Digitonin and sodium dodecylsulfate-solubilized frog rho-dopsin: Behavior under native and denaturing polyacrylamide gel electrophoresis

Abstract

Rhodopsin oligomerization and dissociation in vivo and under experimental conditions is an important topic both for a basic understanding of photoreceptor structure-function but also as a potential eye disease mechanism. In this study, to estimate a state rhodopsin after solubilization with mild and harsh detergents, we applied the native (blue native-PAGE, BN- PAGE) and denaturing electrophoresis (blue-urea- PAGE, BU-PAGE; blue-SDS-PAGE, BSDS-PAGE and SDS- PAGE). After blue BN-PAGE and BSDS- PAGE, rhodopsin and opsin, respectively, were presented in gels a major band of dimer with slight contents of higher oligomers without any traces of monomer, thus testifying in favor dimer-heteromeric state of frog rhodopsin in the photoreceptor membrane. Despite all oligomer bands gave positive staining with the rhodopsin-specific monoclonal antibodies (mAb), subsequent SDS-PAGE in combination with electroelution in denaturing conditions showed that stained bands are not homogenous and besides of opsin oli-gomers contain a small admixture of proteins with unknown function. Unfolding of opsin oligomers by solubilization in SDS, as compared with folded opsin in digitonin, induces their transition to a more compact conformation. It was manifested in a more rapid migration of opsin oligomers toward to anode. Cooling of digitonin/SDS mixed extracts at 4?C for 24 hours led to a partial reverse transition of unfolded opsin dimer to initial folded conformation, thus de- monstrating the entropic nature of this transition. Opsin monomer can be observed in the gels only after harsh dissociation of oligomers under BU-PAGE or SDS-PAGE. The electro elution of the individual opsin oligomers with denaturing buffer followed by SDS-PAGE resulted in dissociation of dimer to monomers. However, unexpectedly, the trimer was dissociated to a prevailing dimer and a small portion of monomer. The products dissociation of both opsin tetramer and pentamer are difficult to determine precisely, but they are neither monomer nor dimer. Dissociation data show that the degree of opsin oli- gomerization by unknown reasons affects the pattern of dissociation of its aggregates. Obtained in this paper data indicate a need for further detailed study the obscure mechanisms of aggregation-dissociation of rhodopsin.