In Dictyostelium discoideum, extracellular K ؉ or Ca 2؉ at a concentration of 40 or 20 mM, respectively, facilitates motility in the absence or presence of a spatial gradient of chemoattractant. Facilitation results in maximum velocity, cellular elongation, persistent translocation, suppression of lateral pseudopod formation, and myosin II localization in the posterior cortex. A lower threshold concentration of 15 mM K ؉ or Na or 5 mM Ca 2؉ is required for chemotactic orientation. Although the common buffer solutions used by D. discoideum researchers to study chemotaxis contain sufficient concentrations of cations for chemotactic orientation, the majority contain insufficient levels to facilitate motility. Here it has been demonstrated that Nhe1, a plasma membrane protein, is required for K ؉ but not Ca 2؉ facilitation of cell motility and for the lower K ؉ but not Ca 2؉ requirement for chemotactic orientation.Extracellular Ca 2ϩ and K ϩ , ubiquitous in the soluble environment of cells, both free-living and in multicellular organisms (5,15,21,36,56,79,80,84,91), have long been known to affect the motility of cells ranging in complexity from bacterial to human (7,8,19,25,31,60,62,65,98). In Dictyostelium discoideum, which serves as a powerful model for studying animal cell motility (4,6,20,34,41,42,43,45,50,82,87,86,92,98), either extracellular K ϩ or Ca 2ϩ at an optimum concentration of 40 mM or 20 mM, respectively, enhances most aspects of basic cell motility in the absence of chemoattractant, including cell elongation, uropod formation, the suppression of lateral pseudopod formation, velocity, directional persistence, and localization of myosin II in the posterior cell cortex (51, 78). These facilitating concentrations of K ϩ or Ca 2ϩ cannot be replaced with other monovalent or divalent cations (51) and are within the range of the soluble concentrations of the two cations found in soil (1,5,32,44,49,56) and manure (9, 66, 83), two common niches in which soil amoebae thrive (12,27,35,67,71). These concentrations of K ϩ and Ca 2ϩ are close to those found to be optimum for enhancing the motility of a variety of other animal cells. For instance, 9 mM Ca 2ϩ has been found to be optimum for flagellated sea urchin sperm motility (99), and Ca 2ϩ ranging in concentration from 1 to 10 mM has been found optimum for vertebrate cell motility (28,58,63). Concentrations of K ϩ greater than 100 mM have been found to be optimum for the polarization and motility of human polymorphonuclear leukocytes (52, 69).The surface molecules and mechanisms regulating K ϩ and Ca 2ϩ facilitation in D. discoideum have not been elucidated. However, Patel and Barber (64) reported a defective behavioral phenotype for cells of the nhe1 null mutant in a facilitating concentration of K ϩ that was strikingly similar to the behavior of wild-type cells in a nonfacilitating concentration of K ϩ (51). Nhe1 is a plasma membrane protein related to cation/H ϩ exchangers (64). We therefore tested the possibility that Nhe1 mediated K ϩ facilitation. First, t...