Material transport takes place actively in cells. As an example, proteins synthesized in ribosomes must be transported to their destinations. Additionally, organelles such as endoplasmic reticulum, mitochondria and chloroplasts frequently move within cells. Specialized carriers in the cytoplasm are involved in such protein transport and organelle movement.
These carriers are called motor proteins, and three groups are known: kinesins and dyneins (two groups of proteins that move along microtubules) and myosins (a group of proteins that move along actin filaments).
Kinesins are complexes consisting of several proteins, and many types are known. A typical kinesin is a complex of two large proteins (heavy chains) and two small proteins (light chains). Heavy chains consist of head and tail structures. The head domains play a central role in movement. Kinesin heads interact with microtubules and move toward the plus end. During movement, the tail binds and carries cargo (e.g., endoplasmic reticulum and mitochondria) (Fig. 6-8A).
Like walking on two legs, kinesins proceed by alternately moving their two heads forward (Fig. 6-8B). This head (ATPase) movement is caused by conformational change through ATP hydrolysis.
Fig. 6-8 Kinesins and dyneins moving along a microtubule
A) Kinesins and dyneins consist of a two-leg-like part that binds to microtubules and a part that binds to cargo. Although kinesins and dyneins travel in opposite directions, they move along microtubules like walking on two legs.
B) A model showing how kinesins walk on two legs on microtubules. Each leg takes a step forward each time ATP is hydrolyzed. → indicates the movement of the heads.
Dyneins are also complexes consisting of multiple proteins, but have more complicated and larger molecular structures than those of kinesins (Fig. 6-8A). They consist of a part that is involved in migration and a part for binding cargo. Like kinesins, they move using structural changes caused by ATP hydrolysis. Dyneins walk on two legs along microtubules in the same way as kinesins, but move toward the minus end.
Dyneins play another important role involving the movement of cilia and flagella. Cilia are well known for the movement of paramecia, and flagella are the structures used in the locomotion of some protozoa, bacteria and animal sperm (Column Fig. 6-2).
There are many types of myosins. All have heads that bind and hydrolyze ATP, but their tails vary among different types. The diversity of their tails reflects the diversity of functions of myosin molecules. Some myosin types function as motor proteins that move along actin filaments; Type I and Type V myosins are well known examples. Their tails differ greatly in structure from those of Type II myosins located in the contractile apparatus of muscle cells (Fig. 6-9A). These differences reflect the varying roles that each myosin molecule plays. Using Type V as an example, the movement of myosin molecule as motor protein is discussed below.
Type V myosin is a complex consisting of two large proteins and several small ones. The head of the large protein binds to actin filaments and the tail binds to cargo. Type V myosin moves by the energy of ATP hydrolysis in the heads. It moves along actin filaments toward the plus end like walking on two legs by alternately changing the head structure (Fig. 6-9B). The width of one step is approximately 37 nm (the width of 6.5 G-actin molecules).
The moving speeds of kinesins, dyneins and myosins (Type V) vary by condition, but they have been confirmed as approximately 1 μm/second. This may seem very slow, but if the size of molecules (which is several dozens of nm) assume as the size of humans, it would be very fast.
Structure of Flagella and their Movement Mechanism
Cilia and flagella are bundles of nine special microtubules surrounding two normal central microtubules. Dyneins are located between the special microtubules. One end of the dynein is fixed to the A-tubule, and the other mobile end adheres to the B-tubule. A sliding motion occurs between the microtubules as a result of dyneins moving toward their minus end. These would simply slide against each other in parallel if they were not fixed together. However, since they are partially fixed by a protein called nexin, the microtubules bend when dyneins move, causing the sine-curve-like movement of cilia and flagella. Sperm and euglena migrate in water using this movement.
Column Fig. 6-2 Sperm flagellum structure and the role of dynein in flagellar movement
A) Cross section of a sperm flagellum. Consisting of nine special microtubules and two normal ones, this is known as the 9 + 2 structure.
B) Dyneins fixed to the A-tubule move on the adjacent B-tubule toward the minus end. As a result, the microtubules bend.