Efficient secretion of the herpes simplex virus tegument protein VP22 from living mammalian cells

Many studies show that a tegument protein, VP22, of herpes simplex virus possesses an unusual capacity for intercellular trafficking, while several studies have reported that the intercellular trafficking was observed only in cells after fixation. Therefore, the trafficking ability in living cells remains controversial. To settle the question, we first examined secretion of VP22 in living cells. In this report, we fused VP22 with beta-galactosidase (betaGal) and investigated the secretion of VP22-betaGal in living cells by monitoring betaGal activity in the culture medium. Under our conditions, a significant amount of VP22-betaGal was detected in the culture medium, and it increased with time. Particularly, 6 days after transfection, 72% of all VP22-betaGal expressed was detected in the culture medium. Lactate dehydrogenase assays revealed that leakage of VP22-betaGal from damaged cells was not the main cause of the high level of secretion. We thus conclude that VP22 possesses a remarkable ability to be secreted from living cells.