Purification of bovine rhodopsin by high-performance size-exclusion chromatography

The Bovine Ex Vivo Retina: A Versatile Model for Retinal Neuroscience

Purpose

The isolated ex vivo retina is the standard model in retinal physiology and neuroscience. During isolation, the retina is peeled from the retinal pigment epithelium (RPE), which plays a key role in the visual cycle. Here we introduce the choroid-attached bovine retina as an in vivo-like model for retinal physiology. We find that-in the bovine eye-the choroid and retina can be peeled from the sclera as a single thin sheet. Importantly, the retina remains tightly associated with the RPE, which is sandwiched between the retina and the choroid. Furthermore, bovine tissue is readily available and cheap, and there are no ethical concerns related to the use of animals solely for research purposes.

Methods

We combine multi-electrode array and single-cell patch-clamp recordings to characterize light responses in the choroid-attached bovine ex vivo retina.

Results

We demonstrate robust and consistent light responses in choroid-attached preparations. Importantly, light responses adapt to different levels of background illumination and rapidly recover from photobleaching. The choroid-attached retina is also thin enough to permit targeted electrophysiological recording from individual retinal neurons using standard differential interference contrast microscopy. We also characterize light responses and membrane properties of bovine retinal ganglion cells and compare data obtained from bovine and murine retinas.

Conclusions

The choroid-attached retinal model retains the advantages of the isolated retina but with an intact visual cycle and represents a useful tool to elucidate retinal physiology.

Solubilization of myelin membranes by detergents

The major proteins of myelin have classically been extracted in organic solvents. Here we investigated some of the characteristics of brain myelin solubilization in aqueous detergent solutions. At comparable molar concentrations, two nonionic detergents, i.e., octyl glucoside and Lubrol PX, proved relatively better myelin solubilizers than the detergents related to the bile salts, i.e., cholate and CHAPS. The two former detergents solubilized more protein than lipid and the two latter ones more lipid than protein from myelin membranes. All four detergents solubilized the phospholipid more efficiently than the cholesterol component of myelin. The detergent concentrations required for myelin solubilization were reduced substantially if the temperature and the salt concentration of the media were increased. As much as 3 mg of lyophilized myelin (about 1 mg of protein) were solubilized readily per milliliter of a solution containing 30 mM octyl glucoside and 0.1 M sodium sulfate in 0.1 M sodium phosphate buffer, pH 6.7. Each of the detergents studied, including the above four, sodium dodecyl sulfate (SDS). Triton X-100, and Zwittergent 3-14, had its own advantages and drawbacks as myelin protein extractors. The nonionic amphiphiles and CHAPS left a small residue mainly composed of proteins of the Wolfgram fraction, as revealed by SDS-polyacrylamide gel electrophoresis. Octyl glucoside was preferred, given its versatility as solubilizer, ultraviolet transparency, and high critical micellar concentration. Observations on possible difficulties that may be encountered are also included.

Analysis and separation of synaptosomal membrane proteins

Synaptosomal membrane proteins solubilized with 8% CHAPS-8 M urea were analyzed with two-dimensional electrophoresis (2DE). The membrane proteins were resolved up to 250 spots on a 2DE map, ranging in isoelectric points (pI) from 3.5 to 10.0 and molecular weights (MW) from 10 kDa to 200 kDa. Comparison of the mapped proteins of synaptosomal membranes with those of myelin and mitochondrial membranes revealed that synaptosomal membrane proteins were characteristic in the area of pI from 4.0 to 7.5 and MW from 20 kDa to 130 kDa, and that at least 30 spots were synaptosomal membrane-specific proteins. Most of these 30 proteins have not been previously described, named, and characterized. Serial numbers (from SY1 to SY30) were assigned to the proteins on the map in order to investigate them systematically. A preliminary attempt to separate synaptosomal membrane proteins was carried out using a reversed-phase HPLC system. Several proteins could either be isolated or enriched. SY10 (pI 4.6; MW 56 kDa) was one of these proteins, and was of particular interest for its unusual behavior on the reversed-phase column, and for its binding to an immobilized protein A-gel.

