Structure of two stable, complex capsidsEnteric and fowl adenoviruses

Resumen

Adenoviruses have a non-enveloped icosahedral capsid with a pseudo T=25 geometry. They are relatively large— with a diameter of 900 Å, enclosing a dsDNA genome in the range of 25-49 kbp (depending on the adenovirus genus) accompanied by virus encoded “histone-like” proteins. Adenoviruses infect a broad range of vertebrate hosts and are traditionally studied due to their pathogenicity in immunocompromised human individuals and in certain animal species. In particular human enteric adenoviruses are of interest because they constitute one of the main causes of viral gastroenteritis in the world and must withstand the harsh conditions found in the gut. This requirement suggests that capsid stability must be different from that of other adenoviruses. On the other hand, the use of non-human adenoviruses has been proposed as alternative vectors in terms of evading the pre-existing immune response against human adenovirus-based vectors. In this thesis we have assessed the thermal stability of an enteric human adenovirus (HAdV-F41) and a fowl adenovirus (FAdV-C4). HAdV-F41 and notably FAdV-C4, are more thermostable than the prototype respiratory adenovirus HAdV-C5. We have determined the structure of HAdV-F41 at 4 Å resolution and of FAdV-C4 at 3.3 Å by single particle averaging cryo-electron microscopy. We have compared the structures with those of other adenoviruses with respiratory (HAdV-C5) and ocular (HAdV-D26) tropisms, as well as adenoviruses infecting lizards (LAdV-2) and cows (BAdV-3). Overall, major coat proteins reveal conservation among adenoviruses, although the organization of surface-exposed loop regions is substantially divergent in HAdV-F41 and FAdV-C4 from the prototype HAdV-C5. Unexpectedly, the organization of the external cementing protein (IX) in HAdV-F41 is unique compared to all previously characterized adenoviruses. In FAdV-C4, internal minor capsid proteins (VIII and IIIa) show important rearranged domains, as compared to other adenovirus structures. Finally, we provide preliminary insights on the different organization of the nucleoproteic core of FAdVC4. Variance in the core organization among adenoviruses might play a role in determining the stability features. All this knowledge could inform on modifications to use HAdV-F41-based oral vaccines with a longer “shelf-life” than drugs relying on HAdV-C5. Design of highly stable vectors, with reduced immunogenic response, based on FAdVs would be also of interest