#cytoskeleton

Basics of Cell Biology

Which of the following is not a function of the cytoskeleton?

Cytoskeletal networks coordinate shape, division, and intracellular logistics. Actin filaments, 7 nm flexible helical polymers of ATP-G-actin nucleated by Arp2/3 at 70 degree branch and formins for linear cables, interact with 50 myosin classes for cortical contraction, formation of contractile ring with myosin II during cytokinesis, lamellipodial protrusion, and vesicle movement near periphery. Microtubules, 25 nm rigid hollow tubes of alpha-beta tubulin heterodimers assembling head to tail with GTP cap regulating dynamic instability, serve as polarized tracks for kinesin families moving outward to plus ends and dynein moving inward, positioning Golgi, endosomes, mitochondria, and assembling mitotic spindle with kinetochore capture ensuring chromosome segregation. Intermediate filaments, 10 nm non-polar rope-like polymers keratin, vimentin, desmin, lamins anchored via plectin and nesprin, provide mechanical resilience against shear stress. Together they govern intracellular transport, mechanical support, and motility. Protein synthesis uses ribosomal peptidyl transferase in cytoplasm and rough ER, not filaments, so translation is not cytoskeletal function.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton Functions Overview.

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Basics of Cell Biology

Which cellular structure plays a role in intracellular transport and is part of the cytoskeleton?

Cytoskeleton provides scaffold and track system built from protein polymers regulated by nucleation factors, GTPases, and post-translational modifications. Actin filaments, 7 nanometer diameter polarized structures polymerizing from ATP-G-actin mediated by Arp2/3 complex branching at 70 degrees and formins nucleating straight cables for filopodia, associate with myosin II for contractility at cortex and myosin V for short-range vesicle transport near cell periphery, driving lamellipodia extension and endocytic invagination. Microtubules, 25 nanometer hollow tubes of alpha-beta tubulin dimers with GTP hydrolysis driving dynamic instability with growth and catastrophe phases, originate at microtubule organizing center centrosome and serve as long-distance highways for kinesin anterograde and dynein retrograde motors carrying organelles, mRNA granules, and lysosomes, forming mitotic spindle via kinetochore attachment through Ndc80 complex ensuring chromosome segregation. Intermediate filaments, 10 nanometer non-polar ropes including keratin in epithelia and vimentin in mesenchyme anchored by plectin, confer tensile strength protecting nucleus. All three together constitute cytoskeletal network supporting transport, mechanical integrity, and motility essential for cell physiology.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton and Intracellular Transport.

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Plasma Membrane Proteins

Which protein is responsible for maintaining the biconcave shape of RBCs?

Maintenance of biconcave disc shape maximizing surface to volume ratio for deformability and gas exchange depends on membrane skeleton elasticity and organization. Spectrin consists of alpha two hundred eighty kilodalton and beta two hundred forty six kilodalton subunits each containing twenty triple-helical spectrin repeats of one hundred six residues forming elongated flexible anti-parallel heterodimer one hundred nanometers long, two heterodimers associate head to head via helical bundle forming tetramer about two hundred nanometers acting as entropic spring with persistence length about ten nanometers. Tetramers interconnected at junctional complexes comprising thirteen subunit short F-actin filaments capped by adducin and tropomodulin plus protein 4.1R dematin tropomyosin forming pseudohexagonal lattice of about thirty five thousand nodes per cell beneath lipid bilayer. Linkage to bilayer via ankyrin-Band 3 and protein 4.1R-glycophorin C ensures force transmission. Hereditary mutations in SPTA1 encoding alpha spectrin and SPTB encoding beta spectrin cause hereditary spherocytosis and elliptocytosis with fragile cells. GPCR seven-pass signaling and aquaporin channel not structural, confirming spectrin role maintaining biconcave geometry and mechanical elasticity under shear stress circulation.

Ref: Lux and Palek, Erythrocyte Membrane Skeleton and Spectrin Elasticity, Blood Cells.

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Plasma Membrane Proteins

Which of the following best describes the function of Ankyrin?

Erythrocyte mechanical stability depends on vertical linkages mediated by ankyrin-R isoform one hundred ninety kilodaltons to two hundred ten kilodaltons comprising three distinct domains enabling integration. N-terminal membrane binding domain contains twenty four tandem ankyrin repeats each thirty three residues forming elongated superhelical solenoid that creates concave binding surface recognizing cytoplasmic loop of Band 3 anion exchanger between residues one hundred seventy five to one hundred eighty five with nanomolar affinity, also binding Na K ATPase alpha subunit, voltage gated sodium channel Nav1.5, and L1 cell adhesion molecule in neurons and muscle. Central spectrin binding domain binds beta spectrin repeat fourteen to fifteen near ankyrin repeat. C-terminal regulatory domain contains death domain and C-terminal extension modulating affinity through autoinhibition. Bridging spectrin-actin lattice to integral proteins prevents membrane vesiculation and maintains deformability required for circulatory passage through three micrometer splenic interendothelial slits. Hereditary mutations account for about fifty percent hereditary spherocytosis with reduced spectrin assembly spherical rigid cells hemolysis and jaundice, illustrating cytoskeletal anchorage role.

