Cytoskeleton: Intermediate Filaments, Microtubules, and Actin

Vocabulary practice flashcards covering Chapter 17 (Cytoskeleton Parts I and II), including intermediate filaments, microtubules, actin filaments, motor proteins, and the mechanism of skeletal muscle contraction.

Intermediate Filaments

A family of strong, flexible, rope-like cytoskeletal protein fibers that lack polarity and provide mechanical support and tensile strength to cells and the nucleus without associated motor proteins.

Keratins

The class of intermediate filament proteins found in epithelial cells that provide tensile strength and protect tissues from mechanical stress.

Desmosomes

Disc-shaped protein plaques that connect neighboring cells and anchor bundles of intermediate filaments across an epithelial sheet.

Nuclear Lamins

Intermediate filament proteins that form a two-dimensional meshwork called the nuclear lamina supporting the inner nuclear membrane and nuclear envelope in all animal cells.

Antiparallel Tetramer (Intermediate Filaments)

The intermediate filament assembly unit formed by two staggered coiled-coil dimers running in opposite directions, which makes the ends of the tetramer identical and abolishes structural polarity.

LINC Complex

A protein bridge spanning the nuclear envelope formed by inner-membrane SUN-domain proteins and outer-membrane KASH-domain proteins, linking the cytoplasmic cytoskeleton directly to the nuclear interior.

Plectin

An accessory linker protein that bundles intermediate filaments and cross-links them to microtubules and actin filament networks for mechanical stability.

Microtubules

Stiff, hollow, directional cylinders with an outer diameter of $25\,\text{nm}$ assembled from $13$ longitudinal protofilaments made of globular $\alpha$- and $\beta$-tubulin heterodimers.

Protofilament

A linear longitudinal strand of alternating $\alpha$- and $\beta$-tubulin heterodimers held together by non-covalent bonds that forms the wall of a microtubule.

Centrosome

The primary microtubule-organizing center (MTOC) in animal cells, consisting of a protein matrix containing $\gamma$-tubulin ring complexes and a pair of centrioles positioned at right angles.

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

Circular protein assemblies embedded in the centrosome matrix that serve as nucleation sites from which microtubules grow at their plus ends while anchoring their minus ends.

Dynamic Instability

The behavior of individual microtubules switching back and forth between polymerization and depolymerization, driven by tubulin GTP binding and subsequent GTP hydrolysis.

GTP Cap

A stabilizing end-structure on a growing microtubule containing GTP-bound tubulin dimers, which prevents protofilaments from peeling away and depolymerizing.

Kinesin

A superfamily of ATP-dependent microtubule motor proteins that move cargo, vesicles, and organelles toward the plus end of microtubules (anterograde transport, toward the cell periphery).

Cytoplasmic Dynein

A multimeric microtubule motor protein with two heavy chains that hydrolyzes ATP to drive retrograde transport of cargo toward the minus end of microtubules (toward the cell body or centrosome).

Taxol

A microtubule-specific drug that binds to and stabilizes microtubules, preventing depolymerization.

Colchicine / Colcemid

Microtubule-affecting drugs that bind free tubulin dimers and prevent their polymerization into microtubules.

Vinblastine / Vincristine

Anti-mitotic drugs that bind to tubulin dimers and block microtubule polymerization.

Actin Filaments

Thin, flexible, two-stranded helical polymers of globular actin monomers with a repeat distance of approximately $37\,\text{nm}$ and structural polarity characterized by plus and minus ends.

Treadmilling (Actin)

The dynamic steady-state process where ATP-actin subunits are added to the filament plus end at the same rate that ADP-actin subunits dissociate from the minus end, maintaining constant overall length.

Phalloidin

An actin-specific mushroom toxin that binds along actin filaments to lock them together and prevent depolymerization.

Cytochalasin

A fungal compound that caps the plus ends of actin filaments, blocking polymerization and triggering filament depolymerization from minus ends.

Latrunculin

A toxin that binds directly to free actin monomers, sequestering them to inhibit actin filament polymerization.

ARP Complex

An actin-binding protein complex that nucleates new actin filaments off the sides of preexisting filaments, creating a dense dendritic (branched) network.

Lamellipodia

Broad, sheet-like exploratory protrusions at the leading edge of a crawling cell supported by a branched meshwork of actin filaments with their plus ends facing the plasma membrane.

Filopodia

Thin, stiff, spike-like membrane extensions containing parallel bundles of unbranched actin filaments used by cells to sense the environment.

Rho Family GTPases

Monomeric molecular switches—including Rho, Rac, and Cdc42—that transmit extracellular signals to orchestrate distinct arrangements of the actin cytoskeleton.

Myosin I

A monomeric actin-based motor protein with a single globular head that walks toward the actin plus end and a tail domain that binds cargo or the plasma membrane.

Myosin II

A two-headed motor protein with coiled-coil tails that self-associates into bipolar filaments with a central bare zone, pulling opposing actin filaments past one another to generate contractile force.

Sarcomere

The repeating contractile unit of a myofibril, approximately $2.5\,\mu\text{m}$ long, delimited by two Z discs and organized into interdigitated actin thin filaments and myosin II thick filaments.

Z Disc

The protein boundary at each end of a sarcomere that anchors the plus ends of actin thin filaments.

Power Stroke

The force-generating conformational change of the myosin II head triggered by the release of inorganic phosphate ($\text{P}_i$) that pulls the actin filament inward toward the center of the sarcomere.

Rigor Configuration

The tightly bound state of a myosin head to an actin filament in the absence of bound ATP or ADP, which is responsible for rigor mortis upon cellular ATP depletion.

Transverse (T) Tubules

Membranous tube invaginations of the muscle cell plasma membrane that convey electrical action potentials rapidly inward around all myofibrils.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum in muscle fibers that sequesters and rapidly releases $\text{Ca}^{2+}$ into the cytosol when triggered by depolarization in adjacent T tubules.

Tropomyosin

A rod-like protein that binds along the groove of the actin filament, physically blocking myosin-binding sites in the absence of calcium.

Troponin Complex

A $\text{Ca}^{2+}$-sensing protein complex associated with tropomyosin that shifts tropomyosin off actin binding sites when bound to cytosolic $\text{Ca}^{2+}$, enabling myosin binding and muscle contraction.