Regulation of intracellular pH homeostasis in response to cAMP, which is essential for chemotaxis. Necessary for F-actin localization and the kinetics of actin polymerization during chemotaxis and cell polarity but not for directional sensing. LY294002, an inhibitor of the catalytic subunit of PI3-kinase, blocks NHE1-dependent (but not NHE1-independent) increase in intracellular pH in response to cAMP. Predominantly localizes to the leading-edge of the chemotaxing polarized cells. Expressed in chemotactically competent cells but in not vegetative cells. Expression is induced 6 hours after starvation. Down-regulated by Pseudomonas aeruginosa infection (strains PAO1 and PA14). Impaired polarity. Starved mutant cells differentiate, and in response to the chemoattractant cAMP retain directional sensing; however, they cannot attain a polarized morphology, but instead extend mislocalized pseudopodia around the cell and exhibit decreased velocity. In response to chemoattractant, mutant cells lack a leading edge localization of F-actin and have significantly attenuated de novo F-actin polymerization but increased abundance of membrane-associated phosphatidylinositol 3,4,5-trisphosphate. However, they retain directional sensing. Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family.