We have shown that proteins within apically enriched fractions of human nasal respiratory epithelium vary their phosphohistidine content with ambient [Cl-] and other anion concentrations. This membrane-delimited phosphorylation cascade includes a multifunctional protein histidine kinase - nucleoside diphosphate kinase (NDPK). NDPK is itself a cascade component in both human and ovine airway, the self-phosphorylation of which is inhibited selectively by [Na+] in the presence of ATP (but not GTP). These findings led us to propose the existence of a dual anion-/cation-controlled phosphorylation-based "sensor" bound to the apical membrane. The present study showed that this cascade uses ATP to phosphorylate a group of proteins above 45 kDa (p45-group, identities unknown). Additionally, the Cl- dependence of ATP (but not GTP) phosphorylation is conditional on phosphatase activity and that interactions exist between the ATP- and GTP-phosphorylated components of the cascade under Cl--free conditions. As a prelude to studies in cystic fibrosis (CF) mice, we showed in the present study that NDPK is present and functionally active in normal murine airway. Since NDPK is essential for UTP synthesis and regulates fetal gut development, G proteins, K+channels, neutrophil-mediated inflammation and pancreatic secretion, the presence of ion-regulated NDPK protein in mouse airway epithelium might aid understanding of the pathogenesis of CF.