Characterizing the Structure and Photocycle of PR 2D Crystals with CD and FTIR Spectroscopy

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Crystallization and preliminary X-ray crystallographic analysis of a blue-light-absorbing proteorhodopsin

Proteorhodopsins (PRs), seven-transmembrane chromoproteins with retinal as a chromophore, are light-driven proton pumps. To elucidate the light-driven proton-pumping mechanism of PRs, a pET28a vector containing the blue-light-absorbing proteorhodopsin (BPR) gene was constructed and the protein was overexpressed in Escherichia coli. The protein was purified by immobilized metal-ion affinity chromatography (IMAC). The purified BPR D97N mutant protein (BPR_D97N) was crystallized using the vapour-diffusion method. Preliminary X-ray diffraction data analysis showed that the crystal belonged to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 161.6, b = 168.6, c = 64.7 Å. A complete data set was collected to 3.3 Å resolution using synchrotron radiation on beamline X06 of the Swiss Light Source (SLS). Molecular replacement was unsuccessful. To solve the structure of BPR_D97N by experimental phasing, selenomethionine-substituted protein crystals were prepared. These crystals diffracted to 3.0 Å resolution and a complete data set was collected on beamline BL17U of the Shanghai Synchrotron Radiation Facility (SSRF). Heavy-atom substructure determination and phasing by SAD clearly showed that the crystal contained five molecules in the asymmetric unit, with a V(M) of 3.26 Å(3) Da(-1) and a solvent content of 62.3%.

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.