Fine-Structural and Related Aspects of Nonmuscle-Cell Motility

This review is concerned principally with the movements of cultured nonmuscle-tissue cells. Although these cells are known to contain tubulin and various other motility proteins, to date there is no single subcellular movement with a mechanism of action that is clearly explicable in terms of the better-understood microtubule-dynein or muscle-cell systems. The aim of this chapter is to examine relevant evidence, particularly that relating to the actin-myosin system. The review begins by considering the motility phenomena the mechanisms of which we are trying to elucidate. Bearing in mind that the better-understood cellmotility systems involve force-generating interactions between structures that are electron-microscopically observable, it has seemed important to review, in some detail, the cultured cells’ fine structure, concentrating on the microtubules and filamentous elements of the cytoplasmic ground substance. Then, since a better understanding of the microtubule-dynein and muscle-cell systems required identification of the proteins in their interacting components, it is planned to outline what is known at present of the protein identities of the various filamentous elements of cultured cells. Even when the aforementioned information is put together, however, it appears that no simple analogies with the microtubule-dynein or the actin-myosin system emerge. Indeed, with respect to the necessity for a codistribution of actin and myosin, there may be some serious deficiencies. And yet, as shown in certain model studies (including those derived from Physarum and Characeae), even though myosin cannot be identified ultrastructurally, there is good evidence that certain movements are generated by actin-myosin interactions. In a number of instances, this evidence has been greatly strengthened through the use of specific inhibitors, and it is suggested that extension of this work with cultured cells is an important future task.