Molecular Aspects of Signal Transduction 
in the Visual System 
James B. Hurley, Ph.D. — Associate Investigator 
Dr. Hurley is also Associate Professor of Biochemistry at the University of Washington School of Medicine, 
He received his undergraduate degree in chemistry from the State University of New York College of Envi- 
ronmental Science and Forestry, Syracuse, and his Ph.D. degree in physiology and biophysics from the 
University of Illinois, Urbana, where he worked with Thomas Ebrey. His postdoctoral research included 
work with Melvin Simon at both the University of California, San Diego, and the California Institute of 
Technology, and with Lubert Stryer at Stanford University. 
OUR laboratory studies molecular aspects of 
signal transduction processes responsible 
for vision. Vertebrate photoreceptor cells re- 
spond to a light flash via G protein-mediated ac- 
tivation of a cyclic GMP phosphodiesterase. A 
light flash hyperpolarizes a photoreceptor be- 
cause hydrolysis of cGMP shuts down dependent 
plasma membrane cation channels. In darkness 
Ca^"^ enters the cell through these channels, but a 
flash blocks its entry. The resulting decrease in 
cytosolic Ca^^ activates guanylate cyclase, which 
resynthesizes cGMP so that the cell recovers. 
Invertebrate photoreceptors respond very dif- 
ferently. In these cells light activates phospholi- 
pase C, which produces inositol triphosphate and 
diacylglycerol as second messengers. 
Much of our work focuses on the actions of G 
proteins, since many aspects of the visual re- 
sponse depend on this class of heterotrimeric sig- 
nal transduction enzymes. G proteins bind GTP 
in response to receptor stimulation, and the GTP- 
charged a-subunit then dissociates from a com- 
plex of 13- and 7-subunits to regulate the activity 
of an effector enzyme or channel. There are at 
least 1 5 different G protein a-subunits and 4 dif- 
ferent types of (S-subunits. This genetic diversity 
suggests that G proteins have a variety of func- 
tions and that G protein activities have been fine- 
tuned to generate specific cellular responses. 
The G proteins that mediate phototransduction 
in rod and cone photoreceptors are called 
transducins. 
Vertebrate Phototransduction 
Ca^^ mediates vertebrate rod cell recovery 
from a light flash. During the past year we identi- 
fied a photoreceptor Ca^"^-binding protein, 
named recoverin, that activates guanylate cyclase 
only at free Ca^"^ concentrations below 300 nM. 
Recoverin couples the light-induced loss of Ca^^ 
from the cell to guanylate cyclase activation and 
recovery from photoexcitation. In collaboration 
with Alexander Dizhoor, a visiting scientist from 
the USSR, we purified this protein, demonstrated 
its Ca^^-binding properties, cloned and se- 
quenced it, and demonstrated its ability to acti- 
vate guanylate cyclase. 
Once stimulated by light to bind GTP, transdu- 
cin slowly loses its bound GTP and its ability to 
activate phosphodiesterase. Photoreceptor cells 
recover from a light flash within a couple of sec- 
onds, but the kinetics of GTP hydrolysis and 
phosphodiesterase deactivation are slower, about 
20-30 seconds. To clarify the role that GTP hy- 
drolysis plays in the photoresponse, we have pro- 
duced transgenic mice that express a mutant 
transducin expected to hydrolyze GTP more 
slowly than its normal counterpart. Preliminary 
analyses suggest that the rod photoresponses 
from these mice are normal except in the pres- 
ence of a high background light. Under those 
conditions, the cells appear to be abnormally 
sensitive. 
Drosophila Vision 
Biochemical and physiological evidence sug- 
gests that a G protein mediates phototransduc- 
tion in the eyes of invertebrates by activating 
phospholipase C. We had previously character- 
ized a Drosophila G protein /3-subunit encoded 
by a gene referred to as GBB, and we expected to 
find this protein in Drosophila eyes. To our sur- 
prise, in situ hybridization studies revealed that 
GBB mRNA is absent from Drosophila photore- 
ceptor cells. 
We also found, however, that a monoclonal an- 
tibody raised against the GBB protein reacts with 
the entire Drosophila nervous system, including 
the eyes. In search of a protein in the eyes that 
reacts with this antibody, we used it to screen a 
Drosophila head cDNA expression library. The 
protein we identified is 50 percent identical to 
the GBB gene product, and the gene that encodes 
it, QBE, is expressed only in the eye. It is likely 
that GBE encodes the 18-subunit of the Drosoph- 
ila phototransduction G protein, but we need to 
do genetic and biochemical experiments to con- 
firm this. 
A Drosophila G protein a-subunit, DGOa, is 
absent from photoreceptors but present in the 
lamina, a layer within the Drosophila eye that 
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