The 21-kDa polypeptide (VAP21) in the rabies virion is a CD99-related host cell protein.

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.