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Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness. Names Rhodopsin was discovered by Franz Christian Boll in 1876. The name rhodospsin derives from Ancient Greek ῥόδον () for "rose", due to its pinkish color, and ὄψις () for "sight". It was coined in 1878 by the German physiologist Wilhelm Friedrich Kühne (1837-1900). When George Wald discovered that rhodopsin is a holoprotein, consisting of retinal and an apoprotein,

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Incorporation of Rhodopsin in Laterally Structured Supported Membranes: Observation of Transducin Activation with Spatially and Time-Resolved Surface Plasmon Resonance

Klaus Peter Hofmann, Horst Vogel

Rhodopsin-transducin coupling was used as an assay to investigate a laterally patterned membrane reconstituted with a receptor and its G protein. It served as a model system to show the feasibility to immobilize G protein-coupled receptors on solid supports and investigate receptor activation and interaction with G proteins by one-dimensional imaging surface plasmon resonance. Supported membranes were formed by the self-assembly of lipids and rhodopsin from detergent soln. onto functionalized gold surfaces. They formed micrometer-sized alternating regions of pure fluid phospholipid bilayers sepd. by bilayers composed of an outer phospholipid leaflet on a gold-attached inner thiolipid. Rhodopsin was found to incorporate preferentially into the phospholipid bilayer regions, whereas transducin was uniformly distributed over the entire outer surface of the supported patterned membrane. The influence of rhodopsin on the dark binding of transducin to lipid membranes was described quant. and compared with previously published data. Coupling reactions with transducin resembled closely the native system, indicating that the native functionality of rhodopsin was preserved in the supported membranes. The spatially varying properties of the membranes resulted in a pattern of rhodopsin activity on the surface. This combination of techniques is very promising for the investigation of the lateral diffusion of transducin, can be extended to include signaling proteins downstream of the G protein, and may be applied to functional screening of other G protein-coupled receptors. In the future, it may also serve as a basis for constructing biosensors based on receptor proteins. [on SciFinder (R)]

1998

Micropatterned immobilization of a G protein-coupled receptor and direct detection of G protein activation

Klaus Peter Hofmann, Horst Vogel

G protein-coupled receptors (GPCRs) constitute an abundant family of membrane receptors of high pharmacol. interest. Cell-based assays are the predominant means of assessing GPCR activation, but are limited by their inherent complexity. Functional mol. assays that directly and specifically report G protein activation by receptors could offer substantial advantages. We present an approach to immobilize receptors stably and with defined orientation to substrates. By surface plasmon resonance (SPR), we were able to follow ligand binding, G protein activation, and receptor deactivation of a representative GPCR, bovine rhodopsin. Microcontact printing was used to produce micrometer-sized patterns with high contrast in receptor activity. These patterns can be used for local referencing to enhance the sensitivity of chip-based assays. The immobilized receptor was stable both for hours and during several activation cycles. A ligand dose-response curve with the photoactivatable agonist 11-cis-retinal showed a half-maximal signal at 120 nM. Our findings may be useful to develop novel assay formats for GPCRs based on receptor immobilization to solid supports, particularly to sensor surfaces. [on SciFinder (R)]

1999

Intrinsic biophysical monitors of transducin activation: fluorescence, UV-visible spectroscopy, light scattering, and evanescent field techniques

Klaus Peter Hofmann, Horst Vogel

Information on the temporal order, the kinetics, and the energetics of each of the reaction intermediates is required in order to fully elucidate the mechanism of catalytic interaction of G proteins with their receptors. Intrinsic monitors, such as light scattering, absorption, and fluorescence, exploit endogenous properties of proteins, and therefore offer the advantage of leaving the system under investigation undisturbed. Such assays currently available for G proteins include the well-known intrinsic fluorescence changes of a tryptophan residue located near the G protein's active center. In addn., the visual system offers UV-visible spectrophotometric assays in which the retinal chromophore itself serves as an intrinsic reporter group. These assays include monitoring the transducin sensitive formation of the active metarhodopsin-II (MII) conformation, as well as the photoregeneration of dark state rhodopsin from MII. A third technique is kinetic light scattering, which makes use of the unique activation-dependent soly. of the rod G protein. While these monitors are highly specialized, more generally applicable monitors have been developed in recent years and successfully tested with the visual system. This family of techniques is based on the evanescent field in the vicinity of a reflecting optical surface, and includes surface plasmon resonance and resonant mirror spectroscopy. Applications and limitations of these techniques are described. (c) 2000 Academic Press. [on SciFinder (R)]

2000

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