Phylogenetic analysis and structural predictions of human adenovirus penton proteins as a basis for tissue-specific adenovirus vector design.

The penton base is a major capsid protein of human adenoviruses (HAdV) which forms the vertices of the capsid and interacts with hexon and fiber protein. Two hypervariable loops of the penton are exposed on the capsid surface. Sequences of these and 300 adjacent amino acid residues of all 51 HAdV and closely related simian adenoviruses were studied. Adjacent sequences and predicted overall secondary structure were conserved. Phylogenetic analysis revealed clustering corresponding to the HAdV species and recombination events in the origin of HAdV prototypes. All HAdV except serotypes 40 and 41 of species F exhibited an integrin binding RGD motif in the second loop. The lengths of the loops (HVR1 and RGD loops) varied significantly between HAdV species with the longest RGD loop observed in species C and the longest HVR1 in species B. Long loops may permit the insertion of motifs that modify tissue tropism. Genetic analysis of HAdV prime strain p17'H30, a neutralization variant of HAdV-D17, indicated the significance of nonhexon neutralization epitopes for HAdV immune escape. Fourteen highly conserved motifs of the penton base were analyzed by site-directed mutagenesis of HAdV-D8 and tested for sustained induction of early cytopathic effects. Thus, three new motifs essential for penton base function were identified additionally to the RGD site, which interacts with a secondary cellular receptor responsible for internalization. Therefore, our penton primary structure data and secondary structure modeling in combination with the recently published fiber knob sequences may permit the rational design of tissue-specific adenoviral vectors.