Eye diseases and proteins controlling visual transduction

Retinal S antigen is a soluble protein found in abundance in photoreceptor cells. Immunization of laboratory animals with this antigen in adjuvant induces experimental autoimmune uveoretinitis. Cellular immunity plays a major role in this condition. Autoimmune responses toward retinal S antigen are often observed in patients with retinal inflammatory disease, however, these responses are usually secondary to local tissue damage. The S antigen is identical to the 48 K protein characterized in rod outer segments by its light-dependent binding to the disk membrane in the presence of ATP. This protein binds specifically to photoexcited and phosphorylated rhodopsin, and quenches the activity of the light-dependent cGMP-phosphodiesterase, most probably because it competes with transducin. There is no evidence for any direct inactivation of phosphodiesterase by 48 K protein. In view of the numerous similarities between the photoreceptor enzyme cascade and hormone-activated cyclase systems, a related protein could be involved in the desensitization of hormonal systems.