Eberhard Bodenschatz, Carsten Beta, Albert Bae, and Gabriel Amselem
MPI for Dynamcis and Self-Organization, Goettingen, Germany

We report on chemotaxis and cell migration of the eukaryote Dic- tyostelium d.(Dicty) under well-controlled spatial and temporal stimuli in microfluidic devices. First the chemotactic response to stationary, linear gradients of cAMP will be reported. In shallow gradients of less than 10?-3 nM/?m, the cells showed no directional response and ex- hibited a constant basal motility. In steeper gradients, cells moved up the gradient on average. In very steep gradients, above 10 nM/?m, the cells lost directionality and the motility returned to the sub-threshold level. We found cells to be able to chemotact well even when the aver- age difference in receptor occupancy at the front and back of the cell is estimated to be only about 10 receptor molecules. Then we report experiments on the intracellular response of of PH-domain proteins to well controlled chematractant gradients. We use the photo-chemical release of caged cAMP in microfluidic devices to expose single chemo- tactic cells to spatio-temporally well controlled chemoattractant stim- uli (switching time approx. 0.5 sec and arbitrarily shaped gradients). We found that the translocation signal sets in with a finite response only for steep gradients. At shallow gradients no translocation signal could be measured. A theory describing polarization of the intracel- lular signaling system will be presented. This work is in collaboration with W. Loomis, H. Levine and W. Rappel at UCSD.