Pineal opsin (P-opsin), an opsin from chick that is highly expressed i
n pineal but is not detectable in retina, was cloned by the polymerase
chain reaction. It is likely that the P-opsin lineage diverged from t
he retinal opsins early in opsin evolution. The amino acid sequence of
P-opsin is 42 to 46 percent identical to that of the retinal opsins.
P-opsin is a seven-membrane spanning, G protein-linked receptor with a
Schiff-base lysine in the seventh membrane span and a Schiff-base cou
nterion in the third membrane span. The primary sequence of P-opsin su
ggests that it will be maximally sensitive to similar to 500-nanometer
tight and produce a slow and prolonged phototransduction response con
sistent with the nonvisual function of pineal photoreception.