Nuclear magnetic resonance investigations of Na+,K+-ATPase

Functionally significant mobile regions of Escherichia coli SecA ATPase identified by NMR

SecA, a 204-kDa homodimeric protein, is a major component of the cellular machinery that mediates the translocation of proteins across the Escherichia coli plasma membrane. SecA promotes translocation by nucleotide-modulated insertion and deinsertion into the cytoplasmic membrane once bound to both the signal sequence and portions of the mature domain of the preprotein. SecA is proposed to undergo major conformational changes during translocation. These conformational changes are accompanied by major rearrangements of SecA structural domains. To understand the interdomain rearrangements, we have examined SecA by NMR and identified regions that display narrow resonances indicating high mobility. The mobile regions of SecA have been assigned to a sequence from the second of two domains with nucleotide-binding folds (NBF-II; residues 564-579) and to the extreme C-terminal segment of SecA (residues 864-901), both of which are essential for preprotein translocation activity. Interactions with ligands suggest that the mobile regions are involved in functionally critical regulatory steps in SecA.

Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

This chapter describes contributions of transition metal-catalyzed oxidative cleavage of Na+,K+-ATPase to our understanding of structure-function relations. In the presence of ascorbate/H2O2, specific cleavages are catalyzed by the bound metal and because more than one peptide bond close to the metal can be cleaved, this technique reveals proximity of the different cleavage positions within the native structure. Specific cleavages are catalyzed by Fe2+ bound at the cytoplasmic surface or by complexes of ATP-Fe2+, which directs the Fe2+ to the normal ATP-Mg2+ site. Fe2+- and ATP-Fe2+-catalyzed cleavages reveal large conformation-dependent changes in interactions between cytoplasmic domains, involving conserved cytoplasmic sequences, and a change of ligation of Mg2+ ions between E1P and E2P, which may be crucial in facilitating hydrolysis of E2P. The pattern of domain interactions in E1 and E2 conformations, and role of Mg2+ ions, may be common to all P-type pumps. Specific cleavages can also be catalyzed by Cu2+ ions, bound at the extracellular surfaces, or a hydrophobic Cu2+-diphenyl phenanthroline (DPP) complex, which directs the Cu2+ to the membrane-water interface. Cu2+ or Cu2+-DPP-catalyzed cleavages are providing information on alpha/beta subunit interactions and spatial organization of transmembrane segments. Transition metal-catalyzed cleavage could be widely used to investigate other P-type pumps and membrane proteins and, especially, ATP binding proteins.

The complex ATP-Fe(2+) serves as a specific affinity cleavage reagent in ATP-Mg(2+) sites of Na,K-ATPase: altered ligation of Fe(2+) (Mg(2+)) ions accompanies the E(1)-->E(2) conformational change

In the presence of ascorbate/H(2)O(2), ATP-Fe(2+) or AMP-PNP-Fe(2+) complexes act as affinity cleavage reagents, mediating selective cleavage of the alpha subunit of Na,K-ATPase at high affinity ATP-Mg(2+) sites. The cleavages reveal contact points of Fe(2+) or Mg(2+) ions. In E(1) and E(1)Na conformations, two major cleavages are detected within the conserved (708)TGDGVNDSPALKK sequence (at V712 and nearby), and one (E(1)Na) or two (E(1)) minor cleavages near V440. In media containing sodium and ATP, Fe(2+) substitutes for Mg(2+) in activating phosphorylation and ATP hydrolysis. In the E(1)P conformation, cleavages are the same as in E(1). Fe(2+) is not bound tightly. By contrast, in the E(2)P conformation, the pattern is different. A major cleavage occurs near the conserved sequence (212)TGES, whereas those in TGDGVNDSPALKK are less prominent. Fe(2+) is bound very tightly. On E(2)P hydrolysis, the Fe(2+) dissociates. The results are consistent with E(1)<-->E(2) conformation-dependent movements of cytoplasmic domains and sites for P(i) and Mg(2+) ions, inferred from previous Fe-cleavage experiments. Furthermore, these concepts fit well with the crystal structure of Ca-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature (London) 405, 647-655]. Altered ligation of Mg(2+) ions in E(2)P may be crucial in facilitating nucleophilic attack of water on the OP bond. Mg(2+) ions may play a similar role in all P-type pumps. As affinity cleavage reagents, ATP-Fe(2+) or other nucleotide-Fe(2+) complexes could be widely used to investigate nucleotide binding proteins.

Binding of nucleotide triphosphates to cardiotoxin analogue II from the Taiwan cobra venom (Naja naja atra). Elucidation of the structural interactions in the dATP-cardiotoxin analogue ii complex

Snake venom cardiotoxins have been recently shown to block the enzymatic activity of phospholipid protein kinase and Na+,K+-ATPase. To understand the molecular basis for the inhibitory effects of cardiotoxin on the action of these enzymes, the nucleotide triphosphate binding ability of cardiotoxin analogue II (CTX II) from the Taiwan cobra (Naja naja atra) venom is investigated using a variety of spectroscopic techniques such as fluorescence, circular dichroism, and two-dimensional NMR. CTX II is found to bind to all the four nucleotide triphosphates (ATP, UTP, GTP, and CTP) with similar affinity. Detailed studies of the binding of dATP to CTX II indicated that the toxin molecule is significantly stabilized in the presence of the nucleotide. Molecular modeling, based on the NOEs observed for the dATP.CTX II complex, reveals that dATP binds to the CTX II molecule at the groove enclosed between the N- and C-terminal ends of the toxin molecule. Based on the results obtained in the present study, a molecular mechanism to account for the inhibition of the enzymatic activity of the phospholipid-sensitive protein kinase and Na+,K+-ATPase is also proposed.

Nuclear Overhauser effect studies of the conformation of Co(NH3)4ATP bound to kidney Na,K-ATPase

Transferred nuclear Overhauser effect measurements (in the two-dimensional mode) have been used to determine the three-dimensional conformation of an ATP analogue, Co(NH3)4ATP, at the active site of sheep kidney Na,K-ATPase. Previous studies have shown that Co(NH3)4ATP is a competitive inhibitor with respect to MnATP for the Na,K-ATPase [Klevickis, C., & Grisham, C.M. (1982) Biochemistry 21, 6979. Gantzer, M.L., et al. (1982) Biochemistry 21, 4083]. Nine unique proton-proton distances on ATPase-bound Co(NH3)4ATP were determined from the initial build-up rates of the cross-peaks of the 2D-TRNOE data sets. These distances, taken together with previous 31P and 1H relaxation measurements with paramagnetic probes, are consistent with a single nucleotide conformation at the active site. The bound Co(NH3)4ATP) adopts an anti conformation, with a glycosidic torsion angle of 35 degrees, and the conformation of the ribose ring is slightly N-type (C2'-exo, C3'-endo). The delta and gamma torsional angles in this conformation are 100 degrees and 178 degrees, respectively. The nucleotide adopts a bent configuration, in which the triphosphate chain lies nearly parallel to the adenine moiety. Mn2+ bound to a single, high-affinity site on the ATPase lies above and in the plane of the adenine ring. The distances from enzyme-bound Mn2+ to N6 and N7 are too large for first coordination sphere complexes, but are appropriate for second-sphere complexes involving, for example, intervening hydrogen-bonded water molecules. The NMR data also indicate that the structure of the bound ATP analogue is independent of the conformational state of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)