STRUCTURAL-CHANGES ASSOCIATED WITH THE COUPLING OF ATP HYDROLYSIS ANDCATION-TRANSPORT BY THE NA PUMP

Most of the residues associated with cation coordination seem to resid e within transmembrane segments of the cc-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 seg ments M4 and M5 (see Lingrel & Kuntzweiler, 1994; Lutsenko & Kaplan, 1 995). The coupling of the two functions of cation transport and Am hyd rolysis involved in the active transport of Na and K ions must involve interactions between these two structural units. This paper summarize s 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 reage nts and a variety of functional assays. The data provide evidence for movements between specific transmembrane segments associated with cati on-binding conformations and coupled changes which take place in the A TP binding domain. The binding of different cations in the cation-bind ing domain is sensed in the ATP binding domain and manifested as a cha nge in reactivity. This occurs at amino acid residues which are widely spaced in primary structure. It is apparent that structural changes a re transmitted through much of the ATP-binding domain as a consequence of the occupancy of the cation-binding domain. We also provide eviden ce that both the number and identity of cations bound are also sensed in the ATP-binding domain.