ratory has characterized a GTP-binding protein, G^if, 
exclusively expressed in the olfactory neurons and 
localized to the sensory cilia. This olfactory-specific 
G protein shares some homology to transducin, the 
G protein involved in visual signal transduction. 
The cyc~ variant of the S49 mouse lymphoma cell 
line is deficient in GTP-stimulated adenylyl cyclase 
activity and has proved to be a useful system to in- 
vestigate G protein function. When G^if is intro- 
duced into this cell line, GTP-dependent adenylyl 
cyclase activity is restored. Moreover, the ability of a 
i8-adrenergic agonist, isoproterenol, to stimulate 
adenylyl cyclase is also restored. These data suggest 
that G„if can couple heterologous receptors to aden- 
ylyl cyclase. 
The third component in the signal transduction 
cascade, adenylyl cyclase, is expected to be abun- 
dant in olfactory cDNA libraries. At the level of en- 
zyme activity, there are 10-fold higher levels of this 
protein in olfactory tissue homogenates than in 
brain tissue. In a collaboration with Dr. Alfred Gil- 
man, Dr. Reed's laboratory has identified cDNA 
clones encoding three distinct forms of adenylyl cy- 
clase. One form is expressed exclusively in brain, 
and a second is expressed in several peripheral tis- 
sues. A third form of the enzyme is expressed only in 
olfactory epithelium. This protein, type III adenylyl 
cyclase, is largely confined to the cilia of rat olfac- 
tory neurons. Biochemical experiments performed 
by expressing type III adenylyl cyclase in a mamma- 
lian expression system revealed that the enzyme has 
unusually low basal activity. This would be advan- 
tageous for a protein that is highly concentrated in 
the cilia and yet must maintain low resting levels of 
intracellular second messenger. 
Regulation of Olfactory-Specific Gene 
Expression 
The expression of components of the olfactory 
signal transduction pathway and other proteins spe- 
cifically expressed in the sensory neurons is likely 
to be regulated coordinately. Dr. Reed's laboratory 
has initiated a series of experiments to identify the 
cis-acting regulatory sequences that control expres- 
sion of this set of genes. An 1 1-base sequence pres- 
ent in each of these genes appears to bind a protein 
present only in extracts from olfactory tissue. By us- 
ing a yeast expression/selection scheme, a candi- 
date protein for a trans-acting transcription factor 
operating at this site has been identified. Exper- 
iments are now under way to establish a func- 
tional role for this protein in olfactory neuronal 
differentiation. 
Genetic Approaches to the Functional 
Analysis of Signal Transduction Components 
Elucidation of a function for the variety of forms 
of adenylyl cyclase expressed in mammalian tissues 
may prove difficult. In research supported by the 
National Institute of Mental Health, Dr. Reed's labo- 
ratory has initiated experiments in Drosophila me- 
lanogaster to identify homologues of the mamma- 
lian proteins. Among the loci that appear to encode 
adenylyl cyclase in Drosophila is the learning and 
memory mutant rutabaga. This gene in Drosophila 
has been cloned and the mutation in memory- 
deficient flies identified. Biochemical studies have 
revealed that a single base change in the coding re- 
gion for the rutabaga protein leads to a complete 
loss of enzyme activity. Demonstration that this de- 
fect is the cause of the defect in learning and mem- 
ory was achieved by analyzing insertions of a trans- 
posable element into the same gene. The patterns of 
gene expression in the fly and the genetic conse- 
quences of these mutations confirm that the cloned 
Drosophila cyclase is rutabaga. The study of the 
molecular defects at this locus may expand under- 
standing of the mechanism of memory processes. 
The identification and molecular characterization 
of components of the cAMP second messenger path- 
way provide important new tools to examine the 
relationship between structure and function of 
these proteins. Additionally, the availability of olfac- 
tory neuron-specific isozymes of many of the pro- 
teins in the cascade will allow the development of 
useful systems for the reconstitution of the odor- 
ant detection pathway in cell lines amenable to 
the study of the mechanisms of specificity and 
sensitivity. 
Dr. Reed is also Professor in the Departments of 
Molecular Biology and Genetics and of Neuro- 
science at the fohns Hopkins University School of 
Medicine. 
Articles 
Colin, S.F., Chang, H.-C, Mollner, S., Pfeufifer, T., 
Reed, R.R., Duman, R.S., and Nestler, E.J. 1991. 
Chronic lithium regulates the expression of ade- 
nylate cyclase and G-protein a subunit in rat cere- 
bral cortex. Proc Natl Acad Sci USA 88:10634- 
10637. 
Cunningham, A.M., and Reed, R.R. 1992. A sense 
of smell. Curr Biol2:\\6-\\S. 
Federman, A.D., Conklin, B.R., Schrader, K.A., 
Reed, R.R., and Bourne, H.R. 1992. Hormonal 
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