Biophysics (communication arising) : is rhodopsin dimeric in native retinal rods?

Individual biological molecules can be imaged under physiological conditions by atomic force microscopy. Our results from AFM, supported by electron microscopy, revealed distinct rows of rhodopsin dimers and paracrystalline arrays in native murine disc membranes. This supramolecular arrangement was also found for the light-activated form, opsin. We counted 30,000-55,000 rhodopsin molecules per square micrometre, a packing density comparable to that measured by optical methods in amphibian discs. From the lattice vectors describing the paracrystalline arrays, a maximum possible packing density of 62,900 rhodopsin molecules per square micrometre was calculated. The rhodopsin dimers seen in AFM topographs have cytosolic protrusions separated by 3.8 nm, providing an ideal docking platform for arrestin, which has two binding grooves that are separated by 3.8 nm as well.