{PDOC00211}
{PS00238; OPSIN}
{BEGIN}
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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.
 - Mammalian  opsin  3  (Encephalopsin)  that  may  play  a role in encephalic
   photoreception.
 - Mammalian  opsin  4  (Melanopsin)  that may mediate regulation of circadian
   rhythms and acute suppression of pineal melatonin.
 - 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: [LIVMFWAC]-[PSGAC]-x-{G}-x-[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: 2.
-Last update: December 2004 / Pattern and text revised.

[ 1] Applebury M.L., Hargrave P.A.
     "Molecular biology of the visual pigments."
     Vision Res. 26:1881-1895(1986).
     PubMed=3303660
[ 2] Fryxell K.J., Meyerowitz E.M.
     "The evolution of rhodopsins and neurotransmitter receptors."
     J. Mol. Evol. 33:367-378(1991).
     PubMed=1663559
[ 3] Shen D., Jiang M., Hao W., Tao L., Salazar M., Fong H.K.W.
     "A human opsin-related gene that encodes a retinaldehyde-binding
     protein."
     Biochemistry 33:13117-13125(1994).
     PubMed=7947717
{END}