Salamander UV cone pigment: Sequence, expression, and spectral properties

The visual pigment from the ultraviolet (UV) cone photoreceptor of the tige r salamander has been cloned, expressed, and characterized. The cDNA contai ns a full-length open reading frame encoding 347 amino acids. The phylogene tic analysis indicates that the highest sequence homology is to the visual pigments in the S group. The UV opsin was tagged at the carboxy-terminus wi th the sequence for the 1D4 epitope. This fusion opsin was expressed in COS -1 cells, regenerated with 11-cis retinal (A1) and immuno-purified, yieldin g a pigment with an absorbance maximum (lambda (max)) of 356 nm which is bl ue shifted from the absorption of retinal itself. The transducin activation assay demonstrated that this pigment is able to activate rod transducin in a light-dependent manner. Regeneration with 11-cis 3,4-dehydroretinal (A2) yielded a pigment with a lambda (max) of 360 nm, only 4 nm red shifted fro m that of the A1 pigment, while bovine rhodopsin generated with A2 showed a 16-nm red shift from the corresponding A1 pigment. These results demonstra te that them trend for a shorter wavelength pigment to have a smaller shift of lambda (max) between the A1 and A2 pigments also fits UV pigments. We h ypothesize that the small red shift with A2 could be due to a twist in the chromophore that essentially isolates the ring double bond(s) from conjugat ion with the rest of the polyene chain.