Cytoskeletal Filaments | Answer Key

University:
University of California, San Diego
Course:
BIMM 100 | Molecular Biology
Academic year:

2024
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148

Pages:

1
Author:

customer-8930767

structure and function of the three major cytoskeletal filaments: microtubules, microfilaments, and intermediate filaments. The cytoskeleton is a dynamic protein network maintaining cell shape, enabling movement, and organizing intracellular transport. Microtubules (25 nm diameter): Polymers of ?/?-tubulin heterodimers (each binds GTP). GTP-bound ?-tubulin promotes assembly; hydrolysis to GDP causes dynamic instability—alternating growth (rescue) and shrinkage (catastrophe). Organize from centrosome (?-tubulin ring complex nucleates). Functions: mitotic spindle (chromosome segregation); ciliary/flagellar axoneme (9+2 arrangement, dynein motors drive sliding); intracellular vesicle transport (kinesin moves toward + end at periphery; dynein moves toward – end at centrosome). Drugs: colchicine (depolymerizes), taxol (stabilizes). Microfilaments (Actin filaments) : 7 nm diameter. G-actin (globular, ATP-bound) polymerizes into Factin (filamentous, helical). ATP hydrolysis weakens filament, promoting treadmilling—assembly at barbed (+ end) and disassembly at pointed (– end). Functions: muscle contraction (with myosin II); cytokinesis (contractile ring); cell crawling (lamellipodia—Arp2/3 complex branches actin; filopodia— formins elongate); microvilli core. Regulated by profilin (promotes assembly), cofilin (severs filaments), and capping proteins. Intermediate filaments (10 nm diameter): Non-polar, rope-like fibers of tissue-specific proteins (keratins in epithelia; vimentin in mesenchymal; desmin in muscle; neurofilaments in neurons; lamins in nucleus). Assembly: monomers form coiled-coil dimers ? staggered tetramers ? unit-length filaments ? elongation. Lack ATPase activity; provide mechanical strength, resist tension. Functions: nuclear lamina (lamins A/C, B, disassemble during mitosis); desmosomes/hemidesmosomes (keratin anchors); axonal radial support (neurofilaments). Mutations cause disease: epidermolysis bullosa simplex (keratin), progeria (lamin A), Alexander disease (GFAP).