MOLECULAR BIOLOGY OF VISUAL PIGMENTS 
Jeremy Nathans, M.D., Ph.D., Associate Investigator 
Visual pigments are the light-absorbing proteins 
that initiate phototransduction. Each consists of an 
integral membrane protein covalently joined to a 
chromophore, 1 l-c« retinal. Light photoisomerizes 
retinal from 1 1-cis to all-trans, which in turn pro- 
duces a conformational change in the attached apo- 
protein, converting it into an activator of a photore- 
ceptor-specific G protein. 
Dr. Nathans is investigating several questions re- 
lated to visual pigment structure and function. 
Which residues comprise the chromophore binding 
site and how do they modify the photochemistry of 
retinal? How do visual pigment genes vary in the 
human population, and what are the consequences 
of this variation for visual function? How does each 
type of photoreceptor cell determine which visual 
pigment to express? 
Color Vision and Color Blindness 
Human color vision is based upon a comparison 
of the extent of excitation of three visual pigments 
that reside in three classes of cone photoreceptor 
cells. The nucleotide sequences of genomic and 
cDNA clones encoding the three human cone pig- 
ments reveal 42% amino acid identity between the 
blue pigment and the red or green pigments, 
whereas the red and green pigment sequences show 
96% mutual identity. 
Shannath Merbs, an M.D.-Ph.D. student, has re- 
cently succeeded in producing the human cone pig- 
ment apoproteins in transfected tissue culture cells. 
Following reconstitution with 1 l-cis retinal, the vi- 
sual pigments show absorption maxima of 426 nm 
(the blue pigment), 530 nm (the green pigment), 
and 552 or 557 nm (two allelic variants of the red 
pigment) . The two red pigment variants differ by an 
alanine-versus-serine substitution at position 180. A 
spectral difference of 5 nm would be predicted to 
produce a readily measurable perceptual difference 
between individuals. Indeed, Dr. Samir Deeb and 
his colleagues at the University of Washington have 
shown that this allelic variation correlates well with 
differences in color-matching ability between indi- 
viduals with normal color vision. In the human gene 
pool, alanine is encoded at this position in ~40% of 
red pigment genes and serine in the remaining 60%. 
In more recent experiments, Ms. Merbs has stud- 
ied a set of hybrid pigments corresponding to those 
produced by homologous unequal recombination 
between red and green pigment genes. The hybrids 
are found in individuals with anomalous trichro- 
macy, a variant form of color vision present in ~6% 
of Caucasian males. The absorption spectra of the 
nine most common hybrids all lie within the inter- 
val defined by the spectra of the red and green pig- 
ments from which they were derived. Amino acid 
differences in exon 5 appear to exert the largest ef- 
fect on spectral tuning. These absorption spectra 
provide a framework for predicting the characteris- 
tics of the various anomalies of red-green color vi- 
sion. Experiments now in progress are aimed at pre- 
cisely defining the contribution to spectral tuning 
of each of the 1 5 amino acids that differ between red 
and green pigments. (The project described above 
was supported in part by a grant from the National 
Eye Institute, National Institutes of Health.) 
Blue Cone Monochromacy 
A rare form of severe color blindness called blue 
cone monochromacy results from mutational events 
at the red and green pigment gene locus that elimi- 
nate the activity of both genes. To date, 40 families 
with blue cone monochromacy have shown large- 
scale rearrangements at the red and green pigment 
gene cluster. One class of rearrangements defines a 
small DNA region adjacent to the visual pigment 
genes that appears to be essential for their correct 
expression. This region is located 3 kb from the ad- 
jacent red pigment gene and 42 kb from the nearest 
green pigment gene. 
Yanshu Wang, a graduate student, has shown that 
in transgenic mice this segment is required for ex- 
pression of a |8-galactosidase reporter gene in cone 
photoreceptors. The essential region coincides with 
a sequence of several hundred bases that are highly 
conserved between humans, mice, and cattle. These 
experiments suggest a model in which the upstream 
controlling region and its associated proteins inter- 
act with either the red or green pigment gene pro- 
moter. If the controlling region can accommodate 
only one such interaction, this mechanism would 
ensure the mutually exclusive expression of red 
and green pigment genes in their respective cone 
photoreceptors. 
Approximately 50% of blue cone monochromats 
have only a single gene in the red and green pigment 
gene array. Most of these single genes carry a 
cysteine-201-to-arginine mutation that disrupts an 
essential disulfide bond. One blue cone monochro- 
mat has two genes in his visual pigment array, both 
of which carry this same mutation. Curiously, sev- 
eral blue cone monochromats have visual pigment 
GENETICS 235 
