17.1Cell Motility

Flagellar movement of bacteria is driven by molecular motors that use a gradient of H+ formed between the inner and outer the cell membrane. By controlling the rotational movement of flagella, bacteria are able to carry out locomotion in reaction to stimulation from the external world (see Column Figure 17-2). Whereas, cytoplasmic streaming of plant cells, the ciliary movement of animal cells and contraction motion of muscle cells, etc., are driven by cytoskeletal activity and the motor proteins that bind to cytoskeletons. The driving force used for this is the energy obtained by the hydrolysis of adenosine 5′-triphosphate (ATP) (see Selection of Chapter 4).

The cytoskeletal components involved in cell movements are actin filaments and microtubules; moreover, many types of motor proteins participate in the cell movements by interacting with these filaments. The motor protein that carries out cell movements by binding to actin filaments is myosin. Muscle contraction is a typical example of movements carried out by myosin. Furthermore, myosins are also present in cells other than muscle cells, and are involved in various cellular movements such as cellular transport, cell locomotion and cell division, to name a few. There are also two types of motor proteins that bind to microtubules to carry out cell movements, namely, dynein and kinesin. These motor proteins are involved in various cell functions such as chromosomal segregation in cell division, organelle transport, and the transport of RNA and proteins.

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There are many types of myosins, varying in their molecular structure and their role in cell movement. One well-known myosin is Type II myosin, a myosin type responsible for the contraction motion of muscle cells. The heavy chain, which makes up the type II myosin body, has a globular head and long tail with an alpha-helical structure. When two molecules polymerize in such a way that the tails of the heavy chain twist, they form a functional unit (Figure 17-1A). Small proteins, called the light chain, are bound to the neck domain located between the head and tail. In muscle cells, many of these functional units polymerize to form thick myosin filaments. The basic unit of the contractile apparatus of muscle cells is a structure called the sarcomere, which consists of an alternating arrangement of actin and myosin filaments (Figure 17-1B, C). When linked, the sarcomeres together form a long fibrous structure called a myofibril. Muscle cells are filled with myofibrils, so when the myofibrils contract all together, a very strong contractile force is generated.

Fig. 17.1 Molecular structure of myosin

A) Pattern drawing of two typical types of myosin. Both consist of two heavy chains and several light chains. The head of the heavy chain includes an ATP-binding site for ATP hydrolysis and an actin-binding site. B) Microscopic photo and pattern drawing of sarcomere of amphibian cytoskeleton. C) Pattern drawing of actin and myosin filaments forming sarcomere. Tropomyosin and troponin C, I, and T, which regulate actin-myosin binding, are bound to actin filaments.

The head of the myosin structure consists of two areas: one that binds to actin filaments and the other that hydrolyzes ATP. Myosin molecule binds each other at the tail region. By utilizing the energy released by the hydrolysis of the ATP bound to the head of the myosin, the three-dimensional structure of myosin is modified (structure from the head to neck domain). As a result, the relative position between the actin and myosin filaments change (contract). This series of processes and their role in muscle contraction has been studied in detail (Figure 17-2A). Apart from the contraction motion of muscle cells, other known roles of myosin include the transport of organelles (mitochondria and endoplasmic reticulum) along actin filaments. The myosin that undertakes this role is myosin Type V. Its basic mechanism of motion is the same as that of Type II myosin, which carries out muscle cell contraction. In other words, myosin binds to the actin filament with its head, and changes its three-dimensional structure using the energy produced by the hydrolysis of ATP to transport the organelles binding to its tail along the actin filament. This mechanism of action resembles that of a person walking on two legs, transporting cargo along the actin filament from the minus to the plus end (Figure 17-2B)

Fig. 17.2 Model of muscle contraction by interaction between actin filaments and myosin molecules

A) The three-dimensional structure of type II myosin is changed through ATP binding, ATP hydrolysis, and ADP and phosphate liberation. These changes are used by muscle contraction. B) Cargo-bound Type V myosin, like a person walking on two legs, moves towards the plus end of the actin filament while binding to the actin filament.


