The T Cell Repertoire 
ture and function of their protein products have 
only recently been determined. They turn out to 
be unusual proteins. Although they are bound to 
the surface membranes of cells, they differ from 
most surface proteins, in that the amino acids of 
their carboxyl terminal rather than amino termi- 
nal lie outside the cell. Only a small amino- 
terminal stretch of the protein lies inside. Experi- 
ments show that this inside portion is not 
essential for the superantigenic properties of the 
viral proteins, but that the proteins must be mem- 
brane bound in order to engage T cell receptors. 
The extreme carboxyl-terminal set of amino acids 
are among those that bind to T cell receptor V/Js. 
In attempts to find out more about how these 
viral superantigens operate, monoclonal antibod- 
ies have been raised to various parts of the pro- 
teins. The antibodies reveal that only a few of 
these proteins are expressed on the cell 
surface — probably about 1 ,000 per cell. Despite 
this small number, the viral superantigens are ex- 
tremely effective stimulators of T cell responses. 
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