Cytoskeletal Elements The ability of eukaryotic cells to adopt a variety of shapes, organize the many components in their interior, interact mechanically with the environment, and carry out coordinated movements depends on the cytoskeleton—an intricate network of protein filaments that extends throughout the cytoplasm. The cytoskeleton is built on a framework of three types of protein filaments: • intermediate filaments microtubules, and actin filaments. Each type of filament has distinct mechanical properties and is formed from a different protein subunit. A family of fibrous proteins forms the intermediate filaments; globular tubulin subunits form microtubules; and globular actin subunits form actin filaments Each cytoskeletal element has distinct properties. Microtubules are long, hollow, unbranched tubes composed of subunits of the protein tubulin. Microfilaments are solid, thinner structures, often organized into a branching network and composed of the protein actin. Intermediate filaments are tough, ropelike fibers composed of a variety of related proteins. Intermediate filaments are ropelike fibers Microtubules are hollow Actin filaments (also known as with a diameter of about 10 nm; they are cylinders made of the protein microfilaments) are helical polymers made of fibrous intermediate filament tubulin. They are long and of the protein actin. They are flexible proteins. One type of intermediate filament straight and typically have one structures, with a diameter of about 7 forms a meshwork called the nuclear lamina end attached to a single nm, that are organized into a variety just beneath the inner nuclear membrane. microtubule-organizing center of linear bundles, two-dimensional Other types extend across the cytoplasm, called a centrosome. With an networks, and three-dimensional gels. giving cells mechanical strength and outer diameter of 25 nm, Although actin filaments are dispersed distributing the mechanical stresses in an microtubules are more rigid than throughout the cell, they are most epithelial tissue by spanning the cytoplasm actin filaments or intermediate highly concentrated in the cortex, the from one cell–cell junction to another. filaments, and they rupture layer of cytoplasm just beneath the Intermediate filaments are very flexible and when stretched. plasma membrane. have great tensile strength. They deform under stress but do not rupture. Actin Filaments The major cytoskeletal protein of most cells is actin, which polymerizes to form actin filaments—thin, flexible fibers approximately 7 nm in diameter and up to several micrometers in length. Within the cell, actin filaments (also called microfilaments) are organized into higher-order structures, forming bundles or three-dimensional networks with the properties of semisolid gels. The assembly and disassembly of actin filaments, their crosslinking into bundles and networks, and their association with other cell structures (such as the plasma membrane) are regulated by a variety of actin-binding proteins, which are critical components of the actin cytoskeleton. Actin filaments are particularly abundant beneath the plasma membrane, where they form a network that provides mechanical support, determines cell shape, and allows movement of the cell surface, thereby enabling cells to migrate, engulf particles, and divide. Each actin monomer (globular [G] actin) has tight binding sites that mediate head-to-tail interactions with two other actin monomers, so actin monomers polymerize to form filaments (filamentous [F] actin). Each monomer is rotated by 166o in the filaments, which therefore have the appearance of a double-stranded helix. Because all the actin monomers are oriented in the same direction, actin filaments have a distinct polarity and their ends (called the plus and minus ends) are distinguishable from one another. The first step in actin polymerization (called nucleation) is the formation of a small aggregate consisting of three actin monomers. Actin filaments are then able to grow by the reversible addition of monomers to both ends, but one end (the plus end) elongates five to ten times faster than the minus end. The actin monomers also bind ATP, which is hydrolyzed to ADP following filament assembly. Although ATP is not required for polymerization, actin monomers to which ATP is bound polymerize more readily than those to which ADP is bound. Because actin polymerization is reversible, filaments can depolymerize by the dissociation of actin subunits, allowing actin filaments to be broken down when necessary. Thus, an apparent equilibrium exists between actin monomers and filaments, which is dependent on the concentration of free monomers. The rate at which actin monomers are incorporated into filaments is proportional to their concentration, so there is a critical concentration of actin monomers at which the rate of their polymerization into filaments equals the rate of dissociation. At this critical concentration, monomers and filaments are in apparent equilibrium Two ends of an actin filament grow at different rates, with monomers being added to the fast- growing end (the plus end) five to ten times faster than to the slow-growing (minus) end. Because ATP-actin dissociates less readily than ADP-actin, this results in a difference in the critical concentration of monomers needed for polymerization at the two ends. This difference can result in the phenomenon known as treadmilling, which illustrates the dynamic behavior of actin filaments. For the system to be at an overall steady state, the concentration of free actin monomers must be intermediate between the critical concentrations required for polymerization at the plus and minus ends of the actin filaments. Under these conditions, there is a net loss of monomers from the minus end, which is balanced by a net addition to the plus end. Treadmilling requires ATP, with ATP-actin polymerizing at the plus end of filaments while ADP-actin dissociates from the minus end. Organization of Actin Filaments Individual actin filaments are assembled into two general types of structures which play different roles in the cell. Actin bundles • In bundles, the actin filaments are crosslinked into closely packed parallel arrays. • The proteins that crosslink actin filaments into bundles (called actin-bundling proteins) usually are small rigid proteins that force the filaments to align closely with one another. Actin networks ▪ In networks, the actin filaments are loosely crosslinked in orthogonal arrays that form three-dimensional meshworks with the properties of semisolid gels. ▪ The proteins that organize actin filaments into networks tend to be large flexible proteins that can crosslink perpendicular filaments. There are two structurally and functionally distinct types of actin bundles, involving different actin-bundling proteins. The first type of bundle, containing closely spaced actin filaments aligned in parallel, supports projections of the plasma membrane, such as microvilli. In these bundles, all the filaments have the same polarity, with their plus ends adjacent to the plasma membrane. An example of a bundling protein involved in the formation of these structures is fimbrin, which was first isolated from intestinal microvilli and later found in surface projections of a wide variety of cell types. Fimbrin is a 68-kd protein, containing two adjacent actin-binding domains. It binds to actin filaments as a monomer, holding two parallel filaments close together The second type of actin bundle is composed of filaments that are more loosely spaced and are capable of contraction, such as the actin bundles of the contractile ring that divides cells in two following mitosis. The looser structure of these bundles (which are called contractile bundles) reflects the properties of the crosslinking protein α-actinin. In contrast to fimbrin, α- actinin binds to actin as a dimer, each subunit of which is a 102- kd protein containing a single actin-binding site. Filaments crosslinked by α-actinin are consequently separated by a greater 40 nm apart distance than those crosslinked by fimbrin of 14 nm). The increased spacing between filaments allows the motor protein myosin to interact with the actin filaments in these bundles, which enables them to contract. The actin filaments in networks are held together by large actin-binding proteins , such as filamin. Filamin (also called actin-binding protein or ABP-280) binds actin as a dimer of two 280-kd subunits. The actin- binding domains and dimerization domains are at opposite ends of each subunit, so the filamin dimer is a flexible V- shaped molecule with actin-binding domains at the ends of each arm. As a result, filamin forms cross-links between orthogonal actin filaments, creating a loose three- dimensional meshwork. Association of Actin Filaments with Plasma Membrane Actin filaments are highly concentrated at the periphery of the cell, where they form a three-dimensional network beneath the plasma membrane. This network of actin filaments and associated actin-binding proteins (called the cell cortex) determines cell shape and is involved in a variety of cell surface activities, including movement. The association of the actin cytoskeleton with the plasma membrane is thus central to cell structure and function. Red blood cells (erythrocytes) used for plasma membrane as they contain no nucleus or internal organelles, so their plasma membrane and associated proteins can be easily isolated without contamination by the various internal membranes that are abundant