Adenovirus serotype 7 retention in a late endosomal compartment prior to cytosol escape is modulated by fiber protein

The intracellular trafficking of adenovirus (Ad) subgroup B (e.g,, Ad7) dif fers from that of subgroup C (e.g., Ad5) in that Ad5 rapidly escapes from e ndocytic compartments following infection whereas Ad7 accumulates in organe lles. To assess the hypothesis that Ad7 is targeted to the lysosomal pathwa y, Ad7 and Ad5 were conjugated with fluorophores and their trafficking in A 549 epithelial cells was analyzed by fluorescence microscopy. Within 1 h af ter infection, Ad7, but not Ad5, accumulated in the cytoplasm of A549 cells . The pH in the environment of Ad5 was nearly neutral (pH 7), while Ad7 occ upied acidic compartments (pH 5) over the first 2 h with a gradual shift to ward neutrality by 8 h. Ad7 partially colocalized with alpha (2)-macroglobu lin and late endosomal and lysosomal marker proteins, including Rab7, manno se-6-phosphate receptor, and LAMP-1. The pH optimum for membrane lysis by A d7, as well as a chimeric Ad5 capsid that expressed the Ad7 fiber (Ad5fiber 7), was pH 5.5, while that for lysis by Ad5 was pH 6.0. Thus, the native tr afficking pathway for Ad7 involves residence in late endosomes and lysosome s, with information encoded in the Ad7 fiber acting as a pa-dependent trigg er for membrane lysis and escape to the cytosol.