Yasuto Todokoro

Purification, characterization and reconstitution into membranes of the oligomeric c-subunit ring of thermophilic FoF1-ATP synthase expressed in Escherichia coli

F(o)F(1)-ATP synthase catalyzes ATP synthesis coupled with proton-translocation across the membrane. The membrane-embedded F(o) portion is responsible for the H(+) translocation coupled with rotation of the oligomeric c-subunit ring, which induces rotation of the γ subunit of F(1). For solid-state NMR measurements, F(o)F(1) of thermophilic Bacillus PS3 (TF(o)F(1)) was overexpressed in Escherichia coli and the intact c-subunit ring (TF(o)c-ring) was isolated by new procedures. One of the key improvement in this purification was the introduction of a His residue to each c-subunit that acts as a virtual His(10)-tag of the c-ring. After solubilization from membranes by sodium deoxycholate, the c-ring was purified by Ni-NTA affinity chromatography, followed by anion-exchange chromatography. The intactness of the isolated c-ring was confirmed by high-resolution clear native PAGE, sedimentation analysis, and H(+)-translocation activity. The isotope-labeled intact TF(o)c-ring was successfully purified in such an amount as enough for solid-state NMR measurements. The isolated TF(o)c-rings were reconstituted into lipid membranes. A solid-state NMR spectrum at a high quality was obtained with this membrane sample, revealing that this purification procedure was suitable for the investigation by solid-state NMR. The purification method developed here can also be used for other physicochemical investigations.

Structure analysis of membrane-reconstituted subunit c-ring of E. coli H+-ATP synthase by solid-state NMR

The subunit c-ring of H+-ATP synthase (Foc-ring) plays an essential role in the proton translocation across a membrane driven by the electrochemical potential. To understand its structure and function, we have carried out solid-state NMR analysis under magic-angle sample spinning. The uniformly [13C, 15N]-labeled Foc from E. coli (EFoc) was reconstituted into lipid membranes as oligomers. Its high resolution two- and three-dimensional spectra were obtained, and the 13C and 15N signals were assigned. The obtained chemical shifts suggested that EFoc takes on a hairpin-type helix-loop-helix structure in membranes as in an organic solution. The results on the magnetization transfer between the EFoc and deuterated lipids indicated that Ile55, Ala62, Gly69 and F76 were lined up on the outer surface of the oligomer. This is in good agreement with the cross-linking results previously reported by Fillingame and his colleagues. This agreement reveals that the reconstituted EFoc oligomer takes on a ring structure similar to the intact one in vivo. On the other hand, analysis of the 13C nuclei distance of [3-13C]Ala24 and [4-13C]Asp61 in the Foc-ring did not agree with the model structures proposed for the EFoc-decamer and dodecamer. Interestingly, the carboxyl group of the essential Asp61 in the membrane-embedded EFoc-ring turned out to be protonated as COOH even at neutral pH. The hydrophobic surface of the EFoc-ring carries relatively short side chains in its central region, which may allow soft and smooth interactions with the hydrocarbon chains of lipids in the liquid-crystalline state.

Theonellamide A, a marine-sponge-derived bicyclic peptide, binds to cholesterol in aqueous DMSO: Solution NMR-based analysis of peptide-sterol interactions using hydroxylated sterol

Theonellamides (TNMs) are antifungal and cytotoxic bicyclic dodecapeptides isolated from the marine sponge Theonella sp. The inclusion of cholesterol (Chol) or ergosterol in the phosphatidylcholine membrane is known to significantly enhance the membrane affinity for theonellamide A (TNM-A). We have previously revealed that TNM-A stays in a monomeric form in dimethylsulfoxide (DMSO) solvent systems, whereas the peptide forms oligomers in aqueous media. In this study, we utilized 1H NMR chemical shift changes (Δδ1H) in aqueous DMSO solution to evaluate the TNM-A/sterol interaction. Because Chol does not dissolve well in this solvent, we used 25-hydroxycholesterol (25-HC) instead, which turned out to interact with membrane-bound TNM-A in a very similar way to that of Chol. We determined the dissociation constant, KD, by NMR titration experiments and measured the chemical shift changes of TNM-A induced by 25-HC binding in the DMSO solution. Significant changes were observed for several amino acid residues in a certain area of the molecule. The results from the solution NMR experiments, together with previous findings, suggest that the TNM-Chol complex, where the hydrophobic cavity of TNM probably incorporates Chol, becomes less polar by Chol interaction, resulting in a greater accumulation of the peptide in membrane. The deeper penetration of TNM-A into the membrane interior enhances membrane disruption. We also demonstrated that hydroxylated sterols, such as 25-HC that has higher solubility in most NMR solvents than Chol, act as a versatile substitute for sterol and could be used in 1H NMR-based studies of sterol-binding peptides.