A unique subpopulation of murine DNA polymerase alpha/primase specifically interacts with polyomavirus T antigen and stimulates DNA replication

Polyomavirus large T antigen mutants affected in viral DNA replication

We have characterized two polyomavirus large T antigen mutants with different properties in viral DNA replication. dl-97, a mutant active in immortalization, exerts a dominant negative effect in viral DNA replication. 13val, which is defective in both immortalization and viral DNA replication, has a lesion in the putative DnaJ domain affecting the block of Rb function.

Enhanced binding to origin DNA at low pH enables easy detection of polyomavirus large T antigen by gel mobility shift assay of unfixed complexes

Enhanced, stable binding by polyomavirus large T antigen to the viral DNA replication origin at pH 6 allowed the development of a gel mobility shift assay for the detection of large T antigen. Such assays were not possible at pH 7.6 without previous fixation, due to instability of the complexes. We demonstrated that the gel mobility shift assay at pH 6 is very sensitive, allowing the detection of as little as 5 ng large T antigen, and is highly specific for DNA containing G(A/G)GGC target sequences. This method was used to detect large T antigen in crude cell lysates from transformed yeast cell lines or nuclear extracts from infected insect cells. Large T antigen-DNA complexes remained at or near the loading well in 5% acrylamide or 1.5% agarose gels, indicating that these complexes are very large. Glycerol gradient analysis showed that protein-DNA complexes formed at pH 6 were massive, and that large T antigen also formed large complexes when incubated at low pH in the absence of DNA. These results show that pH has a major effect on binding of large T antigen to its multiple target sites in the viral origin of DNA replication, presumably by affecting protein-protein interactions that are important for the stability of large T antigen-DNA complexes.

Polyomavirus large T can support DNA replication in human cells

Human cells are generally thought to be nonpermissive for polyomavirus (Py) DNA replication. Using transient transfection, we show that Py large T-antigen (LT) was able to support replication of a Py origin-containing plasmid in two human cell lines. Replication supported by LT in human cells was specific for the Py origin and required its enhancer sequences, as well as the previously reported critical phosphorylation sites within LT. Py replication efficiency was comparable to that of papillomavirus E1 and E2 activated DNA replication in transient assays performed in human 293 and C-33A cells. Previous analysis of DNA replication in vitro has pointed to polymerase alpha-primase as a specificity determinant for polyomavirus. The data presented here imply that in certain cellular environments, Py LT must functionally interact with human polymerase alpha-primase to permit DNA replication.

A DNA polymerase alpha accessory protein exhibits structural and functional similarities to SV40 large tumor antigen

Untransformed cells have been proposed to require a protein homologous to SV40 large tumor antigen (TAg) which functions as a component of the replicase complex during the initiation of DNA synthesis. By definition, this should be a phosphoprotein which interacts with the retinoblastoma protein (pRb) in G0 or early G1, and is capable of binding to and potentiating the activity of DNA polymerase alpha (pol alpha). This protein should also be an ATP-dependent helicase which interacts with the single-stranded DNA (ssDNA) binding protein, RP-A. Because of these requirements, a TAg homologous protein could be expected to contain epitopes with amino acid sequences similar to those of TAg at critical functional sites, such as ATP, pRb and pol alpha binding sites. TAg and a putative cellular homolog of TAg, DNA pol alpha accessory protein (alpha AP), were compared for pRb and pol alpha interaction, and for immunological identity. The analyses utilized immunoaffinity-purified TAg and pRb from a baculovirus expression system, and DNA pol alpha/primase and alpha AP chromatographically isolated from a mouse lymphocytic leukemia cell line. Monoclonal antibodies specific for the pol alpha or pRb binding sites on TAg interacted with alpha AP strongly enough to be employed for immunoaffinity purification of alpha AP. Anti-pRb and anti-TAg reciprocally coimmunoprecipitated pRb bound to TAg and pRb bound to alpha AP. The functional consequences of pol alpha interaction with TAg or alpha AP in the presence or absence of pRb was determined using pol alpha nucleotide incorporation assays. alpha AP exhibited the capacity to stimulate pol alpha activity, a capacity which was diminished in the presence of pRb. Lastly, TAg and alpha AP independently co-purified with pol alpha through a multi-step chromatographic protocol. These data indicate that a pol alpha accessory protein, alpha AP, exhibits functional and immunological similarities to SV40 TAg, suggest that alpha AP is involved in regulation of the initiation of DNA synthesis, and support the proposal that alpha AP may be a normal cell protein homologous to SV40 large T antigen.

Bovine papillomavirus E1 protein binds specifically DNA polymerase alpha but not replication protein A

Extracts prepared from either mouse cells or monkey cells were examined for the ability to support in vitro bovine papillomavirus type 1 (BPV1) DNA replication, and they were used in parallel as a source of host replication proteins for affinity chromatography. DNA synthesis exhibited an absolute requirement for BPV1 E1 protein. In contrast to previous observations, we found that low levels of E1 were highly efficient in initiating DNA replication in the absence of the BPV1 transcription factor E2. Surprisingly, COS-1 cell extract allowed a high rate of BPV1 DNA replication, supporting an efficient production of mature circular DNA molecules, whereas in mouse cell extracts, the replication products mostly consisted of replicative intermediates. Submitting the extracts to affinity chromatography allowed specific binding of DNA polymerase alpha-primase to E1 protein, up to a total depletion of the extract, regardless of the origin of the cell extract. Furthermore, replication protein A was not retained on E1 affinity columns, even when E2 was complexed with E1. These data confirm that the interactions between E1 and DNA polymerase alpha-primase do not exhibit cell-type specificity, as had already been suggested by data from in vivo and in vitro replication assays, but they imply that other cellular proteins may affect the level of E1-dependent replication.