Actin Structures

As we saw in Chapter 5, the actin cytoskeleton is organized into various large structures that extend throughout the cell. Because it is so big, the actin cytoskeleton can easily change cell morphology just by assembling or disassembling itself. In preceding chapters, we have seen examples of large protein complexes in which the number and positions of the sub-units are fixed. For example, all ribosomes have the same number of protein and RNA components, and their three-dimensional geometry is invariant. However, the actin cytoskeleton is different—the lengths of filaments vary greatly, the filaments are cross-linked into imperfect bundles and networks, and the ratio of cytoskeletal proteins is not rigidly maintained. This organizational flexibility of the actin cytoskeleton permits a cell to assume many shapes and to vary them easily. In moving cells, the cytoskeleton must assemble rapidly and does not always have a chance to form well-organized, highly ordered structures.

In this section, we consider the properties of monomeric and polymeric actin, as well as the various proteins that assemble actin filaments into large structures. With this basic understanding of the actin cytoskeleton established, we examine in Section 19.2 how a cell can tailor this framework to carry out various tasks requiring motion of the entire cell or subcellular parts.

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