formation.

Guanine-Nucleotide Exchange Factors (GEFs)

Regulatory proteins that stimulate Rho family GTPases by facilitating the exchange of bound GDP for GTP, converting them into their active states.

GTPase Activating Proteins (GAPs)

Regulatory proteins that inactivate Rho GTPases by stimulating their intrinsic enzymatic activity to hydrolyze bound GTP to GDP.

Nuclear Lamins (Class V IFs)

Intermediate filament proteins (Lamin A, Lamin B, and Lamin C) found in all animal cell types that form a supporting scaffold along the inner nuclear membrane to provide shape and mechanical stability to the nucleus.

Hutchinson-Gilford Progeria Syndrome (HGPS)

An autosomal dominant disorder caused by a single $C\rightarrow T$ point mutation in the Lamin A gene that creates a cryptic splice site, leading to a 50-amino acid deletion, loss of the endoproteolytic cleavage site, and failure to remove the farnesyl group, which weakens the nuclear envelope.

Plectin

A multifunctional cytoskeletal linker protein that binds intermediate filaments and connects them to microfilaments and microtubules, creating an integrated mechanical network.