Spectral evidence for cytochrome o in the brown membrane of Halobacterium halobium

The brown membrane domain of Halobacterium halobium is a developmental precursor of the purple membrane and it contains a b-type cytochrome in addition to bacterio-opsin. In this report we provide spectroscopic evidence that the majority of the cytochrome content is a halobacterial cytochrome o. This cytochrome has absorption spectral properties in the oxidized, reduced, and CO liganded states which are characteristic of cytochrome o. The CD spectra show a complex bilobed pattern in the Soret spectral region which reflects the similarity of the heme environment to those of other b-type cytochromes involved in electron transport. We have also demonstrated a positive cooperativity of CO binding which, combined with CD spectral results, suggests two interacting heme moieties per cytochrome o. Size exclusion HPLC of solubilized brown membrane preparations shows a heme b containing protein with an Mr 42,000-46,000. The cytochrome may be easily separated from bacterio-opsin using a hydroxyapatite elution method on Triton X-100 solubilized preparations. The relative ease with which brown membrane disks may be oriented in high optical quality films should make the brown membrane a valuable model system for future spectroscopic investigations of cytochrome o.

Effect of detergents on the structure of integral membrane proteins of Sendai virus studied with size-exclusion high-performance liquid chromatography and monoclonal antibodies

The integral membrane proteins of Sendai virus, the fusion protein F (Mr = 65,000) and the haemagglutinin-neuraminidase protein HN (Mr = 68,000), were used as a model protein mixture. They were subjected to size-exclusion high-performance liquid chromatography on Superose 6HR columns with eluents containing various additives in order to solubilize the proteins. The effect of the additives on the structure of the membrane proteins was investigated with conformation-dependent monoclonal antibodies, either directed against F or HN protein, and by determination of the haemagglutinating capacity of the HN protein. The results show that the structure of the HN protein is more easily disturbed by eluents than that of the F protein. When the elution conditions are mild, e.g., 0.1% octylglucoside, the structure of both proteins is conserved but no separation is obtained. Elution with a buffer containing 0.05% sarkosyl (dodecyl methylglycine sodium salt) did not affect the structure and resulted in pure F protein. Pretreatment of the Amberlite XAD-2-treated Sendai virus envelope extract with 4% sodium dodecyl sulphate (SDS) and elution with 0.1% SDS in 50 mM sodium phosphate (pH 6.5) altered the structure of the HN protein but resulted in purification of the tetramer and the dimer of the HN protein, and the monomer of the F protein.

Column liquid chromatography of integral membrane proteins

Biological membranes have as a major function the compartmentation of biological processes in cells and organelles. They consist of a bilayer of phospholipid molecules in which proteins are embedded. These integral membrane proteins, which cross the bilayer once or several times, generally have a higher than average hydrophobicity and tend to aggregate. Detergents are needed to remove integral membrane proteins from the lipid bilayer and they have to be present during further chromatographic purification. Predominantly, four modes of HPLC have been used alone or in combination for the purification of integral membrane proteins. These are based on differences of proteins in size (size-exclusion chromatography, SEC), electrostatic interaction (ion-exchange chromatography, IEC), bioaffinity (bioaffinity chromatography, BAC) and hydrophobic interaction (reversed-phase chromatography, RPC, and hydrophobic-interaction chromatography, HIC). SEC, IEC, BAC and HIC are used under relatively mild conditions, and buffer systems generally contain a non-ionic detergent. RPC generally has a denaturing effect on the protein and should preferably be used for the purification of integral membrane proteins smaller than 50 kD.

Isolation of detergent-solubilized monomers of bacteriorhodopsin by size-exclusion high-performance liquid chromatography

Bacteriorhodopsin, an integral membrane protein of purple membranes, was solubilized with n-octylglucoside and isolated as intact monomeric micelles by high-performance size-exclusion chromatography. It was shown that separation was obtained between these micelles and either those containing bacterio-opsin or retinal as well as bacterio-opsin in the aggregated state. Estimates of the apparent molecular weights and Stokes radii were obtained by comparison with water-soluble proteins with known properties. Thermal denaturation of the native protein micelle induced the formation of a denatured species, which was similar to that found from denaturation at 4 degrees C.