Structure of the fiber head of Ad3, a non-CAR-binding serotype of adenovirus

Adenoviruses of serotype Ad3 (subgenus B) use a still-unknown host cell receptor for viral attachment, whereas viruses from all other known subgenera use the coxsackie and adenovirus receptor (CAR). The receptor binding domain (head) of the Ad3 fiber protein has been expressed in Escherichia coli inclusion bodies. After denaturation and renaturation using a rapid dilution method, crystals of trimeric head were obtained. The 1.6 A resolution X-ray structure shows a strict conservation of the beta-sheet scaffold of the protein very similar to the head structures of the CAR-binding serotypes Ad2, Ad5, and Ad12. The conformation of the loops is different, with the exception of the AB loop, which forms the center of the interface in the Ad12-CAR complex structure. The structure explains why a mutation in Ad5 of one residue in the AB loop to glutamic acid, as in Ad3, abrogates binding to CAR. It is possible that the Ad3 receptor binding site is nevertheless situated similar to the CAR binding site, although it cannot be excluded that other regions of the relatively hydrophobic head surface may be used.