How well can an amoeba climb?

We report here our efforts to measure the crawling force generated by cells undergoing amoeboid locomotion. In a centrifuge microscope, acceleration w as increased until amoebae of Dictyostelium discoideum were "stalled" or no longer able to "climb up." The "apparent weight" of the amoebae at stallin g rpm in myosin mutants depended on the presence of myosin II (but not myos ins IA and IB) and paralleled the cortical strength of the cells. Surprisin gly, however, the cell stalled not only in low-density media as expected bu t also in media with densities greater than the cell density where the buoy ant force should push the amoeba upward. We find that the leading pseudopod is bent under centrifugal force in all stalled amoebae, suggesting that th is pseudopod is very dense indeed, This finding also suggests that directi onal cell locomotion against resistive forces requires a turgid forward-poi nting pseudopod, most likely sustained by cortical actomyosin II.