There are three types of cytoskeleton, each of which plays unique roles within cells. These are actin filaments,microtubules and intermediate filaments. All have thin, fibrous structures and are polymers of basic unit proteins. They have unique characteristics and perform various functions according to their nature.
The cytoskeleton has both structural roles and functional roles. The former refers to the roles played by filaments in cells tomaintain the cell shape and the arrangement of organelles. The latter refers to the functional roles played through theinteraction with other proteins, such as muscle contraction, cell locomotion, cell division and intracellular transport.
In the next sections, the structure and functional roles of the three types of cytoskeletal filament is discussed.
The cytoskeleton has both structural roles and functional roles. The former refers to the roles played by filaments in cells tomaintain the cell shape and the arrangement of organelles. The latter refers to the functional roles played through theinteraction with other proteins, such as muscle contraction, cell locomotion, cell division and intracellular transport.
In the next sections, the structure and functional roles of the three types of cytoskeletal filament is discussed.
Actin Filaments
The basic unit of actin filaments is a protein called G-actin, whose structure is very similar in many organisms includingameba, plants and humans. G-actin polymerizes to form actin filaments with a diameter of around 7 nm (Fig. 6-1). Since G-actin molecule has plus end and minus end, the polymerized filament also has plus end and minus end.Actin filaments are found in all types of cells, and are particularly abundant in the contractile apparatus of muscle cells.The main constituents of such apparatus are myosin and actin filaments. Additionally, in normal animal cells other thanmuscle cells, actin filaments are abundant immediately below the plasma membrane and in cell processes. The actin filaments located below the plasma membrane stabilize it and tether membrane proteins by forming a network structure. Theactin filaments in cell processes are involved in the formation of the pseudopodia (processes) of moving cells and processesknown as microvilli often found in usual cells.In cells, compared with an in vitro environment, polymerization and depolymerization of actin filaments take place faster and more accurately. This is due to the action of many types of regulatory protein that bind to actin filaments to regulate theirpolymerization. These proteins are called actin-binding proteins.
The basic unit of actin filaments is a protein called G-actin, whose structure is very similar in many organisms includingameba, plants and humans. G-actin polymerizes to form actin filaments with a diameter of around 7 nm (Fig. 6-1). Since G-actin molecule has plus end and minus end, the polymerized filament also has plus end and minus end.Actin filaments are found in all types of cells, and are particularly abundant in the contractile apparatus of muscle cells.The main constituents of such apparatus are myosin and actin filaments. Additionally, in normal animal cells other thanmuscle cells, actin filaments are abundant immediately below the plasma membrane and in cell processes. The actin filaments located below the plasma membrane stabilize it and tether membrane proteins by forming a network structure. Theactin filaments in cell processes are involved in the formation of the pseudopodia (processes) of moving cells and processesknown as microvilli often found in usual cells.In cells, compared with an in vitro environment, polymerization and depolymerization of actin filaments take place faster and more accurately. This is due to the action of many types of regulatory protein that bind to actin filaments to regulate theirpolymerization. These proteins are called actin-binding proteins.
Fig. 6-1 Actin filaments
G-actin (the basic unit of actin filaments) and actin filaments (polymers of G-actin molecules) are shown here.Each actin filament has two stranded helix of polymerizedG-actin molecules. Since G-actin has polarity, an actin filament also has polarity.
Fig. 6-2 Formation of actin filaments and therecycling of G-actin
ATP-G-actin binds to the plus end of actin filaments.Hydrolysis of the ATP facilitates the depolymerization,removing G-actin molecules from its minus end. If the ADPof the dissociated G-actin is replaced with ATP, the G-actinis able to polymerize again.
Microtubules
The basic unit of microtubules is dimer of α- and β-tubulin. Microtubules - long, thin fibrous structures - are polymers of these dimers (Fig. 6-3). A microtubule is a tubular filament of approximately 25 nm in diameter, with each turn of the helixcontaining 13 dimers. A microtubule has polarity; one end of the filament with β-tubulin is the plus end, and the opposite end is the minus end.
Microtubules in cells frequently repeat polymerization and depolymerization in the similar way as actin filaments. Dimerswhose β-tubulin is bound with GTP are more stably polymerize than those bound with GDP. Polymerization is more likely tooccur at the plus end of microtubules. After polymerization, hydrolysis of GTP bound to β-tubulin into GDP destabilizes thedimer, depolymerizing the tubulins from the minus end.
Microtubules in cells frequently repeat polymerization and depolymerization in the similar way as actin filaments. Dimerswhose β-tubulin is bound with GTP are more stably polymerize than those bound with GDP. Polymerization is more likely tooccur at the plus end of microtubules. After polymerization, hydrolysis of GTP bound to β-tubulin into GDP destabilizes thedimer, depolymerizing the tubulins from the minus end.
Fig. 6-3 Tubulins and the formation of microtubules
The basic unit of microtubules is dimer consisting of α- and β-tubulins. Microtubules have a long tubular structure, with each turnof the helix containing 13 dimers. They have plus and minus ends, and polymerization occurs at the plus end.
Fig. 6-4 Microtubules in a cell
In cells, microtubules radiate from the centrosome - the origin of polymerization. The minus end ofmicrotubules is the side of the centrosome.
An organelle that serves as the polymerization origin of microtubulesexists in cells. This structure is called the centrosome, localized nearthe nucleus. Special protein complexes that serve as the starting pointin the polymerization of microtubules are found in the centrosome. In most cells, microtubules radiate from the centrosome (Fig. 6-4).Therefore, their growth ends (i.e., those opposite from the centrosome)are the plus ends.
One of the important roles of microtubules is to segregatechromosomes during cell division. Centrioles are replicated into two during the DNA replication phase of the cell cycle, and form twocentrosomes (spindle poles) prior to the mitotic phase. In the mitotic phase, microtubules extending from two spindle poles form mitotic spindles that bind to sister chromatids (see Chapter 12). Themicrotubules segregate chromosomes by pulling apart pairs of sister chromatid to spindle poles (Fig. 6-5). Motor proteins, which will bediscussed later, are involved in this process.
In microtubules, as in actin filaments, proteins that play various functions by binding to microtubules are found. These areknown as microtubule-associated proteins, and well known examples are those regulating the polymerization/depolymerization of microtubules and motor proteins that transport cargo in cells.
One of the important roles of microtubules is to segregatechromosomes during cell division. Centrioles are replicated into two during the DNA replication phase of the cell cycle, and form twocentrosomes (spindle poles) prior to the mitotic phase. In the mitotic phase, microtubules extending from two spindle poles form mitotic spindles that bind to sister chromatids (see Chapter 12). Themicrotubules segregate chromosomes by pulling apart pairs of sister chromatid to spindle poles (Fig. 6-5). Motor proteins, which will bediscussed later, are involved in this process.
In microtubules, as in actin filaments, proteins that play various functions by binding to microtubules are found. These areknown as microtubule-associated proteins, and well known examples are those regulating the polymerization/depolymerization of microtubules and motor proteins that transport cargo in cells.
Fig. 6-5 Chromosome segregation and microtubules
A) Microtubules extend from the centrosome and bind to sister chromatids. The structure is called mitotic spindle due to spindle-like shape.
B) Microtubules bind to pairs of chromatids, pulling them apart to opposite sides of the cell. During this process, motor proteinslocated in some parts of microtubules serve as the generating force for the separation. → indicates the direction of the force.
B) Microtubules bind to pairs of chromatids, pulling them apart to opposite sides of the cell. During this process, motor proteinslocated in some parts of microtubules serve as the generating force for the separation. → indicates the direction of the force.