Actin in the cytoskeleton

Living cells have the ability to change their shape and move upon stimuli in the environment. There is a network, called the cytoskeleton, a complex of filamentous proteins, which is the engine of biological adaptation. Actin is the key component of the cytoskeleton, because it exists in two forms, globular and fibrous, and it can also form a gel; these properties are ideal for restructuring the cytoskeleton in response to a variety of signals. For instance, growth factor stimulation promotes actin assembly at the plasma membrane to generate movement, whereas apoptotic signals cause cytoskeletal destruction to elicit characteristic membrane blebbing and morphological changes.

The dynamic role of actin in modifying the cytoskeleton is regulated by the actin binding proteins. Fig. CM 12 illustrates the multiple pathways involved in the regulation. Cellular movements are initiated by extracellular stimuli and the signals transduced through the cell membrane couple the messenger response to actin assembly in the cell. This is illustrated in Fig. CM13.

Fig. CM12. Regulation of actin polymerization by the actin-binding proteins. Symbols used: "C" for monomeric binding proteins; bracket for capping and severing proteins; squiggle for cross-linking proteins. (From Pollard and Cooper, with permission from the Annual Review of Biochemistry,. vol. 35, 1986, by Annual Reviews, http://www.AnnualReviews.org).

Fig. CM12. Regulation of actin polymerization by the actin-binding proteins. Symbols used: "C" for monomeric binding proteins; bracket for capping and severing proteins; squiggle for cross-linking proteins. (From Pollard and Cooper, with permission from the Annual Review of Biochemistry,. vol. 35, 1986, by Annual Reviews, http://www.AnnualReviews.org).

Fig. CM13. Schematic overview of how actin-binding proteins might link stimulus-induced signal generation to actin assembly in the cell. The sequence of postulated events is indicated by the numbers in the circles. PI, phosphatidylinositol; DAG, diacylglycerol. (From Stossel, 1989).

The understanding of the mechanism of actin-based motility in cells has been greatly advanced by the work of Loisel et al.(1999), who were able to reconstitute bacterial motility from pure protein components. In addition to actin, only three other components were absolutely needed: the Arp2/3 complex, ADF, and capping protein, all which are actin-binding proteins. These results demonstrate that actin-based propulsion is driven by the free energy released by ATP hydrolysis coupled to actin polymerization, and does not require myosin.

Work on nonmuscle cells has been recently applied to muscle as well. Thus, it was reported that actin dynamics at pointed ends regulates thin filament length in striated muscle (Littlefied et al., 2001).

It was thought until recently that bacteria lacked the actin network that organize eukaryotic cytoplasm. The mreB gene is involved in determining cell shape in rod-like bacteria and recently van den Ent et al. (2001) showed that bacterial MreB protein assembles into filaments with a subunit repeat similar to that of F-actin in eukaryotic cells. In addition, the MreB crystal structure reveals a shape similar to that of actin. Accordingly, prokaryotes possess homologues of both tubulin and actin, suggesting that the building blocks for the actin cytoskeleton originated in prokaryotes before becoming the mainstay of eukaryotic cells.

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  • marino
    How does the cytoskeleton involves in muscle contraction?
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