Organization of rhodopsin in photoreceptor membranes. 2. Transmembrane organization of bovine rhodopsin: evidence from proteolysis and lactoperoxidase-catalyzed iodination of native and reconstituted membranes.

Proteolysis of reconstituted membranes with papain and thermolysin reveals the existence of two rhodopsin populations: one susceptible to proteolysis and the other protected. The susceptible population corresponds to rhodopsin molecules with the same orientation as rhodopsin in the native membrane, while the protected population corresponds to "inverted" rhodopsin molecules only found in reconstituted membranes. Using an iodination enhancement probe, we demonstrate that lactoperoxidase catalyzes iodination of rhodopsin exclusively on the external surface of these sealed reconstituted vesicles. Furthermore, we find that both rhodopsin populations in reconstituted membranes (normal and inverted) are readily iodinated by lactoperoxidase, providing definitive evidence that the rhodopsin polypeptide spans the membrane thickness. Additional conclusions from these experiments are discussed in terms of a model for the folding of the rhodopsin polypeptide in the membrane.