MOLECULAR BIOLOGY OF VISUAL PIGMENTS 
Jeremy Nathans, M.D., Ph.D., Assistant Investigator 
Visual pigments are the light-absorbing proteins 
in the retina that mediate the first step in visual ex- 
citation. They are members of a large family of cell 
surface receptors that transduce external stimuli by 
activation of G proteins. 
One area of interest in Dr. Nathans' laboratory is 
the chemistry of visual pigment photoactivation. Each 
pigment consists of a chromophore, 11-cis retinal, 
joined covalently to an integral membrane protein, 
opsin. Photon absorption leads to isomerization of 
the chromophore to an all-trans configuration. This 
isomerization drives a series of conformational 
changes in the attached opsin. These changes can be 
foUov^^ed spectroscopically, and the intermediates in 
photoactivation can be trapped at low temperature. 
As a first step in defining the 1 1-cis retinal-binding 
pocket and characterizing its electronic environ- 
ment, Dr. Nathan and his co-workers have studied a 
series of site-directed mutants carrying alterations 
in the putative retinal-binding pocket of bovine 
rhodopsin. Fourteen mutants were constructed 
that replaced charged amino acids predicted to 
contact retinal. These alterations were chosen be- 
cause retinal is known to undergo a significant 
change in dipole moment upon photoexcitation 
and therefore should be sensitive to electronic per- 
turbations. The expectation was that these mutants 
would combine with 11-cis retinal to form visual 
pigments with novel absorbance spectra. Instead 
the experiment shows that each has a nearly nor- 
mal spectrum. These results suggest that either the 
current binding pocket model is incorrect or the 
commonly held hypothesis of retinal tuning by 
nearby negatively charged residues is incorrect. To 
test the latter possibility, which is thought to be the 
most likely, 1 5 site-directed mutants have been con- 
structed, in which each of the remaining negatively 
charged amino acids in bovine rhodopsin has been 
replaced by a neutral amino acid. 
PUBLICATIONS 
A second area of interest is the study of inherited 
variations in human vision. In one set of expe- 
riments. Dr. Charles Weitz has initiated a study of 
inherited variation in blue sensitivity. Affected per- 
sons in two families from Japan show the same 
single-amino acid change in their blue pigment 
genes, resulting in a glycine-to-arginine change in 
the putative second transmembrane segment. Link- 
age analysis, together with expression and char- 
acterization of the mutant protein, will be required 
to test definitively whether this change causes 
the mutant phenotype. In another set of exper- 
iments the genetic alterations responsible for a rare 
X-linked trait, blue cone monochromacy (also 
called incomplete achromatopsia), have been de- 
termined. Affected individuals show 1) nonfunc- 
tional red and green cone systems, leading to a 
complete absence of color sense; 2) photophobia; 
3) low acuity typically 20/80 to 20/200; 4) nystag- 
mus; and 5) in some families a slowly progressive 
macular scarring. Fourteen families have been 
analyzed: ten carry deletions in the red and green 
pigment gene cluster that range in size from 0.6 
to 54 kb; four have lost all but one of their red 
and green pigment genes, and sequence analysis 
shows that the remaining gene has suffered a point 
mutation. The deleted regions all remove a com- 
mon 0.6 kb segment that lies ~4 kb upstream of 
the red and green pigment gene cluster. This region 
may contain a long-range enhancer analogous to 
the one defined by Grosveld and his colleagues 
adjacent to the P-globin gene cluster. Transgenic 
mouse experiments in which this region is joined 
to a reporter gene are in progress to test this hy- 
pothesis. 
Dr. Nathans is also Assistant Professor of Molecu- 
lar Biology and Genetics and of Neuroscience at 
The Johns Hopkins University School of Medicine. 
Articles 
Nathans, J. 1989. The genes for color vision. SciAm 260:42-49. 
Nathans, J., Weitz, C J., Agarwal, N., Nir, I., and Papermaster, D.S. 1989. Production of bovine rhodopsin by 
mammalian cell lines expressing cloned cDNA: spectrophotometry and subcellular localization. Vision Res 
29:907-914. 
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