Epstein-Barr virus single-stranded DNA-binding protein: purification, characterization, and action on DNA synthesis by the viral DNA polymerase

Abstract

Epstein-Barr virus (EBV) BALF2 gene product is one of the essential components in the lytic phase of the EBV DNA replication. The BALF2 protein was purified to near homogeneity from the nuclear extract of B95-8 cells with virus productive cycle induced by chemical agents. SDS-polyacrylamide gel electrophoresis showed the presence of a single polypeptide with a molecular weight of 130 K, which was identified as BALF2 protein by Western immunoblot analysis. On Superose 6 HR 10/30 gel filtration the BALF2 protein eluted at a position corresponding to an apparent molecular mass of approximately 128 K, indicating that the BALF2 protein behaves as a monomer in solution. The purified BALF2 protein bound to single-stranded DNA preferentially over double-stranded DNA or single-stranded RNA. Replication of singly primed M13 single-stranded DNA by the EBV DNA polymerase complex in the absence of the BALF2 protein exhibited a highly processive mode of replication and generated full length products in addition to some bands of pausing sites. Although the addition of the BALF2 protein did not affect the replication rate, the average chain length of the replication products was slightly increased with eliminating bands of pausing sites. Similar effects were observed with the reconstituted polymerase complex composed of the BALF5 and BMRF1 Pol subunits. On the other hand, in the absence of the BALF2 protein, the BALF5 Pol catalytic subunit alone extended the primer slightly and paused at specific sites on M13 ssDNA template where stable secondary structure is predicted. However, addition of the BALF2 protein, in contrast to the case of herpes simplex virus ICP8 which does not affect the overall distribution of length of the replication products synthesized by the HSV Pol catalytic subunit (Gottlieb et al., 1990, J. Virol. 64, 5976-5987), stimulated DNA synthesis and yielded a distribution of replication products with long lengths in addition to full length products. Although the BALF2 protein behaved as if it converts a low processive enzyme of the EBV Pol catalytic subunit to a highly processive form like the BMRF1 Pol accessory subunit, challenger DNA experiments revealed that the EBV Pol catalytic subunit is transferred to challenger DNA even in the presence of the BALF2 protein. It is therefore likely that the EBV BALF2 protein functions to melt out the regions of secondary structure on the single-stranded DNA template, thereby reducing and eliminating pausing of the EBV DNA polymerase at specific sites. These properties indicate that the EBV BALF2 protein acts as a single-stranded DNA-binding protein during lytic phase of EBV DNA replication.