The eukaryotic cytoplasm contains a fibrous structure called cytoskeleton (Fig. 11-9A). The cytoskeleton is involved in various functions such as maintaining cell shape, cell movement, intracellular transport of substances, and cell division. This fibrous structure is dynamic and frequently and repeatedly undergoes break-up (depolymerization) and restructuring (polymerization). Three types of fibers are found in the cytoskeleton: actin filaments, microtubules, and intermediate filaments. Each of these filaments is characteristically different in their distribution throughout the cell, structure, and function. On the other hand, no such structure is found in prokaryotic cells.
Actin filaments (with a diameter of approximately 5–9 nm) are loosely helical structure formed by a sequence of two lines of unit proteins called G-actin (Fig. 11-9B). G-actin comprises three types of subunits, i.e., α, β, and γ. α-Actin is predominantly found in muscle cells, and β- and γ-actin are found in a broad range of cells. G-actin also has the property of binding to ATP and ADP, and it changes to a tertiary structure depending on whether it has bound to ATP or ADP. These tertiary structure changes play a vital role in G-actin polymerization and depolymerization.
The ATP-bound G-actin needs ions such as Mg2+, K+, and Na+ to polymerize and form fibrous structures. This ATP-bound G-actin has the property of binding stably to other ATP-bound G-actin structures. In addition, after polymerization, once ATP is hydrolyzed to ADP, the bonds between G-actin become unstable and prone to depolymerization. Because G-actin stretches out unidirectionally when it polymerizes, the fibers formed have a set orientation. This orientation is called the plus end and minus end; actin polymerization is rapid at the plus end and slow at the minus end (approximately 1/10th the rate of the plus end). In other words, while polymerization of the ATP-bound G-actin occurs from the plus end, the G-actin that has been destabilized by the hydrolyzing action of ATP is depolymerized from the minus end. Moreover, the depolymerized G-actin is recycled by exchanging ADP with ATP.
Cells contain various proteins collectively referred to as actin-binding proteins. These proteins regulate polymerization and depolymerization of the actin filaments by matching them to their cell functions. For example, cofilin binds to G-actin and promotes its depolymerization, and profilin binds to G-actin and promotes its polymerization. The actin filaments found in cells construct the three-dimensional structure required in various functions such as maintaining cell shape, cell movement, cell division, and the intracellular transport of materials. Actin-binding proteins are also involved in forming this three-dimensional structure. For example, α-actin arranges the actin filaments in parallel, and profilin arranges the actin filaments perpendicularly.
Microtubules are tube-shaped structures formed by protein units polymerizing into a cylindrical shape, with a diameter of 24–25 nm (Fig. 11-9C). The protein units are dimers composed of two types of proteins called α- and β-tubulin. α- and β-Tubulin have a fairly similar structure but different properties. α-Tubulin can bind to GTP but cannot hydrolyze it nor can it convert GTP to GDP. In other words, it is usually bound to GTP. In contrast, β-tubulin can bind to both GTP and GDP. It also functions as a GTPase, which is an enzyme hydrolyzing GTP. The microtubules have an orientation wherein the tip having β-tubulin is the plus terminus and the opposite end is the minus terminus.
When tubulin polymerizes to form microtubules, polymerization and depolymerization may both occur from the plus or minus terminus, but the polymerization rate is higher at the plus terminus. In addition, when GTP is bound to β-tubulin with dimers that bind one another, a more stable bond is formed than that formed when GDP is bound to β-tubulin. Because of these properties, the dimers to which GTP binds polymerize stably from the plus terminus, while hydrolysis of GTP bound to β-tubulin causes destabilization and thus depolymerization of dimers from the minus terminus. The tubulin dimers that have been depolymerized are recycled by exchanging GDP with GTP.
Polymerization of microtubules in a cell is controlled by the microtubule-binding protein. This protein plays various roles such as stabilizing the microtubular structure, cleaving the microtubules, promoting tubulin depolymerization, and transporting substances.
The microtubules found in the cell consider the centrosome as the base point for polymerization and are found extending radially outward from it into the cell. In addition, the microtubules extending out into the cell act as the transport route in intracellular transport (see Column Selection 6 of Chapter 12). Moreover, during cell division, spindle fibers, which are composed of microtubules, extending out from the two centrosomes play an important role in separation of sister chromatids and bringing them to the two poles (see Selection 1 ofChapter 13, Fig. 13-3).
The diameter of intermediate filaments is approximately between the diameters of the actin filaments and microtubules (approximately 10 nm); this is why these filaments are called intermediate filaments (Fig. 11-9D). Intermediate filaments are also the same as actin filaments and microtubules in that they are filaments formed by polymerization of protein units. However, intermediate filaments vary according to cell type. Examples of these include keratin found in epidermal cells, desmin and vimentin in muscle cells, and neurofilaments in nerve cells. In addition, their mechanism of polymerization is different from that of actin filaments and microtubles in that nucleotides are not required during polymerization and there is no orientation for the filament structure. Moreover, in contrast to actin filaments and microtubules, which are constantly repeating the process of polymerization and depolymerization, intermediate filaments are found in the cell in a comparatively more stabilized state. This is because intermediate filaments are involved in functions that are not as critical to dynamic changes, such as reinforcing cell adhesion and maintaining the structures found in the cell at all times.
Fig. 11-9 Cytoskeleton
(A) Distribution of the three types of cytoskeletal fibers. (B) Actin filaments. An electron microscope photograph showing actin filaments (⇦) constituting the myofibrils of skeletal muscles. The dark sections are Z-rays. (C) Microtubules. The electron microscope photograph shows spindle microtubules (⇦; Refer to Chapter 18). (D) Intermediate filaments. Intermediate filaments are formed when dimers polymerize, and the formed filaments bundle together. The electron microscope photograph shows intermediate filaments (←) connecting to the desmosome of an epidermal cell.