Practice question

Three cytoskeletal filament systems differ chemically mechanically and kinetically defining cytoplasmic organization. Actin filaments polar 7 nm helical polymers of ATP G actin assembling barbed plus fast pointed minus slow exhibiting treadmilling ATP hydrolysis dependent turnover myosin motors. Microtubules polar 25 nm tubes of GTP tubulin dimers showing dynamic instability and kinesin dynein transport highways plus end tracking. Intermediate filaments 10 nm rope like nonpolar exceptionally stable structures without polarity unique to metazoans. Lack of polarity stems from assembly pathway: parallel dimers form antiparallel tetramers neutralizing polarity tetramers associate laterally into unit length filaments of 60 nm that anneal end to end producing filament without distinct plus minus ends no nucleotide binding required for polymerization. Consequently motor proteins kinesin dynein myosin have no directional track on intermediate filaments for directional walking; instead filaments serve as stable scaffold anchored at desmosomes hemidesmosomes providing tensile strength resisting shear stress up to 300 percent strain. Stability greater than actin or microtubules not more dynamic no motor function no ATP requirement.