The fiber knob carries the type-specific gamma-antigen which can be demonstrated in hemagglutination inhibition tests. In order to characterize the gamma-determinant we selected subgenus DI adenovirus serotypes 9 and 19 (Ad9 and Ad19) which exhibited 29 amino acid exchanges in the knob domain. Like all subgenus DI adenoviruses they showed a complete hemagglutination pattern with rat and human erythrocytes. We constructed a total of 14 chimeric Ad9/Ad19 and Ad19/Ad9 fiber proteins, which possessed fiber knobs with progressively exchanged Ad9 and Ad19 amino acids. Furthermore, we created 39 fiber proteins with distinct amino acid exchanges in the knob regions by primer-directed mutagenesis. The proteins were expressed in Escherichia coli and tested in hemagglutination and hemagglutination inhibition tests. From our results we can conclude that the type-specific gamma-determinant is not restricted to a distinct region on the adenovirus fiber knob but is composed of at least 17 amino acids. Most of the amino acids contributing to the Ad9 and Ad19 gamma-determinants are located on the fiber knob loops.

The adenovirus fibre carries the type-specific gamma determinant the existence of which was suggested by haemagglutination inhibition tests. Furthermore, the fibre is thought to be responsible for the agglutination of monkey, rat and human erythrocytes. In order to verify that the haemagglutination properties and the type-specific gamma determinant are located on the fibre knob domain, several recombinant fibre proteins of subgenus D adenoviruses were constructed, expressed in HeLa cells and tested in haemagglutination and haemagglutination inhibition tests. Our data showed that the epitopes responsible for the interaction with erythrocytes and the gamma determinant are located on the fibre knob domain.

The fiber and hexon genes of the immunological adenovirus (Ad) variant strains Ad9/Hx and Ad15/Hx, belonging to subgenus D, were sequenced and compared to the corresponding sequences of the prototypes Ad9 and Ad15. It was found that the variants possessed a novel and identical hemagglutinin as they could not be distinguished by hemagglutination-inhibition tests. These serological data were now confirmed on DNA and protein level. The analyzed hexon regions of Ad15 and Ad15/Hx, and Ad9 and Ad9/Hx, respectively, were 100% identical on the amino acid level. The comparison between the variant fibers revealed that they possessed an identical fiber, which was distinct from the Ad9 and Ad15 fiber polypeptides. It seems likely that the donor of the novel fiber protein was a so far unidentified subgroup D adenovirus. The fiber polypeptides of Ad9 and the variants revealed the highest homology (73.7%), whereas the variants and the Ad15 fiber had only 64.2% of the amino acids in common. Most of the differences between the adenovirus serotypes were detected in the fiber knob. Seven conserved sequences found for subgenus D fibers could be confirmed for the fibers of the variants. Furthermore, the data presented complement the knowledge on the organization of subgenus D fiber genes.