Signal Transduction by Adrenergic Receptors 
Brian K. Kobilka, M.D. — Assistant Investigator 
Dr. Kobilka is also Assistant Professor of Medicine, Cardiology, and Molecular and Cellular Physiology at 
the Stanford University Medical Center. He received his undergraduate degree in biology and chemistry 
from the University of Minnesota, Duluth, and his M.D. degree from Yale University. After his residency 
in internal medicine at Barnes Hospital, St. Louis, he joined the laboratory of Robert Lefkowitz as a re- 
search fellow in cardiology at Duke University. Four years later he was appointed Assistant Professor in 
the Department of Medicine at Duke University, and the following year he assumed his present positions 
at Stanford. 
THE autonomic nervous system serves as the 
master control center for the cardiovascular 
system. It monitors the effectiveness of the latter 
system in providing nutrients and oxygen to the 
rest of the body and appropriately adjusts the 
heart rate, blood pressure, and blood flow. These 
adjustments are made via nerves that serve the 
heart, blood vessels, and kidneys. 
Adrenergic receptors form the interface be- 
tween these nerves (of the sympathetic sub- 
system) and the organs they innervate. Cate- 
cholamines released from sympathetic nerve 
terminals bind to adrenergic receptors on the sur- 
face of target cells, and the activated receptors 
modify the function of these cells. 
When a catecholamine occupies its binding 
site, the receptor activates a GTP-binding protein 
(G protein) inside the cell. The activated G pro- 
tein may then modulate the activity of a cellular 
enzyme or ion channel. The genes (or corre- 
sponding cDNAs) for eight types of adrenergic 
receptors have been cloned. These receptors 
have different functional properties and play dif- 
ferent roles in the sympathetic nervous system. 
All of these receptors, however, are struc- 
turally similar, having seven hydrophobic do- 
mains that are thought to be membrane spanning. 
Mutagenesis studies have revealed that the hydro- 
phobic domains are involved in forming the cate- 
cholamine-binding pocket. The cytoplasmic do- 
mains, which are hydrophilic, are involved in 
activating G proteins and in desensitizing the 
receptors. 
Adrenergic Receptor Structure 
A major focus in my laboratory is to learn more 
about the three-dimensional structure of adrener- 
gic receptors and to determine how they transmit 
signals across the cell membrane's lipid bilayer. 
We are taking several approaches to study the re- 
ceptor structure. Mutagenesis studies are identi- 
fying amino acid residues involved in binding 
subtype-specific ligands. These studies will help 
to define the boundaries of the ligand-binding 
pocket and possibly provide insight into the dif- 
ferences between agonist and antagonist binding. 
A long-range goal in my laboratory is to charac- 
terize the three-dimensional structure of the 182' 
adrenergic receptor and to understand how the 
structure changes during signal transduction. 
Our current efforts are focused on developing 
methods to produce large quantities of pure, 
functional receptor protein for biochemical and 
biophysical studies. We are attempting to in- 
crease production of ^2 receptor and to improve 
the purification procedure. 
Efforts include designing expression vectors 
and determining the optimum conditions for 
growth of tissue culture cells. Efforts at im- 
proving the efficiency of purification involve de- 
veloping new ligand affinity resins and making 
recombinant receptors with minor structural 
modifications that facilitate purification. 
Receptor Biosynthesis 
The primary amino acid sequence of a receptor 
contains all of the essential information needed 
for the receptor's proper folding, post-transla- 
tional processing, and cellular targeting. Under- 
standing the process by which receptors are 
folded and processed should provide insights 
into receptor structure and may identify factors 
that will enhance the production of functional 
receptor protein. 
We have developed a cell-free expression sys- 
tem capable of synthesizing functional iS2-adren- 
ergic receptor. This system was used to study the 
process by which the 182 receptor is inserted into 
the endoplasmic reticulum and folded into a 
functional protein. Research thus far has shown 
that the receptor is nonfunctional immediately 
after translation and translocation into the endo- 
plasmic reticulum. To produce a functional re- 
ceptor, additional processing is needed. ATP, 
intact microsomal membranes, and a high-molec- 
ular-weight cytosolic factor are required for this 
processing. We are attempting to determine the 
nature of this processing by identifying structural 
differences between a functional receptor and 
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