Structural changes associated with the coupling of ATP hydrolysis and cation transport by the Na pump

Most of the residues associated with cation coordination seem to reside within transmembrane segments of the alpha-subunit of the Na,K-ATPase, whereas amino acids which appear to be involved in the coordination of ATP are found in the major cytoplasmic loop between transmembrane segments M4 and M5 (see Lingrel & Kuntzweiler, 1994; Lutsenko & Kaplan, 1995). The coupling of the two functions of cation transport and ATP hydrolysis involved in the active transport of Na and K ions must involve interactions between these two structural units. This paper summarizes recent experimental results and conclusions of studies on the renal Na,K-ATPase which have employed controlled proteolysis in the presence of physiological ligands, chemical modification with a range of reagents and a variety of functional assays. The data provide evidence for movements between specific transmembrane segments associated with cation-binding conformations and coupled changes which take place in the ATP binding domain. The binding of different cations in the cation-binding domain is sensed in the ATP binding domain and manifested as a change in reactivity. This occurs at amino acid residues which are widely spaced in primary structure. It is apparent that structural changes are transmitted through much of the ATP-binding domain as a consequence of the occupancy of the cation-binding domain. We also provide evidence that both the number and identity of cations bound are also sensed in the ATP-binding domain.