For cells to function properly, they must organize themselves in space and interact mechanically with each other and with their environment. They have to be correctly shaped, physically robust, and properly structured internally. Many have to change their shape and move from place to place. All cells have to be able to rearrange their internal components as they grow, divide, and adapt to changing circumstances. These spatial and mechanical functions depend on a remarkable system of filaments called the cytoskeleton (Figure 16–1).
The cytoskeleton’s varied functions depend on the behavior of three families of protein filaments—actin filaments, microtubules, and intermediate filaments. Each type of filament has distinct mechanical properties, dynamics, and biological roles, but all share certain fundamental features. Just as we require our ligaments, bones, and muscles to work together, so all three cytoskeletal filament systems must normally function collectively to give a cell its strength, its shape, and its ability to move.
In this chapter, we describe the function and conservation of the three main filament systems. We explain the basic principles underlying filament assembly and disassembly, and how other proteins interact with the filaments to alter their dynamics, enabling the cell to establish and maintain internal order, to shape and remodel its surface, and to move organelles in a directed manner from one place to another. Finally, we discuss how the integration and regulation of the cytoskeleton allows a cell to move to new locations.
The three major cytoskeletal filaments are responsible for different aspects of the cell’s spatial organization and mechanical properties. Actin filaments determine the shape of the cell’s surface and are necessary for whole-cell locomotion; they also drive the pinching of one cell into two. Microtubules determine the positions of membrane-enclosed organelles, direct intracellular transport, and form the mitotic spindle that segregates chromosomes during cell division. Intermediate filaments provide mechanical strength. All of these cytoskeletal filaments interact with hundreds of accessory proteins that regulate and link the filaments to other cell components, as well as to each other. The accessory proteins are essential for the controlled assembly of the cytoskeletal filaments in particular locations, and they include the motor proteins, remarkable molecular machines that convert the energy of ATP hydrolysis into mechanical force that can either move organelles along the filaments or move the filaments themselves.
In this section, we discuss the general features of the proteins that make up the filaments of the cytoskeleton. We focus on their ability to form intrinsically
Figure 16–1 The cytoskeleton. (A) A cell in culture has been fixed and labeled to show its cytoplasmic arrays of microtubules (green) and actin filaments (red). (B) This dividing cell has been labeled to show its spindle microtubules (green) and surrounding cage of intermediate filaments (red). The DNA in both cells is labeled in blue. (A, courtesy of Albert Tousson;  B, courtesy of Conly Rieder.) polarized and self-organized structures that are highly dynamic, allowing the cell to rapidly modify cytoskeletal structure and function under different conditions.
Cytoskeletal Filaments Adapt to Form Dynamic or Stable Structures
Cytoskeletal systems are dynamic and adaptable, organized more like ant trails than interstate highways. A single trail of ants may persist for many hours, extending from the ant nest to a delectable picnic site, but the individual ants within the trail are anything but static. If the ant scouts find a new and better source of food, or if the picnickers clean up and leave, the dynamic structure adapts with astonishing rapidity. In a similar way, large-scale cytoskeletal structures can change or persist, according to need, lasting for lengths of time ranging from less than a minute up to the cell’s lifetime. But the individual macromolecular components that make up these structures are in a constant state of flux. Thus, like the alteration of an ant trail, a structural rearrangement in a cell requires little extra energy when conditions change.