Infectious Adenovirus Type 2 Transport Through Early but not Late Endosomes

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Receptor-mediated endocytosis is a major gate for pathogens into cells. In this study, we analyzed the trafficking of human adenovirus type 2 and 5 (Ad2/5) and the escape-defective temperature-sensitive Ad2-ts1 mutant in epithelial cancer cells. Ad2/5 and Ad2-ts1 uptake into endosomes containing transferrin, major histocompatibility antigen 1 and the Rab5 effector early endosome antigen 1 (EEA1) involved dynamin, amphiphysin, clathrin and Eps15. Cointernalization experiments showed that most of the Ad2/5 and Ad2-ts1 visited the same EEA1-positive endosomes. In contrast to Ad2/5, Ad2-ts1 required functional Rab5 for endocytosis and lysosomal transport and was sensitive to the phosphatidyl-inositol-3 (PI3)-kinase inhibitor wortmannin or the ubiquitin-binding protein Hrs for sorting from early to late endosomes. Endosomal escape of Ad2 was not affected by incubation at 19°C, which blocked membrane sorting in early endosomes and inhibited Ad2-ts1 transport to lysosomes. Unlike Semliki Forest Virus (SFV), sorting of Ad2-ts1 to late endosomes was independent of Rab7 and Ad2/5 infection independent of EEA1. The data indicate that Ad2/5 and Ad2-ts1 use an invariant machinery for clathrin-mediated uptake to early endosomes. We suggest that the infectious Ad2 particles are either directly released from early endosomes to the cytosol or sorted by a temperature-insensitive and PI3-kinase-independent mechanism to an escape compartment different from late endosomes or lysosomes.

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