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* Visual pigments (opsins) retinal binding site *
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Visual pigments [1,2] are the light-absorbing  molecules that  mediate vision.
They consist of  an apoprotein, opsin,  covalently  linked  to the chromophore
cis-retinal.  Vision is  effected through  the absorption of a  photon by cis-
retinal  which is isomerized to  trans-retinal.  This isomerization leads to a
change  of conformation  of the protein. Opsins are integral membrane proteins
with  seven transmembrane regions that belong to family 1 of G-protein coupled
receptors (see <PDOC00210>).

In vertebrates four different pigments are generally found.   Rod cells, which
mediate vision in dim light, contain the pigment rhodopsin.  Cone cells, which
function in bright light, are responsible  for  color vision and contain three
or more color pigments (for example, in mammals: red, blue and green).

In Drosophila, the  eye   is composed   of 800   facets  or   ommatidia.  Each
ommatidium contains eight photoreceptor cells (R1-R8):  the R1 to R6 cells are
outer cells,  R7  and R8 inner cells. Each of the three types of cells (R1-R6,
R7 and R8) expresses a specific opsin.

Proteins evolutionary related to opsins include squid retinochrome, also known
as retinal  photoisomerase, which converts various isomers of retinal into 11-
cis retinal and mammalian retinal pigment  epithelium (RPE) RGR [3], a protein
that may also act in retinal isomerization.

The attachment  site  for  retinal in the above proteins is a conserved lysine
residue in  the  middle  of  the  seventh  transmembrane helix. The pattern we
developed includes this residue.

-Consensus pattern: [LIVMWAC]-[PGAC]-x(3)-[SAC]-K-[STALIMR]-[GSACPNV]-[STACP]-
                    x(2)-[DENF]-[AP]-x(2)-[IY]
                    [K is the retinal binding site]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in SWISS-PROT: NONE.
-Last update: July 1998 / Pattern and text revised.

[ 1] Applebury M.L., Hargrave P.A.
     Vision Res. 26:1881-1895(1986).
[ 2] Fryxell K.J., Meyerowitz E.M.
     J. Mol. Evol. 33:367-378(1991).
[ 3] Shen D., Jiang M., Hao W., Tao L., Salazar M., Fong H.K.W.
     Biochemistry 33:13117-13125(1994).
