Practice question

Cytoskeletal polymers provide complementary mechanical functions. F actin, 7 nanometer double helical polymer of ATP loaded G actin, drives protrusion and contraction via myosin II and generates branched networks via Arp2/3 at 70 degrees. Microtubules, 25 nanometer hollow cylinders of alpha beta tubulin heterodimers with GTP cap, resist compression and act as long range highways for kinesin plus end and dynein minus end motors transporting organelles and chromosomes. Intermediate filaments, 10 nanometer non polar fibers, provide outstanding tensile strength and resilience. Assembly hierarchy involves parallel coiled coil dimers with central rod domain of heptad repeats, antiparallel tetramers, unit length filaments and final 32 mer compact fibers. Types include acidic and basic keratins in epithelia anchored to desmosomes and hemidesmosomes, vimentin in fibroblasts, desmin linking Z discs and mitochondria in muscle, neurofilaments regulating axonal caliber and lamin meshwork underlying nuclear envelope. Lacking polarity and ATPase activity, they stretch several fold without breaking, distributing mechanical load across tissues and preventing cell lysis under tension. This mechanical resilience explains accumulation at sites of mechanical stress.