Human herpesvirus-6A (HHV-6A) and HHV-6B integrate their genomes into the
telomeres of human chromosomes, however, the mechanisms leading to integration
remain unknown. HHV-6A/B encode a protein that has been proposed to be
involved in integration termed U94, an ortholog of adeno-associated virus type
2 (AAV-2) Rep68 integrase. In this report, we addressed whether purified
recombinant maltose-binding protein (MBP)-U94 fusion proteins of HHV-6A/B
possess biological functions compatible with viral integration. We could
demonstrate that MBP-U94 efficiently binds both dsDNA and ssDNA containing
telomeric repeats using gel shift assay and surface plasmon resonance. MBP-U94
is also able to hydrolyze adenosine triphosphate (ATP) to ADP, providing the
energy for further catalytic activities. In addition, U94 displays a 3′ to 5′
exonuclease activity on dsDNA with a preference for 3′-recessed ends. Once the
DNA strand reaches 8–10 nt in length, the enzyme dissociates it from the
complementary strand. Lastly, MBP-U94 compromises the integrity of a synthetic
telomeric D-loop through exonuclease attack at the 3′ end of the invading
strand. The preferential DNA binding of MBP-U94 to telomeric sequences, its
ability to hydrolyze ATP and its exonuclease/helicase activities suggest that
U94 possesses all functions required for HHV-6A/B chromosomal integratio
