Non-classical export of an adenovirus structural protein.

The icosahedral capsids of Adenoviruses (Ads) consist of the hexon and stabilizing proteins building the facettes, and of the vertex protein penton base (Pb) anchoring the protruding fibers. The fibers bind to the Coxsackie virus B Ad cell surface receptor (CAR) and Pb to integrins. Here we describe a novel property of the Ad2 Pb. Pb was found to leave the infected cell and, upon exit, it attached to the surrounding noninfected cells forming a radial gradient with highest Pb levels on cells adjacent to the infected cell. The producer cells remained intact until at least 30 h post infection. At this point, Pb was not recovered from the extracellular medium, suggesting that its cell-cell spread might not involve free Pb. When viral particles were released at late stages of infection, soluble Pb was found in the extracellular medium and it randomly bound to noninfected cells. Nonlytic export of Pb occurred upon transient transfection with plasmid DNA, but plasmid-encoded fiber was not exported, indicating that cell-cell spread of Pb is autonomous of infection. Pb export was not affected by Brefeldin A-induced disruption of the Golgi apparatus, suggesting that it occurred via a nonclassical mechanism. Interestingly, the coexpression of Pb and fiber leads to both Pb and fiber export, termed 'protein abduction'. We suggest that fiber abduction might support viral dissemination in infected tissues by interfering with tissue integrity.