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 Farber 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 and Harvard Medical School 
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- 
binding protein, called a receptor, are not suscep- 
tible to infection by a particular virus. Indeed, 
the ability of many pathogenic viruses, such as 
human immunodeficiency virus, poliovirus, and 
certain herpesviruses, to infect specific host tis- 
sues has been closely correlated with the expres- 
sion of specific receptors. 
Our laboratory has been interested in the mech- 
anism of infection utilized by Moloney murine 
leukemia virus (MoMuLV) , a member of a group 
of related leukemogenic retroviruses found in vir- 
tually all vertebrates. We have isolated a molecu- 
lar clone, MCAT, which confers MoMuLV infectiv- 
ity upon introduction into mammalian cells that 
are not normally susceptible to infection. Subse- 
quent experiments have demonstrated that MCAT 
encodes for a membrane protein that serves as the 
MoMuLV receptor. Our current research is ad- 
dressed toward dissecting the molecular details 
of the virus-receptor interaction that mediates in- 
fection and understanding the function of the re- 
ceptor in normal cell metabolism. 
The MoMuLV receptor is not present in mice 
for the convenience of the virus, but rather pro- 
vides a portal for entry of lysine, arginine, and 
ornithine, amino acids that carry a net positive 
charge. There is a similarity between MCAT and 
two membrane proteins in yeast that are also 
amino acid transporters. This suggests conserva- 
tion of a single mechanism for transport of these 
amino acids over evolutionary time and predicts 
that proteins similar to the MoMuLV receptor are 
used by all animals. Inherited disorders of cat- 
ionic amino acid transport have been described 
in patients that may be explained by mutations in 
MCAT genes, a hypothesis we are now examining. 
A protein that is closely related to the MoMuLV 
receptor has been identified in liver tissue. We 
have demonstrated that this protein is also an 
amino acid transporter with properties that may 
help explain the specific requirements for argi- 
nine metabolism by the liver: the capacity of he- 
patocytes to clear the portal vein of the high con- 
centration of amino acids present after a big meal 
and the regulation of the urea cycle, the meta- 
bolic pathway used to eliminate nitrogen waste. 
Related forms of this protein are expressed in T 
and B lymphocytes and macrophages that are ac- 
tivated as part of the host response to bacterial 
and parasitic infection. Within the past few years, 
arginine has been identified as the substrate for 
nitric oxide, an important mediator of the host 
defense against these pathogens. Currently we 
are investigating how the family of related amino 
acid transporters can influence nitric oxide pro- 
duction by regulating arginine availability. As 
part of these studies, we have identified a molecu- 
lar clone that encodes nitric oxide synthase, the 
enzyme that is responsible for the synthesis of 
nitric oxide from arginine. Expression of the gene 
encoding this enzyme is stimulated in macro- 
phages that are activated in response to infection. 
Our laboratory remains interested in how retro- 
viruses interact with MCAT to gain entry into the 
cell. Recently we have prepared an antibody that 
can recognize the MCAT protein and thereby per- 
mits examination of its synthesis in infected and 
uninfected cells. These studies have demon- 
strated that MCAT is normally modified by the 
addition of carbohydrate during its transit from 
the ribosome to the plasma membrane. In cells 
that have been infected with MoMuLV, we have 
identified an interaction between MCAT and the 
virus envelope protein that impairs MCAT matura- 
tion and decreases amino acid uptake. This de- 
crease may result from failure of the mature 
MCAT protein to arrive at the plasma membrane. 
This finding has important implications for un- 
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