Identification and analysis of a novel heparin-binding glycoprotein encoded by human herpesvirus 7

Human herpesvirus 6 (HHV-6) and HHV-7 are closely related betaherpesviruses that encode a number of genes with no known counterparts in other herpesvi ruses. The product of one such gene is the HHV-6 glycoprotein gp82-105, whi ch is a major virion component and a target for neutralizing antibodies. A 1.7-kb cDNA clone from HHV-7 was identified which contains a large open rea ding frame capable of encoding a predicted primary translational product of 468 amino acids (54 kDa) with 13 cysteine residues and 9 potential N-linke d glycosylation sites. This putative protein, which we have termed gp65, wa s homologous to HHV-6 gp105 (30% identity) and contained a single potential membrane-spanning domain located near its amino terminus. Comparison of th e cDNA sequence with that of the viral genome revealed that the gene encodi ng gp65 contains eight exons, spanning almost 6 kb of the viral genome at t he right (3') end of the HHV-7 genome. Northern (RNA) blot analysis with po ly(A)(+) RNA from HHV-7-infected cells revealed that the cDNA insert hybrid ized to a single major RNA species of 1.7 kb. Antiserum raised against a pu rified, recombinant form of gp65 recognized a protein of roughly 65 kDa in sucrose density gradient-purified HHV-7 preparations; treatment with PNGase F reduced this glycoprotein to a putative precursor of approximately 50 kD a. Gp65-specific antiserum also neutralized the infectivity of HHV-7, while matched preimmune serum did not do so. Finally, analysis of the biochemica l properties of recombinant gp65 revealed a specific interaction with hepar in and heparan sulfate proteoglycans and not with closely related molecules such as N-acetylheparin and de-N-sulfated heparin, At least two domains of the protein were found to contribute to heparin binding. Taken together, t hese findings suggest that HHV-7 gp65 may contribute to viral attachment to cell surface proteoglycans.