Motors Based on Microtubules

Abstract

The association of microtubules with movement is, with one important exception, rather more an article of faith than a well documented reality but the exception is a major motor system, the microtubules of eukaryotic cilia and flagella. To minimize confusion between bacterial flagella, which are totally different both in structure and mode of action, I will use cilia as the general term for the eukaryotic appendage. The ciliary motor system, as the best understood microtubule-associated movement, depends upon two classes of proteins, tubulins and dyneins, whose roles are analogous to those of actin and myosin respectively. Tubulin, when assembled to form a microtubule, provides a linear element with which a discontinuous interaction occurs, involving conformational change in the dynein-ATPase. We will discuss this motor system in detail in Section 3.5. The reason for supposing that there are other microtubule-associated motor systems derives from their location within cells and the sensitivity of certain cellular movements to microtubule blocking agents such as colchicine. The classic example is, of course, the microtubules of the mitotic spindle which lie parallel to the movements of chromatids at anaphase. If the assembly of these microtubules is blocked then chromatid separation occurs, but not poleward movement, and mitosis halts at metaphase — a convenient phenomenon for the karyologist.