Molecular Mechanisms of Olfaction 
Randall R. Reed, Ph.D. — Associate Investigator 
Dr. Reed is also Associate Professor in the Departments of Molecular Biology and Genetics and of Neuro- 
science at the Johns Hopkins University School of Medicine. He received his bachelor's degree in biophysics 
from Johns Hopkins and his Ph.D. degree from Yale University. His postdoctoral research was done with 
Philip Leder at Harvard Medical School. 
OLFACTION is among the oldest of the sen- 
sory systems. All multicellular and many 
unicellular organisms have evolved sensitive 
chemosensory systems able to detect and identify 
natural chemical substances. The olfactory sys- 
tem of vertebrates and analogous systems for the 
other senses — vision, hearing, taste, and touch 
— allows the conversion of external stimuli into 
nerve impulses. In mammals the olfactory system 
is exquisitely sensitive, capable of detecting 
some odorants present at a concentration of only 
a few parts per trillion. 
The ability of the olfactory system to discrimi- 
nate thousands of different odorants suggests a 
complex coding mechanism. However, the bio- 
logical basis for this coding remains a mystery. 
Unlike the visual and auditory systems, which 
need only encode information on frequency (or 
wavelength) and intensity, the olfactory sense re- 
quires multidimensional information. These con- 
siderations suggest a complex signal transductory 
process. 
The signal transductory pathway for olfaction 
can be divided temporally and spatially into sev- 
eral distinct steps. The first consists of the solubi- 
lization and concentration of airborne odorants. 
Considerable experimental data suggest that com- 
ponents of the mucus are able to concentrate 
odorants several thousandfold. We previously 
identified cDNA clones encoding proteins that 
are present at high concentration in the mucus 
and that appear to bind odorants. These proteins, 
from rat and frog, are members of the retinol- 
binding protein family, many of which have been 
shown to solubilize hydrophobic ligands in 
serum. They are likely to play a similar role in the 
olfactory system. 
The recognition of the chemical structure of an 
odorant and transduction of that information 
across the plasma membrane is a poorly under- 
stood process. Some investigators hypothesize 
that odorants interact directly with the lipid 
membrane, but it is difficult to see how the abil- 
ity to discriminate stereoisomeric compounds 
could be accommodated by such a system. Sev- 
eral years ago, we began to test an alternative hy- 
pothesis: that binding of odorants to specific 
membrane-associated proteins leads to intracel- 
lular changes in the primary sensory neuron. 
The detection of odorant-stimulated activation 
of adenylyl cyclase in olfactory neurons sug- 
gested an analogy to visual signal transduction. 
Moreover, activation of adenylyl cyclase in prepa- 
rations of rat olfactory cilia depends on the pres- 
ence of guanine nucleotides. The observation of 
odorant-stimulated GTP-dependent adenylate cy- 
clase activity argues strongly for a GTP-binding 
protein-coupled signal transductory pathway. 
We have identified a GTP-binding protein ex- 
pressed exclusively in olfactory sensory neurons. 
Moreover, this G protein, Goif, is localized to the 
olfactory cilia, where the initial events in olfac- 
tory signal transduction are thought to occur. The 
Golf protein, which is highly homologous to a 
GTP-binding protein that stimulates adenylyl cy- 
clase in other systems, is expected to interact di- 
rectly with olfactory receptors. We have demon- 
strated that Golf can couple receptor activation to 
increases in intracellular cAMP in cell lines defi- 
cient for Gsj. 
Does the identification of an olfactory-specific 
G protein provide any insight into the nature of 
the olfactory receptor? A number of G protein- 
coupled receptors have been cloned and charac- 
terized; each consists of a single glycosylated 
polypeptide chain containing seven membrane 
crossing regions. There is limited but significant 
homology among members of this family, and we 
are presently attempting the direct cloning of the 
olfactory receptors based on these similarities. 
We have recently identified several novel G pro- 
tein-coupled receptors expressed in olfactory ep- 
ithelium. Their role in olfaction remains unclear. 
Identification of genes encoding olfactory recep- 
tors may reveal how receptors are able to detect 
thousands of different odors. Specifically, does 
each of the several million olfactory sensory cells 
encode a distinct receptor protein? 
The final step in the transduction of odorant 
stimuli is the generation of the intracellular sig- 
nal and the firing of an action potential. Special- 
ized forms of second messenger-generating en- 
363 