Changes in three-dimensional structure of myosin molecules during muscle contraction

The head of myosin consists of an ATP-binding site and actin-binding site. It is connected to a thick myosin filament by its neck and tail which form an alpha-helix structure. The three-dimensional structure of the head and neck of the myosin dramatically changes with the hydrolysis of ATP and dissociation of inorganic phosphoric acid from adenosine 5′-diphosphate (ADP) (Column Figure 17-1). These changes serve as the driving force of muscle cell contraction. The sliding movement between myosin and actin filament caused by one set of structural change is only of a short distance; however, when this occurs repeatedly at a high speed, it causes muscle contractions of a long distance in a short period of time.

Column Figure 17-1 Three-dimensional structure changes and contraction of myosin molecule

When inorganic phosphoric acid and ADP dissociate from the myosin head, the three-dimensional structure of myosin binding to the actin filament changes considerably, resulting in a sliding movement between the actin and myosin filaments. This sliding movement serves as the basis of muscle contraction. Large changes in the three-dimensional structure can be seen in the neck and neck-head binding domains, as shown by the red dotted lines.

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Dynein and kinesin

The structure of kinesin very closely resembles that of myosin, but that of dynein does not (Figure 17-3). A common trait shared by dynein and kinesin is that they both consist of a region that binds to microtubules, a region that binds to an organelle as cargo and a region that binds and hydrolyzes ATP. The basic mechanism of movement of dynein and kinesin is the same as that of myosin; it uses the energy produced by hydrolysis of ATP to change its three-dimensional structure to move along microtubules. Dynein and kinesin move in opposite directions; dynein towards the minus end of the microtubule and kinesin towards the plus end. As these motor proteins move, the organelles binding to them are transported along the microtubules. In this way, like type V myosin, kinesin and dynein play an important role as intracellular transport proteins relocating materials within the cell. Dynein also binds with the microtubules constituting cilia and flagella, and activates ciliary and flagellar movement (Figure 17-4). These motor proteins also play a role in chromosomal segregation during cell division and cell locomotion (see Column Figure 17-3).


Flagellar movement of bacteria

Most prokaryotic cells are capable of locomotion, and the most well understood method to achieve this is through the use of a flagellum. The flagella of prokaryotic cells have a completely different structure and mechanism of movement from those of eukaryotic cells. While the cilia and flagella of eukaryotic cells use the energy produced by ATP hydrolysis, the flagella of prokaryotic cells are moved using the energy produced from an H+ gradient across the membrane (Column Figure 17-2). This mechanism closely resembles that of the F-type ATP synthase present in the mitochondria (see Selection 3 of Chapter 16, Figure 16-7). Bacteria move around by controlling the movement of their flagella in reaction to environmental stimuli. Examples of this include chemotaxis to and avoidance of certain substances. The direction to which the bacterium moves is determined by the direction of the flagellar rotation. To determine and control its locomotion, bacteria have receptors that detect the presence of certain substances and systems to transmit such information to control the direction of flagellar rotation.

Column Figure 17-2 Flagellar movement of bacteria

A) Locomotion of bacteria by flagellar rotation
B) Pattern drawing of motor rotating a flagellum. The motor rotor consists of positively and negatively charged areas, which are arranged alternately. H+ activates these areas to rotate the rotor. The source of this rotational energy is the movement of H+ induced by the H+ gradient across the cell membrane (inner membrane). This energy is able to rotate the flagellum at 100–1000 revolutions per minute. C) Bacteria sense chemical substances using sensors in the cell membrane to control the rotation of flagella. The H+ gradient (low concentration inside cell) is maintained by the pump in the cell membrane.

Figure 17-3 Movements of dynein and kinesin, and transport of cargo

A) Both dynein and kinesin bind to microtubules at one end and to a cargo, such as an organelle, at the other end, and move in a constant direction along the microtubules. In this way, dynein and kinesin play the role of transporting a cargo along microtubules. B) Kinesin moves along microtubules by changing its three-dimensional structure using energy produced by ATP hydrolysis. This movement model resembles a man walking on two legs.

Figure 17-4 Structure and movement of cilias

A) Electron microscopic photo of the microtubules arranged in a “9 + 2” structure in the cilium. B) The microtubles are deformed and bent by the movement of the dyneins between special microtubules forming a doublet structure in the flagellum. This bending causes the ciliary movement.

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