5 The Cytoskeleton

A dynamic framework for cells

The cytoskeleton is a dynamic network of protein filaments, not a rigid scaffold. Cells can assemble, disassemble, and rearrange its components as they change shape or respond to their surroundings. Its three main filament systems have distinct building blocks and roles: actin microfilaments support surface changes and force generation, intermediate filaments strengthen cells, and tubulin microtubules organize the interior and support transport.

Three filament systems, three main roles

The three filament systems differ in their composition, mechanical behavior, and typical functions.

• Microfilaments, also called actin filaments, are thin and flexible. They are common just beneath the plasma membrane, where they help support cell shape. Rapid assembly and disassembly let them reshape the cell surface. Growing actin filaments can push the membrane outward, while interactions with myosin generate contraction.

• Intermediate filaments are rope-like fibers whose proteins vary by cell type. Keratins are common in epithelial cells, and vimentin occurs in many connective-tissue cells. These generally less-dynamic fibers resist mechanical stress and help tissues withstand pulling or stretching. The nuclear lamina is a specialized intermediate-filament network that supports the inner surface of the nuclear envelope.

• Microtubules are the largest filaments. They are hollow tubes assembled from α- and β-tubulin dimers. Their two ends differ, and adding or removing tubulin subunits allows them to grow or shrink. Microtubules help position organelles and provide tracks for cargo. They also form the structural core of cilia and eukaryotic flagella; in many motile examples, they have a characteristic 9 + 2 arrangement.

A useful comparison is that actin is especially associated with surface change and contraction, intermediate filaments with tensile strength, and microtubules with internal organization and transport.

Motors turn energy into directed movement

Motor proteins use energy from ATP to move along a cytoskeletal filament. Some carry cargo, such as vesicles or organelles; others cause filaments to slide past one another.

• Myosins generally move along actin filaments. Actin and myosin work together in muscle contraction, and an actin–myosin ring tightens during the final separation of many dividing animal cells.

• Kinesins generally move toward a microtubule’s plus end, often transporting cargo toward the cell’s periphery.

• Dyneins generally move toward a microtubule’s minus end, often transporting cargo toward the cell center. Dynein also helps generate the bending of motile cilia and flagella.

These directions are general patterns: the behavior of a motor depends on the specific motor and how the filaments are organized in a cell.

How the cytoskeleton works as a team

Cell shape and movement emerge from coordinated work across the cytoskeleton. For example, a crawling cell can extend an actin-rich protrusion at its leading edge, attach that protrusion to its surroundings, and contract to move its body forward. Actin and myosin help generate force, while intermediate filaments strengthen the cell as it experiences mechanical strain. Microtubules organize the interior and help deliver materials where they are needed.

Movement can also occur without moving the whole cell. In motile cilia, dynein-driven interactions cause microtubules to slide relative to one another; this produces bending that can move fluid across a cell’s surface. Together, these examples show how the cytoskeleton supports both changes in cell shape and movement of cells or their parts.

Takeaway: Cell movement depends on coordinated roles: actin helps reshape and contract, intermediate filaments resist strain, microtubules organize and provide tracks, and motor proteins produce directed motion.