If Dictyostelium amoebae are not the Ferraris of moving cells, they are at least a respectable entrant in the cell motility Grand Prix. Amoebae can move as fast as 10–15 μm/min (Varnum and Soll, 1984). They do not do this in a random walk, but move directionally, up gradients of cAMP or folate. D. discoideum responds to gradients that vary by as little as 2% from the front to the back of a cell, as we will describe in Chapter 8. This astonishing chemical sensitivity raises a number of questions. How are signal transduction pathways connected to the cytoskeleton such that stimulation of chemotactic receptors leads to movement up a chemical gradient? How are the components of the cytoskeleton organized to promote sudden movement of cells? How does the mobilization of the cytoskeleton differ in the various protrusions, such as pseudopodia and fllopodia? How are responses terminated? In this chapter we will review how the actin-based cytoskeleton is mobilized during cell movement, while in Chapter 8 we will consider how it reorganizes to drive the cells up a gradient of cAMP, toward centers of aggregation.

Dictyostelium is one of the few organisms with impressive motility and tractable genetics. The cells move toward folate during growth and toward cAMP during development. The amoebae are useful for optical observation, so that with a few tricks, the movements of macromolecules within the cells can be observed by a variety of microscopic methods.