Multiple isoelectric forms of detergent-solubilized bovine rhodopsin. I. Identity, composition and properties

A Usual G-Protein-Coupled Receptor in Unusual Membranes

G-protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and constitute about 50% of all known drug targets. They offer great potential for membrane protein nanotechnologies. We report here a charge-interaction-directed reconstitution mechanism that induces spontaneous insertion of bovine rhodopsin, the eukaryotic GPCR, into both lipid- and polymer-based artificial membranes. We reveal a new allosteric mode of rhodopsin activation incurred by the non-biological membranes: the cationic membrane drives a transition from the inactive MI to the activated MII state in the absence of high [H(+)] or negative spontaneous curvature. We attribute this activation to the attractive charge interaction between the membrane surface and the deprotonated Glu134 residue of the rhodopsin-conserved ERY sequence motif that helps break the cytoplasmic "ionic lock". This study unveils a novel design concept of non-biological membranes to reconstitute and harness GPCR functions in synthetic systems.

Proteomic analysis of the retinal rod outer segment disks

The initial events of vision at low light take place in vertebrate retinal rods. The rod outer segment consists of a stack of flattened disks surrounded by the plasma membrane. A list of the proteins that reside in disks has not been achieved yet. We present the first comprehensive proteomic analysis of purified rod disks, obtained by combining the results of two-dimensional gel electrophoresis separation of disk proteins to MALDI-TOF or nLC-ESI-MS/MS mass spectrometry techniques. Intact disks were isolated from bovine retinal rod outer segments by a method that minimizes contamination from inner segment. Out of a total of 187 excised spots, 148 proteins were unambiguously identified. An additional set of 61 proteins (partially overlapping with the previous ones) was generated by one-dimensional (1D) gel nLC-ESI-MS/MS method. Proteins involved in vision as well as in aerobic metabolism were found, among which are the five complexes of oxidative phosphorylation. Results from biochemical, Western blot, and confocal laser scanning microscopy immunochemistry experiments suggest that F 1F o-ATP synthase is located and catalytically active in ROS disk membranes. This study represents a step toward a global physiological characterization of the disk proteome and provides information necessary for future studies on energy supply for phototransduction.

Reductively methylated, tritiated rhodopsin of high specific activity; a convenient sensitive tracer for use in the radioimmunoassay of rhodopsin

Bovine rhodopsin was subjected to reductive methylation in the dark using formaldehyde and high specific activity sodium borotritide. After purification by gel filtration and affinity chromatography on Concanavalin A-Sepharose, the product retained its immunoreactive properties. [3H]-Reductively methylated rhodopsin (specific activity, 32 Ci/mmole) was suitable for use in radioimmunoassays for rhodopsin, having many advantages over radioiodinated rhodopsin for this purpose. The site of the reductive methylation was shown to be the non-active site lysines with the production of tritiated N-epsilon-dimethyllysine and tritiated N-epsilon-methyllysine in a molar ratio of about 1.3:1, respectively. In terms of stability, ease of preparation, and specificity, tritiated, reductively methylated rhodopsin presents itself as a preferable ligand to radioiodinated rhodopsin in many applications, such as the radioimmunoassay.

Varieties of rhodopsin in frog rod outer segment membranes: analysis by isoelectric focusing

Purified ROS membranes from cattle, R. pipiens and R. catesbeiana adults and tadpoles were investigated by analytical and preparative isoelectric focusing and SDS-polyacrylamide gel electrophoresis. The rhodopsin from single specimens of R. pipiens displayed two closely-spaced bands with Mr at 34.7 and 37.0 k on SDS polyacrylamide gels. Both were found to be phosphorylated when prepared from retinas incubated with 32Pi and then exposed to light. When purified ROS membranes were solubilized in octyl glucoside and examined by isoelectric focusing followed by SDS-polyacrylamide gel electrophoresis, the low-Mr component focused in two bands (I, IIa) at pH 8.8 and 8.1. Band I and a trace amount of band IIa were observed if band I was eluted and refocused. The high-Mr component focused in one band (IIb) at pH 8.0. Identical isoelectric focusing patterns were observed with ROS membranes from R. catesbeiana tadpoles and the dorsal and ventral retina areas from R. catesbeiana adults. Bovine rhodopsin, on the other hand, had a single Mr component and focused in a major band at pH 6.2.

Multiple isoelectric forms of detergent-solubilized bovine rhodopsin. II. Spectral analysis

The absorption and circular dichroic spectra of three stable isoelectric forms of purified rhodopsin (in Emulphogene BC 720) with isoelectric points of 5.19, 5.58 and 6.14 were analysed over the accessible wavelength region of 190-800 nm. It was found from the spectral analysis of the 5.58 and 6.14 forms that the tertiary structure of these isoelectric forms was different, without any change being observed in the secondary structure of these proteins. The difference in structure was removed by denaturation with SDS. Specifically, the aromatic amino acid residues of the 6.14 isoelectric form were suggested to be in a more polar environment as compared to those for the 5.58 isoelectric form. The changes of structure in the microenvironment of retinal in these proteins were minor in comparison to the tertiary changes observed in the proteins. This indicated that the site of charge perturbation is not localized in the retinylidene microenvironment. Furthermore, the spectral features of these isoelectric forms were features common to the spectra of both purified unfocused rhodopsin and crude rhodopsin, suggesting that the isoelectric forms are not a consequence of purification. Spectral analysis of the 5.19 isoelectric form indicated that this form of rhodopsin had an increased tendency to aggregate after focusing. Based on the denaturation properties of these isoelectric forms, it was shown that the 5.19 and 5.58 isoelectric forms had different conformations. It was concluded from this study that preparations of 'purified' rhodopsin are a heterogeneous mixture of stable proteins with different tertiary structures and different isoelectric points.