6.1Types of Cytoskeleton and their Molecular Structures
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 to maintain the cell shape and the arrangement of organelles. The latter refers to the functional roles played through the interaction 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 basic unit of actin filaments is a protein called G-actin, whose structure is very similar in many organisms including ameba, 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 than muscle 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. The actin filaments in cell processes are involved in the formation of the pseudopodia (processes) of moving cells and processes known as microvilli often found in usual cells.
G-actin has a binding site for ATP or ADP, and ATP bound G-actin molecules (ATP-G-actin) polymerize stably. However, after polymerization, when bound ATP is hydrolyzed into ADP, the polymer becomes unstable and is easily depolymerized. After depolymerization, when ADP is replaced with ATP, G-actin molecules again become able to bind to actin filaments. In this way, G-actin is recycled (Fig. 6-2).
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 their polymerization. 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 polymerized G-actin molecules. Since G-actin has polarity, an actin filament also has polarity.
Fig. 6-2 Formation of actin filaments and the recycling 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 ADP of the dissociated G-actin is replaced with ATP, the G-actin is able to polymerize again.
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 helix containing 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. Dimers whose β-tubulin is bound with GTP are more stably polymerize than those bound with GDP. Polymerization is more likely to occur at the plus end of microtubules. After polymerization, hydrolysis of GTP bound to β-tubulin into GDP destabilizes the dimer, depolymerizing the tubulins from the minus end.
Fig. 6-4 Microtubules in a cell
In cells, microtubules radiate from the centrosome - the origin of polymerization. The minus end of microtubules is the side of the centrosome.
An organelle that serves as the polymerization origin of microtubules exists in cells. This structure is called the centrosome, localized near the nucleus. Special protein complexes that serve as the starting point in 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 segregate chromosomes during cell division. Centrioles are replicated into two during the DNA replication phase of the cell cycle, and form two centrosomes (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). The microtubules segregate chromosomes by pulling apart pairs of sister chromatid to spindle poles (Fig. 6-5). Motor proteins, which will be discussed later, are involved in this process.
In microtubules, as in actin filaments, proteins that play various functions by binding to microtubules are found. These are known 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 proteins located in some parts of microtubules serve as the generating force for the separation. → indicates the direction of the force.
When G-actin bound with ATP is kept at a constant concentration in vitro, it is polymerized into the plus end of actin filaments. On the other hand, G-actin whose ATP is hydrolyzed into ADP is depolymerized from minus end of actin filaments. In this condition, G-actin polymerized into the plus end appears to move toward the minus end in a phenomenon known as treadmilling (Column Fig. 6-1).
The name intermediate comes from the diameter (10 nm) of these filaments. Like actin filaments and microtubules, intermediate filaments are polymers of elementary unit protein (Fig. 6-6). However, their polymerization mechanism differs. Polymerization does not require nucleotides such as ATP and GTP. Other differences include a lack of polarity- the plus and minus ends - in filaments.
Intermediate filaments in cells form complex networks with actin filaments and microtubules. They are abundant in cells to which physical tension is applied (e.g., epidermal cells and muscle cells) and neurons. Generally, intermediate filaments exist stably (degradation and reconstruction are infrequent), but when significant changes (such as cell division) occur, their degradation and reconstruction become very active.
Fig. 6-6 Intermediate filaments
The polymerization steps of intermediate filaments are shown here. As the first step, two basic units (i.e., monomers) associate in the same direction to form a dimer. Two dimers running in the reverse direction are line up to form a tetramer, with the two strands slightly offset in their opposite directions. Tetramers are arranged side by side to form a protofilament. Eight protofilaments are bundled to form an intermediate filament with a diameter of approximately 10 nm.