Mechanism of Retrovirus Infection 
James M. Cunningham, M.D. — Assistant Investigator 
Dr. Cunningham is also Assistant Professor of Medicine at Brigham and Women's Hospital and Harvard 
Medical School. He received a B.S. degree in chemistry from the University of Michigan and an M.D. degree 
from Stanford University School of Medicine. After clinical training in internal medicine (Peter Bent 
Brigham Hospital) and oncology (Dana Farher Cancer Institute), he was a postdoctoral fellow in the 
laboratory of Robert Weinberg at the Massachusetts Institute of Technology. Dr. Cunningham was an 
HHMI Associate at Brigham and Women's Hospital before assuming his current appointment. 
VIRUSES are parasites. They cannot produce 
progeny on their own, but must rely on the 
machinery provided by the host cell to replicate 
the viral genome and assemble new virus parti- 
cles. Infection is initiated by attachment of the 
virus to the host cell — the first step in a complex 
reaction that results in transfer of the viral ge- 
nome through the cell membrane and into the 
cytoplasm. This attachment, or binding, is a con- 
sequence of the interaction between proteins ex- 
posed on the surface of the virus and the host cell 
plasma membrane. 
Cells that do not express a suitable virus-bind- 
ing protein, called a receptor, are not susceptible 
to infection by a particular virus. Indeed, the abil- 
ity of many pathogenic viruses, such as human 
immunodeficiency virus, poliovirus, and certain 
herpesviruses, to infect specific host tissues has 
been closely correlated with the expression of 
specific receptors. 
Our laboratory has been interested in the mech- 
anism of infection utilized by Moloney murine 
leukemia virus (Mo-MuLV) , a member of a group 
of related leukemogenic retroviruses found in vir- 
tually all vertebrates. We have isolated a molecu- 
lar clone, Rec- 1, which confers Mo-MuLV infectiv- 
ity upon introduction into mammalian cells that 
are not normally susceptible to infection. Subse- 
quent experiments have demonstrated that Rec- 1 
encodes for a membrane protein that serves as the 
Mo-MuLV receptor. Our current research is ad- 
dressed toward dissecting the molecular details 
of the virus-receptor interaction that mediates 
infection. 
The Mo-MuLV receptor is not present in mice 
for the convenience of the virus, but rather must 
have a function essential to normal cell metabo- 
lism. We have identified the receptor protein on 
the plasma membrane of all mouse cells, suggest- 
ing it participates in basic cell metabolism and 
does not perform a specialized function limited 
to a subset of tissues. Examination of the amino 
acid sequence encoded by Ree l reveals that it 
comprises an abundance of extremely hydropho- 
bic amino acid residues. This implies the recep- 
tor protein must exist primarily within the lipid 
environment of the membrane. A molecular 
model of the Mo-MuLV receptor predicts that it 
traverses the plasma membrane as many as 14 
times, a structure similar to a protein found in 
yeast that transports the amino acids arginine and 
lysine. Arginine and lysine are 2 of the 20 amino 
acids that are the building blocks of proteins. 
Frog eggs injected with Rec- 1 RNA demonstrate a 
large increase in the transport of arginine and ly- 
sine across the cell membrane. More detailed ex- 
periments have confirmed that the Mo-MuLV re- 
ceptor is the transporter for all cationic amino 
acids, i.e., amino acids that carry a net positive 
charge. Amino acid transporters have been 
known to exist in mammalian cells, but none 
have previously been isolated or their structure 
known. The similarity between the yeast and 
mouse cationic amino acid transporter is consis- 
tent with conservation of a single mechanism for 
transport of cationic amino acids over evolution- 
ary time and predicts that proteins similar to the 
Mo-MuLV receptor are used by all animals. In- 
herited disorders of cationic amino acid transport 
have been described in patients; mutations in 
Ree l genes may explain these disorders, a hy- 
pothesis we are now examining. 
In addition to protein synthesis, cationic amino 
acids have other roles. For example, arginine and 
ornithine are important intermediates in the urea 
cycle, a metabolic pathway found in liver cells 
that rids animals of nitrogen waste. A protein that 
is closely related to the Mo-MuLV receptor has 
been identified in liver tissue, and we are examin- 
ing its role in transport of ornithine across the 
mitochondrial membrane of hepatocytes, an im- 
portant step in this pathway. Also within the past 
few years, arginine has been identified as the sub- 
strate for nitric oxide, the proximal molecule in a 
cell-signaling pathway that is important in the 
control of blood pressure, nerve transmission, 
and host immune defense. Currently we are in- 
vestigating how the Mo-MuLV receptor/trans- 
porter can influence nitric oxide production by 
regulating arginine availability. 
The laboratory remains interested in how retro- 
viruses interact with the Mo-MuLV receptor to 
permit fusion with the target cell membrane and 
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