Cryo-Electron Microscopy Structure of Adenovirus Type 2 Temperature-Sensitive Mutant 1 Reveals Insight into the Cell Entry Defect

Abstract

The structure of the adenovirus type 2 temperature-sensitive mutant 1 (Ad2 ts 1) was determined to a resolution of 10 Å by cryo-electron microscopy single-particle reconstruction. Ad2 ts 1 was prepared at a nonpermissive temperature and contains the precursor forms of the capsid proteins IIIa, VI, and VIII; the core proteins VII, X (mu), and terminal protein (TP); and the L1-52K protein. Cell entry studies have shown that although Ad2 ts 1 can bind the coxsackievirus and Ad receptor and undergo internalization via αv integrins, this mutant does not escape from the early endosome and is targeted for degradation. Comparison of the Ad2 ts 1 structure to that of mature Ad indicates that Ad2 ts 1 has a different core architecture. The Ad2 ts 1 core is closely associated with the icosahedral capsid, a connection which may be mediated by preproteins IIIa and VI. Density within hexon cavities is assigned to preprotein VI, and membrane disruption assays show that hexon shields the lytic activity of both the mature and precursor forms of protein VI. The internal surface of the penton base in Ad2 ts 1 appears to be anchored to the core by interactions with preprotein IIIa. Our structural analyses suggest that these connections to the core inhibit the release of the vertex proteins and lead to the cell entry defect of Ad2 ts 1.