MOLECULAR BIOLOGY OF ROTAVIRUS INFECTION 
Carlos F. Arias, Ph.D., International Research Scholar 
Acute, infectious diarrhea is the commonest cause 
of morbidity and mortality among young children 
living in developing countries, accounting for as 
many as 1 billion illnesses and between 4 and 5 
million deaths each year. Rotaviruses are the leading 
cause of severe gastroenteritis in children under 
three years of age, and there is considerable interest 
in developing effective vaccination strategies. 
Fundamental to the construction of an effective 
vaccine is a basic understanding of the viral determi- 
nants that elicit protective immunity. In addition, a 
detailed knowledge of the molecular biology of the 
virus, especially of the initial events that lead to in- 
fection of the target cell, should contribute to the 
rational design of rotavirus vaccines and broaden 
the approaches to control of the viral infection. 
The main interest of Dr. Arias and his colleagues is 
to study the molecular biology of rotaviruses, with 
particular emphasis on the interactions of the virion 
with the host cell early in the infection cycle. In 
addition, they are interested in the host immune re- 
sponse to rotavirus infection and the epidemiology 
of rotaviruses in Mexico. 
Attachment and Penetration of Rotaviruses 
The surface of the virus is formed by two proteins, 
VP4 and VP7. VP4 forms spikes that extend from the 
viral surface and is involved in a variety of viral 
functions, including virulence in vivo, agglutina- 
tion of red blood cells (hemagglutination), and 
penetration of the virion into target epithelial cells. 
In contrast, the protein responsible for the initial 
attachment of rotavirus to the target cell remains 
controversial; both VP4 and VP7 have been pro- 
posed to play this role. 
Hemagglutination and the attachment of animal 
rotaviruses to epithelial cells are mediated by sialic 
acid-containing compounds, since treatment of 
cells with sialidases inhibits hemagglutination and 
greatly reduces the binding of virus to the host cell 
surface. In addition, hemagglutination and the bind- 
ing to target cells can be inhibited by incubation of 
the virus with a wide range of sialoglycoproteins, 
including glycophorin A, the protein in the red 
blood cell membrane that mediates the binding of 
the virus. Recently several different sialic acid- 
containing membrane components have been pro- 
posed to serve as binding sites for rotavirus particles 
in epithelial cells. 
Virus mutants whose infectivity of epithelial cells 
is no longer inhibited by treatment of cells with si- 
alidases or incubation with glycophorin have been 
isolated by Dr. Arias and his co-workers. Paradoxi- 
cally, these mutants are still able to hemagglutinate, 
and this hemagglutination is inhibited by glyco- 
phorin. Apparently glycophorin is able to bind to 
the mutant viruses but no longer prevents infection, 
suggesting that there is a second site on their sur- 
face, independent of the sialic acid-binding site, 
that is primarily responsible for interaction with the 
host cell. 
Preliminary reassortment experiments indicate 
that this mutant phenotype segregates with the VP4 
gene. The group has proposed as a working hypoth- 
esis that there are at least two sites on the VP4 pro- 
tein of animal rotaviruses that interact with the cell 
surface during virus entry. In this model the first site 
mediates the initial interaction between the virus 
and the cell through a cell membrane sialic acid- 
containing structure, but this interaction alone does 
not lead to infection. After this initial event the sec- 
ond site mediates an interaction, probably with an- 
other cell membrane component, leading to pene- 
tration of the virion into the cell. 
Previous reports indicate that rotaviruses bind to a 
wide variety of cell types. Productive infection, 
however, has only been observed to occur in a sub- 
set of the tested cells. This suggests that the first 
interaction is of a promiscuous nature, while the 
second is more specific and probably determines, at 
least in some cases, whether the cell is susceptible 
to infection. 
The ability of the mutant viruses to infect in the 
presence of glycophorin or sialidase-treated ceils 
could be explained if the mutation(s) had increased 
the affinity of the second interaction, such that the 
second site were able to interact effectively with the 
cell surface even in the absence of the first interac- 
tion. The infection of human rotaviruses is not de- 
pendent on sialic acids and is not inhibited by glyco- 
phorin, which would suggest that in this case the 
interaction of the human virus with the cell surface 
could be directly through a site similar to the sec- 
ond site identified in animal rotaviruses. Experi- 
ments are in progress to map the domains on VP4 
responsible for these interactions. 
The infectivity of rotaviruses is increased, and is 
probably dependent on, trypsin treatment of the 
virus. Previously, Dr. Arias and his co-workers iden- 
tified two specific cleavage sites on VP4, at con- 
served arginines 241 and 247, and proposed that 
cleavage at these sites enhances infectivity. Recently 
INTERNATIONAL RESEARCH SCHOLARS 499 
