Molecular scale conductance photoswitching in engineered bacteriorhodopsin.

Bacteriorhodopsin (BR) is a robust light-driven proton pump embedded in the purple membrane of the extremophilic archae Halobacterium salinarium . Its photoactivity remains in the dry state, making BR of significant interest for nanotechnological use. Here, in a novel configuration, BR was depleted from most of its endogenous lipids and covalently and asymmetrically anchored onto a gold electrode through a strategically located and highly responsive cysteine mutation; BR has no indigenous cysteines. Chemisorption on gold was characterized by surface plasmon resonance, reductive striping voltammetry, ellipsometry, and atomic force microscopy (AFM). For the first time, the conductance of isolated protein trimers, intimately probed by conducting AFM, was reproducibly and reversibly switched under wavelength-specific conditions (mean resistance of 39 ± 12 MΩ under illumination, 137 ± 18 MΩ in the dark), demonstrating a surface stability that is relevant to potential nanodevice applications.

Molecular dynamics of proteorhodopsin in lipid bilayers by solid-state NMR.

Environmental factors such as temperature, hydration, and lipid bilayer properties are tightly coupled to the dynamics of membrane proteins. So far, site-resolved data visualizing the protein's response to alterations in these factors are rare, and conclusions had to be drawn from dynamic data averaged over the whole protein structure. In the current study, high-resolution solid-state NMR at high magnetic field was used to investigate their effects on the molecular dynamics of green proteorhodopsin, a bacterial light-driven proton pump. Through-space and through-bond correlation experiments were employed to identify and characterize highly mobile and motionally restricted regions of proteorhodopsin. Our data show that hydration water plays an essential role for enhancing molecular dynamics of residues in tails and interhelical loops, while it is found less important for residues in transmembrane domains or rigid, structured loop segments. In contrast, switching the lipids from the gel to their liquid crystalline phase enhances molecular fluctuations mainly in transmembrane helices on a time scale of 10(-6) s, but has little effect on loop and tail residues. Increased mobility is especially observed in helices C, F, and G, but also in the EF loop. Fluctuations in those regions are relevant to structural dynamics during the photocycle of proteorhodopsin. Our data are important for the functional understanding of proteorhodopsin, but also offer an important contribution to the general understanding of site-resolved effects of water and lipid bilayers onto the dynamic properties of membrane proteins.

Studying the stoichiometries of membrane proteins by mass spectrometry: microbial rhodopsins and a potassium ion channel.

In the present work we demonstrate the advantages of LILBID mass spectrometry in the mass analysis of membrane proteins with emphasis on ion-pumps and channels. Due to their hydrophobic nature, membrane proteins have to be solubilized by detergents. However, these molecules tend to complicate the analysis by mass spectrometry. In LILBID, detergent molecules are readily tolerated which allows for the study of solution phase quaternary structures of membrane proteins. This is shown for the proton-pump bacteriorhodospin and the potassium channel KcsA where in both cases the stoichiometries found by LILBID reflect the known structures from 2D or 3D crystals. With proteorhodopsin we demonstrate a preliminary detergent screening showing different structures in different detergents and the implications for the functionality of this protein. We show that Triton-X 100 prevents the formation of the pentamer of proteorhodopsin. Furthermore, the quaternary structures of proteorhodopsin cloned without the signal peptide and of the cation channel channelrhodopsin-2 were studied. The intrinsic properties of channelrhodopsin-2 allow for mass spectrometric analysis in very high salt concentrations up to 100 mM of NaCl. In summary we demonstrate that LILBID is an alternative mass spectrometric method for the analysis of membrane proteins from solution phase.

Solid-state NMR and functional studies on proteorhodopsin.

Proteins of the proteorhodopsin (PR) family are found abundantly in many marine bacteria in the photic zone of the oceans. They are colour-tuned to their environment. The green absorbing species has been shown to act as a light-driven proton pump and thus could form a potential source of energy. The pK(a) of the primary proton acceptor is close to the pH of seawater which could also indicate a regulatory role. Here, we review and summarize our own recent findings in the context of known data and present some new results. Proton transfer in vitro by PR is shown by a fluorescence assay which confirms a pH dependent vectoriality. Previously reported low diffracting 2D crystal preparations of PR are assessed for their use for solid-state NMR by two dimensional (13)C-(13)C DARR spectra. (15)N-(1)H HETCOR MAS NMR experiments show bound water in the vicinity of the protonated Schiff base which could play a role in proton transfer. The effect of highly conserved H75 onto the properties of the chromophore has been investigated by single site mutations. They do show a pronounced effect onto the optical absorption maximum and the pK(a) of the proton acceptor but have only a small effect onto the (15)N chemical shifts of the protonated Schiff base.

Advances towards resonance assignments for uniformly--13C, 15N enriched bacteriorhodopsin at 18.8 T in purple membranes.

Solid state NMR spectra from uniformly (13)C, (15)N enriched bacteriorhodospin (bR) purified from H. salinarium were acquired at 18.8 T using magic angle spinning methods. Isolated resonances of 2D (13)C-(13)C spectra exhibited 0.50-0.55 ppm line-widths. Several amino acid types could be assigned, and at least 12 out of 15 Ile peaks could be resolved clearly and identified based on their characteristic chemical shifts and connectivities. This study confirms that high resolution solid state NMR spectra can be obtained for a 248 amino acid uniformly labeled membrane protein in its native membrane environment and indicates that site-specific assignments are likely to be feasible with heteronuclear multidimensional spectra.