Ref: Mohandas and Peters, Erythrocyte Cytoskeleton and Ankyrin Function, Annu Rev Med.

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Cytoskeleton

What is a major function of intermediate filaments in cells?

Intermediate filaments constitute one of three major cytoskeletal polymers alongside actin microfilaments and microtubules, classified by ten nanometer diameter and apolar rope-like assembly through parallel coiled-coil dimers antiparallel tetramers laterally associating into filaments without polarity. Major classes include keratins in epithelia, vimentin in mesenchyme, desmin in muscle, glial fibrillary acidic protein in astrocytes, neurofilaments in neurons, lamins in nucleus. Their primary mechanical role is tensile resistance to shear and stretching, absorbing deformation through flexible networks anchored at cell-cell desmosomes and cell-matrix hemidesmosomes linked by plectin and desmoplakin. This protects tissues experiencing repetitive strain, as seen in blistering diseases upon keratin mutation. They do not generate ATP, actively transport vesicles, or directly regulate microtubule growth, though they cross-talk via plectin. Knockout studies show fragility rather than motility defects, indicating specialization toward structural support enabling tissue cohesion, nuclear scaffolding for chromatin organization and integration of mechanical signals over long timescales beyond dynamic actin and microtubule remodeling.

Ref: Herrmann J Cell Sci; IF 10nm tensile strength, anchored at desmosomes hemidesmosomes, no polarity.

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Cytoskeleton

Which statement about dynein function is FALSE?

Dynein heavy chains are among the largest AAA+ machines, containing six concatenated AAA modules forming an asymmetric ring. AAA1 is primary catalytic site where ATP binding closes interface with AAA2, straightening linker and detaching stalk from microtubule. Hydrolysis and phosphate release trigger linker swing from pre to post power stroke, pulling tail forward while microtubule-binding domain reattaches downstream. This cycle strictly depends on ATP concentration and hydrolysis; nonhydrolyzable analogs such as AMP-PNP arrest motility in strong binding state, vanadate traps ADP-Pi intermediate. Dynein moves toward minus ends, positioning Golgi ribbon adjacent to centrosome and driving retrograde transport of autophagosomes, endosomes and interphase nuclei. Disruption fragments Golgi into scattered ministacks. Claiming independence from ATP contradicts enzymology and single molecule assays showing velocity dependence on Mg-ATP with Michaelis-Menten kinetics, while other statements regarding Golgi positioning, organelle transport and directionality align with extensive siRNA, dominant negative p150 and knockout studies in mammalian culture and in vivo models.

Ref: Alberts Ch 16 Motor proteins; dynein AAA+ requires ATP hydrolysis, minus-end directed transport.

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Cytoskeleton

What happens when dynein loses its ability to bind microtubules?

Cytoplasmic dynein generates minus-end directed transport using a microtubule-binding domain at the tip of an antiparallel coiled-coil stalk that extends from the AAA ring. The binding interface undergoes ATP-dependent registry shift between strong and weak states. When mutations, deletion or antibody blockade abolishes microtubule attachment, the motor continues to bind ATP and hydrolyze it via AAA1, but force cannot be transmitted to the filament, uncoupling ATPase activity from movement. Cargo adaptors such as BICD2, HOOK and dynactin still recruit dynein via tail and intermediate chains, but vesicles including late endosomes, lysosomes, Golgi elements and messenger ribonucleoprotein granules remain stationary, while kinesin-mediated plus-end movement proceeds unopposed. This results in peripheral accumulation and loss of centripetal concentration near the microtubule-organizing center. The phenotype mimics genetic dynein loss, demonstrating that stable microtubule engagement is mandatory for productive power stroke rather than for ATP hydrolysis, separating chemical and mechanical cycles in mechanochemical coupling. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Vale 2003 Cell; Roberts Cell 2013 dynein stalk MTBD binding essential for cargo translocation.

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Cytoskeleton

Which protein links intermediate filaments to actin filaments and microtubules?

Plectin is a giant cytolinker of the plakin family that simultaneously recognizes actin, microtubules and intermediate filaments, integrating cytoskeletal networks into a cohesive mechanical continuum. Its structure includes an N-terminal actin-binding domain composed of two calponin-homology motifs, a central 200-nm coiled-coil rod dimerization domain, and a C-terminal repeat domain that binds vimentin, keratin, desmin and glial fibrillary acidic protein with distinct isoform specificity. Alternative splicing of first exons targets isoforms to hemidesmosomes via integrin beta4, to focal adhesions, desmosomes, mitochondria and nuclear envelope. At these sites plectin recruits microtubule-associated proteins and directly contacts tubulin, coordinating plus-end dynamics with intermediate filament anchorage. Deletion produces skin blistering with muscular dystrophy due to failure of stress transfer. In contrast spectrin forms tetramers capping actin at membranes, fimbrin bundles actin in microvilli, filamin crosslinks actin orthogonally, none containing high-affinity intermediate filament repeats. Thus plectin alone provides universal bridging activity essential for epithelial and muscle integrity.

