Opsins with mutations at the site of chromophore attachment constitutively activate transducin but are not phosphorylated by rhodopsin kinase.

More than 70 mutations in the gene encoding the visual pigment rhodopsin have been identified in patients with autosomal dominant retinitis pigmentosa. Most of these mutations are thought to interfere with proper folding of the membrane protein. However, families with a severe phenotype of retinitis pigmentosa have been identified and shown to carry a mutation at the site of chromophore attachment, Lys-296. This mutation disrupts the inactive conformation of opsin and results in a constitutively active protein that can activate the rod-specific GTP-binding protein, transducin, in the absence of light and in the absence of the chromophore 11-cis-retinal. It has been suggested that this mutant opsin molecule may cause rod degeneration by depletion of the components used to inactivate rhodopsin, such as rhodopsin kinase. In this work we test this idea by determining whether two constitutively active opsin mutants are phosphorylated by rhodopsin kinase. We found that opsin mutants where Lys-296 is replaced either by Glu (K296E) or by Gly (K296G) are not substrates of rhodopsin kinase in the absence of chromophore. However, when K296G is regenerated with a Schiff base complex of 11-cis-retinal and n-propylamine and exposed to illumination, phosphorylation of opsin occurs. These experiments suggest that in the rod photoreceptors of patients with retinitis pigmentosa carrying a mutation at Lys-296, there is persistent activation of the GTP-binding protein-mediated cascade. This may result in a situation that mimics long-term exposure to continuous illumination and results in the degeneration of photoreceptors.