Characterization of a CD99-related 21 -kDa cellular protein(VAP21) incorporated into rabies virion

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The VAP21, a CD99-related 21-kDa cellular transmembrane protein, was expressed in various types of organs of Syrian hamster as well as the hamster-derived cell lines (BHK-21 and HmLu-1). The protein displayed heterogeneous electrophoretic mobilities depending on the organs and cell lines probably due to different posttranslational modifications. The VAP21, however, was not expressed in other animal species we tested, including Chinese hamster, mouse, dog, monkey and human. We tried to express the VAP21 in VAP21-negative cell lines, such as CHO-K1 , HeLa, COS-7 and MDCK, using a tetracycline-regulated gene expression system. All of the trials, however, resulted in obtaining no permanently positive nor inducible cell lines : at the beginning of the cDNA transfection, cells were transiently positive for VAP21 expression, but the positive population decreased to undetectable levels within a few weeks, even though not a few G418-resistant clones were obtained. To the contrary, we could easily establish the VAP21-overexpressing cell lines from Syrian hamster cells, which were successfully maintained without any loss of VAP21 expression even under the induced culture conditions. Furthermore, although transfection of the VAP21 cDNA affected the recipient cells differently depending on the cell types, the VAP21 expression was somewhat of benefit to infection with the vesicular stomatitis virus as seen in the increased yield of progeny viruses. Interestingly, although VAP21 antigen was diffusely distributed as very fine dots over the entire cell surface of BHK-21 and HmLu-1 cells, alteration of the distribution pattern such as the development of bleb-shaped spots on the cell surface was observed both in BHK-21/VAP21 and HmLu-1/VAP21 cells. These results provide important implications for potential role of VAP21 in cellular function.