Time-resolved responses to chemoattractant, characteristic of the front and tail of Dictyostelium cells.

In a gradient of chemoattractant, Dictyostelium cells are orientated with their front directed toward the source and their tail pointing into the opposite direction. The front region is specified by the polymerization of actin and the tail by the recruitment of filamentous myosin-II. We have dissected these front and tail responses by exposing cells to an upshift of cyclic AMP. A sharp rise and fall of polymerized actin within 10s is accompanied by the recruitment of proteins involved in turning actin polymerization on or off. The cortical accumulation of myosin-II starts when the front response has declined, supporting the concept of divergent signal transmission and adaptation pathways.