Is rhodopsin dimeric in native retinal rods?

Abstract

The model of vertebrate rhodopsin that has emerged from a combination of traditional biophysical techniques^1,2,3,4,5,6 defines it as an archetypal monomeric receptor protein that is randomly dispersed and freely diffusing in a fluid lipid matrix in the rod-disc membrane. A quite different arrangement has been demonstrated by Fotiadis et al.⁷, however, who use atomic force microscopy (AFM) to reveal rows of rhodopsin molecules packed as dimers in isolated murine disc membranes at twice the surface density estimated from previous in situ measurements^3,4,6; lipids are probably excluded from these rhodopsin packs⁸. We contend that these 'dimers' are in fact long double rows of equally spaced proteins, packed in partially ordered microcrystalline arrays, and suggest that the arrangement of irregular packs of protein and patches of pure lipid with a free-floating border may be an artefact resulting from the conditions used in sample preparation for AFM.