Ref: Alberts Ch 16; Fuchs & Cleveland, Science 1998 plakin plectin crosslinks IF, actin, microtubules.

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Cytoskeleton

How does phosphorylation affect intermediate filaments?

Intermediate filaments are apolar polymers of tetrameric coiled-coil dimers that lack intrinsic nucleotide turnover, so dynamic regulation relies on post-translational modification. Phosphorylation within the N-terminal head domain by mitotic kinases such as Cdk1, Plk1, Aurora B, as well as PKA, PKC and MAP kinases, introduces dense negative charge that disrupts head-to-rod electrostatic interactions essential for filament elongation. The result shifts equilibrium toward soluble tetramers and monomers, enabling mitotic disassembly of vimentin networks and nuclear lamina breakdown at prometaphase, which is required for envelope rupture and chromosome access. Upon mitotic exit phosphatases PP1 and PP2A remove phosphates, promoting rapid reassembly into 10-nm ropes anchored at desmosomes and hemidesmosomes to restore tensile resilience. Phosphorylation also modulates binding to plectin and 14-3-3 adaptors during cell migration. Because assembly requires no ATP or GTP, kinase-phosphatase balance provides primary control over local turnover, solubility, and interaction with signaling scaffolds under stress. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Alberts Ch 16 Cytoskeleton; Lodish Ch 18; IF phosphorylation controls disassembly via head domain serine.

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Cytoskeleton

Which motor protein moves cilia and flagella?

Cilia and flagella motility requires active sliding filament machine within conserved axoneme evolutionary ancient. Structure nine outer doublet microtubules surrounding central pair retains dynein motors as exclusive driver of beating: outer dynein arms contain two or three heavy chains per arm inner arms more heterogeneous but also axonemal dynein isoforms both anchored to A tubule via docking complex and reaching to B tubule of neighboring doublet executing ATP hydrolysis cycle. Power stroke toward minus end attempts to slide doublets relative each other but nexin dynein regulatory complex NDRC and radial spokes constrain sliding converting linear force into bending moment propagated along length in oscillatory fashion regulated by calcium calmodulin kinases and central pair rotation. Kinesin-1 transports mitochondria outward on cytoplasmic microtubules also drives anterograde intraflagellar transport constructing flagellum but not bending itself Myosin II actin based contractility not involved in axonemal bending. Hence dynein powers motion with coordinated activation producing wave form for mucus propulsion sperm swimming and left right patterning nodal flow.

Ref: King, J Cell Sci 2012 – Dynein motor protein moves cilia and flagella beating machinery.

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Cytoskeleton

In epithelial cells, intermediate filaments are mainly composed of:

Epithelial tissues covering skin intestine lung alveoli renal tubules subject to friction stretch requiring robust mechanical buffering and polarity maintenance. Keratin intermediate filaments provide this resilience forming dense cable network 10 nm diameter spanning cytoplasm inserting into desmosomes and hemidesmosomes creating continuous transcellular network distributing tensile forces across sheet preventing cell separation under shear. Expression program tissue specific precisely regulated: basal epidermal keratinocytes keratin 5 and 14 pair suprabasal differentiating keratin 1 and 10 simple epithelia like hepatocytes kidney keratin 8 and 18 and intestinal keratin 20. Synthesis as obligate heterodimer of acidic type I and basic type II ensures assembly control prevents homopolymer formation. Vimentin normally mesenchymal origin desmin muscle lamin nuclear scaffold not epithelial primary. Network also regulates signaling via 14-3-3 binding organelle positioning including mitochondria wound healing migration. Detection of keratin isoforms widely used immunohistochemistry tumor marker distinguishing carcinoma of epithelial origin from sarcoma mesenchymal origin underscoring epithelial specificity abundance representing majority protein content.

Ref: Herrmann et al., Nat Rev Mol Cell Biol 2007 – Keratin main intermediate filament protein in epithelial cells.

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Cytoskeleton

Which of the following proteins is a component of intermediate filaments?

Intermediate filament protein family comprises over 70 members classified by sequence homology and expression pattern regulated developmentally. Signature central rod domain about 310 amino acids alpha helical coiled coil flanked by variable head tail domains mediating assembly and interactions. Major subgroups include type I acidic keratins 28 genes and type II basic keratins 26 genes obligate heteropolymers in epithelia forming extensive cytoskeleton; type III includes vimentin mesenchymal fibroblasts endothelial desmin skeletal cardiac smooth muscle GFAP astrocytes peripherin peripheral neurons; type IV neurofilaments NF light medium heavy in central neurons determining axon caliber; type V lamins nuclear envelope; type VI nestin progenitor cells. Keratin represents classic component forming dense tonofilament networks resisting mechanical stress chemical insult and apoptosis. Tubulin alpha beta heterodimers make microtubules actin forms thin filaments myosin motor domain binds actin not intermediate. Therefore keratin is recognized intermediate filament constituent essential for epithelial integrity mutations causing epidermolysis bullosa simplex due to cell fragility under friction trauma.

Ref: Eriksson et al., Physiol Rev 2009 – Intermediate filament proteins including keratin as component classification.

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