In our monoclonal antibody (MAb) stocks prepared against the BHK-21 cell 
antigens, two (#11875 and 28276) recognized a 21-kDa polypeptide (referred to as 
VAP21) which is efficiently incorporated into the rabies virion. By using these 
MAbs, we isolated the cDNA clones that encoded a polypeptide of 144 amino acids 
from our BHK-21 cell cDNA library. Based on the following evidence, the cDNA was 
assumed to encode a full-length sequence of VAP21 antigen: i) expression of the 
cDNA in animal cells resulted in the production of a polypeptide recognized by 
the two MAbs, and its electrophoretic mobility was the same as that of authentic 
VAP21 antigen; and ii) immunization with the products from the cDNA-transformed 
E. coli cells raised specific antibodies in rabbits that recognized a 21-kDa 
polypeptide in the virion. From the deduced amino acid sequence, it is suggested 
that the VAP21 antigen has a molecular structure of type-I transmembrane protein 
containing characteristic proline-rich and glycine-rich regions in its 
ectodomain. Homology searches resulted in finding homologous sequences (totally 
about 40% homology) in the human MIC2 gene product (CD99; 32-kDa) of T 
lymphocytes. These results suggest that the VAP21 antigen in the rabies virion 
is a cellular CD99-related transmembrane protein.