Calorimetric Studies of Bovine Rod Outer Segment Disk Membranes Support a Monomeric Unit for Both Rhodopsin and Opsin
Thomas Craighead Edrington 1, Michael Paul Bennett 2 and Arlene D Albert 1*
1 University of Connecticut
2 NIH
* To whom correspondence should be addressed. E-mail: arlene.albert{at}uconn.edu.
Submitted on January 4, 2008
Revised on February 17, 2008
Accepted on 10 June 2008
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Abstract |
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The photoreceptor rhodopsin is a G-protein coupled receptor (GPCR) 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 a ~15°C difference in the thermal denaturation temperatures (Tm) of rhodopsin and opsin as measured by differential scanning calorimetry (DSC). Here we use DSC to investigate the effect of partial bleaching of disk membranes on the Tm 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 Tms of rhodopsin and opsin are expected to be perturbed if mixed oligomers are present. The Tm remained constant for rhodopsin and opsin in native disks regardless of the level of bleaching. In contrast, the Tm of cross-linked rhodopsin in disk membranes was dependent on the extent of bleaching. The energy of activation for denaturation (Eact) of rhodopsin and cross-linked rhodopsin was calculated. Cross-linking rhodopsin significantly decreased the Eact. We conclude that in native disk membranes rhodopsin behaves predominantly as a monomer.
Key Words:
calorimetry, denaturation temperature, monomeric GPCR, opsin, rhodopsin
