Proteinase-treated photoreceptor discs. Photoelectric activity of the partially-digested rhodopsin and membrane orientation

Structure of bovine rhodopsin in a trigonal crystal form

We have determined the structure of bovine rhodopsin at 2.65 A resolution using untwinned native crystals in the space group P3(1), by molecular replacement from the 2.8 A model (1F88) solved in space group P4(1). The new structure reveals mechanistically important details unresolved previously, which are considered in the membrane context by docking the structure into a cryo-electron microscopy map of 2D crystals. Kinks in the transmembrane helices facilitate inter-helical polar interactions. Ordered water molecules extend the hydrogen bonding networks, linking Trp265 in the retinal binding pocket to the NPxxY motif near the cytoplasmic boundary, and the Glu113 counterion for the protonated Schiff base to the extracellular surface. Glu113 forms a complex with a water molecule hydrogen bonded between its main chain and side-chain oxygen atoms. This can be expected to stabilise the salt-bridge with the protonated Schiff base linking the 11-cis-retinal to Lys296. The cytoplasmic ends of helices H5 and H6 have been extended by one turn. The G-protein interaction sites mapped to the cytoplasmic ends of H5 and H6 and a spiral extension of H5 are elevated above the bilayer. There is a surface cavity next to the conserved Glu134-Arg135 ion pair. The cytoplasmic loops have the highest temperature factors in the structure, indicative of their flexibility when not interacting with G protein or regulatory proteins. An ordered detergent molecule is seen wrapped around the kink in H6, stabilising the structure around the potential hinge in H6. These findings provide further explanation for the stability of the dark state structure. They support a mechanism for the activation, initiated by photo-isomerisation of the chromophore to its all-trans form, that involves pivoting movements of kinked helices, which, while maintaining hydrophobic contacts in the membrane interior, can be coupled to amplified translation of the helix ends near the membrane surfaces.

Probing the "message:address" sites for chemoattractant binding to the C5a receptor. Mutagenesis of hydrophilic and proline residues within the transmembrane segments

The C5a anaphylatoxin ligand-receptor interaction on polymorphonuclear granulocytes stimulates chemotaxis, degranulation, and the oxidative burst. The receptor is a member of the large G-protein-coupled family. The ligand is a cationic peptide of 72 amino acids derived from the C5 component of complement and has been shown to have a number of structural requirements for interaction with the receptor. In order to probe the potential interaction sites between ligand and receptor, we constructed a series of mutated receptor molecules, targeting cysteines, prolines, and additional amino acids of interest because of combinations of charge or hydrophobicity and putative location with respect to the membrane. Transfected mutant receptors were analyzed for cell surface expression, ligand binding, and ligand-activated phospholipase C activity. The receptors created can be placed generally in four distinct classes: those which bind and signal like the natural receptor; those which bind but fail to transduce signals; those which are expressed but neither bind nor transduce signal; and those which are not expressed at the cell surface.

Retinal Schiff base position relative to the surfaces of photoreceptor disk

Surface-enhanced Raman spectra of native and photobleached bovine rod outer segment disks as well as inside-out (inverted) photoreceptor disks adsorbed on silver hydrosol have been analyzed. Surface-enhanced spectra of inverted disks and disk-monoclonal antibody complexes reveal the short-range mechanism of enhancement. The distance between retinal Schiff base and the cytoplasmic side of native disk has been shown to be 5-10 A.