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Calorimetric Studies of Bovine Rod Outer Segment Disk Membranes Support a Monomeric Unit for Both Rhodopsin and Opsin

Thomas C. Edrington, 5th ^*, Michael Bennett  and Arlene D. Albert ^*

^* Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06250; and Section of Fluorescence Studies, Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, Maryland 20892

Correspondence: Address reprint requests to Arlene D. Albert, E-mail: arlene.albert{at}uconn.edu.

The photoreceptor rhodopsin is a G-protein coupled receptor that has recently been proposed to exist as a dimer or higher order oligomer, in contrast to the previously described monomer, in retinal rod outer segment disk membranes. Rhodopsin exhibits considerably greater thermal stability than opsin (the bleached form of the receptor), which is reflected in an 15°C difference in the thermal denaturation temperatures (T_m) of rhodopsin and opsin as measured by differential scanning calorimetry. Here we use differential scanning calorimetry to investigate the effect of partial bleaching of disk membranes on the T_m of rhodopsin and of opsin in native disk membranes, as well as in cross-linked disk membranes in which rhodopsin dimers are known to be present. The T_ms of rhodopsin and opsin are expected to be perturbed if mixed oligomers are present. The T_m remained constant for rhodopsin and opsin in native disks regardless of the level of bleaching. In contrast, the T_m of cross-linked rhodopsin in disk membranes was dependent on the extent of bleaching. The energy of activation for denaturation of rhodopsin and cross-linked rhodopsin was calculated. Cross-linking rhodopsin significantly decreased the energy of activation. We conclude that in native disk membranes, rhodopsin behaves predominantly as a monomer.

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