Zinc is present at high concentrations in the photoreceptor cells of the retina where it has been proposed to play a role in the visual phototransduction process. In order to obtain more information about this role, the study of the effect of zinc on several properties of the visual photoreceptor rhodopsin has been investigated. A specific effect of Zn 2؉ on the thermal stability of rhodopsin, obtained from bovine retinas and solubilized in dodecyl maltoside detergent, in the dark is reported. Rhodopsin is the photoreceptor protein of the vertebrate retina (1-3) belonging to the G-protein-coupled receptor (GPCR) 1 superfamily (4 -6). It is the main protein component of the rod outer segments (ROS) of the retinal photoreceptor cells, and its easy isolation from bovine retinas has made of this receptor a widely used model for the GPCR superfamily. Rhodopsin is a key molecule in the biochemistry of vision and alterations in its sequence have been associated with retinal disease. In particular, a high number of mutations in the opsin gene have been associated with the autosomal form of the retinal degenerative disease retinitis pigmentosa (7,8). A number of factors have been proposed to be related to retinal function, and among them a possible role for Zn 2ϩ in the retina and its metabolism has been proposed (9). Zn 2ϩ is present at particularly high concentrations in the retina (10) being a component of the disc membranes in the rod outer segments of the photoreceptor cells (11). Zn 2ϩ has been histochemically localized to ROS (12) and it has been shown to copurify with ROS proteins as well (13). Despite this presence, the role of Zn 2ϩ in the visual cycle and specifically in its interaction with rhodopsin remains unclear.Zn 2ϩ is required for the function of numerous proteins, serving both as a part of the active site in, for example, metalloenzymes, and acting to stabilize protein domains, such as the Zn 2ϩ fingerbinding motif in transcription factors (14, 15). The structure of many Zn 2ϩ -binding sites is known from x-ray crystallography of Zn 2ϩ -binding proteins and, thus, the geometry of the interaction between Zn 2ϩ and different coordinating residues is well characterized (14, 15). This, together with the small size of the zinc (II) ion, makes artificially generated Zn 2ϩ -binding sites a highly useful approach for probing structure-function relationships in proteins. In GPCRs, for example, construction of bis-His Zn 2ϩ -binding sites has led to important information about both the organization of the transmembrane helices and their movements during receptor activation to be obtained (16 -22). Therefore, engineered Zn 2ϩ binding sites created by site-directed mutagenesis is an interesting tool for probing intramolecular interactions in these types of receptors (23). Particularly in the case of rhodopsin, important structural information about helical orientation, connectivities, and the conformational change upon light activation, has been obtained from engineered Zn 2ϩ -binding sites in this visual photore...