Isotopic labeling of proteins by utilizing photosynthetic bacteria

Isotope labeling for solution and solid-state NMR spectroscopy of membrane proteins

In this chapter, we summarize the isotopic labeling strategies used to obtain high-quality solution and solid-state NMR spectra of biological samples, with emphasis on integral membrane proteins (IMPs). While solution NMR is used to study IMPs under fast tumbling conditions, such as in the presence of detergent micelles or isotropic bicelles, solid-state NMR is used to study the structure and orientation of IMPs in lipid vesicles and bilayers. In spite of the tremendous progress in biomolecular NMR spectroscopy, the homogeneity and overall quality of the sample is still a substantial obstacle to overcome. Isotopic labeling is a major avenue to simplify overlapped spectra by either diluting the NMR active nuclei or allowing the resonances to be separated in multiple dimensions. In the following we will discuss isotopic labeling approaches that have been successfully used in the study of IMPs by solution and solid-state NMR spectroscopy.

Overexpression and characterization of the Rhodobacter sphaeroides PufX membrane protein in Escherichia coli

Heterologous expression of the PufX membrane protein from purple photosynthetic bacterium Rhodobacter sphaeroides was attempted by using Escherichia (E.) coli cells. The PufX was overexpressed as a recombinant protein with a histidine tag added to the carboxyl terminus, and can be extracted from the cell membrane by various detergents. Circular dichroism measurements showed that the expressed PufX protein had alpha-helix contents of 29% in organic solvents and 22-26% in 0.8-2.0% (w/v) n-octyl beta-D-glucopyranoside solutions, suggesting that the PufX contains a substantial alpha-helical region composed of 18-22 amino acids. The PufX expressed in E. coli was examined by reconstitution experiments with LH1 alpha- and beta-polypeptides and bacteriochlorophyll a. It was shown that the PufX inhibited not only the reconstitution of the LH1 complex, but also the formation of the B820 subunit type complex at high concentrations, indicating that the expressed PufX is biologically active. Large-scale expression of the functional PufX membrane protein provides sufficient quantity for further biophysical and structural analyses of its biological function, and adds another example for producing highly hydrophobic integral membrane proteins using the E. coli expression system.

Solution Structure of the Rhodobacter sphaeroides PufX Membrane Protein: Implications for the Quinone Exchange and Protein−Protein Interactions,

PufX membrane protein is found in Rhodobacter species of purple photosynthetic bacteria and has been known to play an essential role in ubiquinone/ubiquinol exchange between the reaction center and cytochrome bc1 complex and also contribute to the dimerization of the reaction center-light-harvesting core complex. We have determined the solution structure of the Rhodobacter sphaeroides PufX using multidimensional NMR spectroscopy. The PufX, functionally expressed in Escherichia coli, forms a stable alpha helix consisting of 21 residues over the central transmembrane domain. The overall structure of the PufX is very similar to those of the LH1 alpha- and beta-polypeptides from Rhodospirillum rubrum and LH2 polypeptides. A short segment (Lys28-Gly35) rich in Gly and Ala residues revealed a relatively fast exchange between the backbone amide protons and deuteriums in the hydroxyl groups of the solvent, indicating that the backbone of this segment is more easily accessible to the surrounding solvent molecules compared to those of its neighboring portions. The Gly- and Ala-rich segment is located in the middle of the central helix and forms an extensive groove-like conformation on the surface with the neighboring residues, where the residues with large side chains are aligned on one side of the helix, and small residues are aligned on the other face. Such a structural motif may serve as a functional site responsible for ubiquinol transport from the core complex to the membrane phase and for sequence-specific helix-helix interactions with the neighboring polypeptides.