Some proteins and organelles are rapidly transported over long distances along microtubules, which serve as tracks that direct and facilitate the motion. Study of transport along axons (long projections that allow one nerve to contact another) reveals small vesicles or whole organelles that can actually be seen moving along microtubule bundles in both directions. Two "molecular motors" are involved. One, called cytoplasmic dynein, resembles the dynein involved in the motion of cilia and flagella. It is involved in transport from the plus end of the microtubule toward the minus end. The other molecular motor protein is called kinesin. It transports objects in the opposite direction from cytoplasmic dynein. The two proteins are similar in structure and resemble myosins (Figure 8.26).

Careful studies of the motion of kinesin and dynein on microtubules indicate that they "walk" along the microtubule track, with a step size of about 8 nm-the distance from one tubulin dimer to the next. Figure 8.27 depicts a model for the motion of kinesin on a microtubule.

Evidence exists for another kind of transport in the cytoplasm. This one involves not microtubules, but actin fibers. Thus, microtubules may represent the "superhighways" for intracellular transport, whereas actin fibers may serve as the "country roads".