Several physical and biochemical properties distinguish intermediate filaments from microfilaments and microtubules. To begin with, intermediate filaments are extremely stable. Even after extraction with solutions containing detergents and high concentrations of salts, most intermediate filaments in a cell remain intact, whereas microfilaments and micro-tubules depolymerize into their soluble subunits. In fact, most IF purification methods employ these treatments to free intermediate filaments from other proteins. Intermediate filaments also differ in size from the other two cytoskeletal fibers. Indeed, their name derives from their 10-nm diameter—smaller than microtubules (24 nm) but larger than microfilaments (7 nm) (see Figure 5-29). Moreover, in contrast with the globular actin and tubulin subunits, which polymerize into microfilaments and hollow microtubules, respectively, IF subunits are a-helical rods that assemble into ropelike filaments. Finally, IF subunits do not bind nu-cleotides, and their assembly into intermediate filaments does not involve the hydrolysis of ATP or GTP, as does the polymerization of G-actin and tubulin. However, many of the details concerning the assembly of intermediate filaments in cells remain speculative.
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