Chapter 18: Cell Organization and Movement II: Microtubules and Intermediate Filaments

Intermediate Filaments Are Dynamic

Although intermediate filaments are much more stable than microtubules and microfilaments, IF protein subunits have been shown to be in dynamic equilibrium with the existing IF cytoskeleton. In one experiment, biotin-labeled keratin was injected into fibroblasts; within 2 hours, the labeled protein had been incorporated into the already existing keratin cytoskeleton (Figure 18-49). The results of this experiment and others demonstrate that IF subunits in a soluble pool are able to add themselves to preexisting filaments and that subunits are able to dissociate from intact filaments.

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EXPERIMENTAL FIGURE 18-49 Keratin intermediate filaments are dynamic, as soluble keratin is incorporated into filaments. Monomeric type I keratin was purified, chemically labeled with biotin, and microinjected into living epithelial cells. The cells were then fixed at different times after injection and stained with an antibody to biotin and with antibodies to keratin. (a) At 20 minutes after injection, the injected biotin-labeled keratin is concentrated in small foci scattered throughout the cytoplasm (left) and has not been integrated into the endogenous keratin cytoskeleton (right). (b) After 4 hours, the biotin-labeled keratin (left) and the keratin filaments (right) display identical patterns, indicating that the microinjected protein has become incorporated into the existing cytoskeleton.