ABSTRACT
DNA replication is a central process in all living organisms. Polyomavirus DNA replication serves as a model system for eukaryotic DNA replication and has considerably contributed to our understanding of basic replication mechanisms. However, the details of the involved processes are still unclear, in particular regarding lagging strand synthesis. To delineate the complex mechanism of coordination of various cellular proteins binding simultaneously or consecutively to DNA to initiate replication, we investigated single-stranded DNA (ssDNA) interactions by the SV40 large T antigen (Tag). Using single molecule imaging by atomic force microscopy (AFM) combined with biochemical and spectroscopic analyses we reveal independent activity of monomeric and oligomeric Tag in high affinity binding to ssDNA. Depending on ssDNA length, we obtain dissociation constants for Tag-ssDNA interactions (KD values of 10-30 nM) that are in the same order of magnitude as ssDNA binding by human replication protein A (RPA). Furthermore, we observe the formation of RPA-Tag-ssDNA complexes containing hexameric as well as monomeric Tag forms. Importantly, our data clearly show stimulation of primase function in lagging strand Okazaki fragment synthesis by monomeric Tag whereas hexameric Tag inhibits the reaction, redefining DNA replication initiation on the lagging strand.
Subject(s)
Antigens, Polyomavirus Transforming/metabolism , DNA Replication , DNA, Single-Stranded/metabolism , Replication Protein A/metabolism , Adenosine Triphosphate/metabolism , DNA/metabolism , DNA Polymerase I/metabolism , DNA Primase/metabolism , DNA, Single-Stranded/chemistry , Protein Binding , Simian virus 40/immunologyABSTRACT
The replication factor Cdc45 has essential functions in the initiation and elongation steps of eukaryotic DNA replication and plays an important role in the intra-S-phase checkpoint. Its interactions with other replication proteins during the cell cycle and after intra-S-phase checkpoint activation are only partially characterized. In the present study, we show that the C terminal part of Cdc45 may mediate its interactions with Claspin. The interactions of human Cdc45 with the three replication factors Claspin, replication protein A and DNA polymerase δ are maximal during the S phase. Following UVC-induced DNA damage, Cdc45-Claspin complex formation is reduced, whereas the binding of Cdc45 to replication protein A is not affected. We also show that treatment of cells with UCN-01 and phosphatidylinositol 3-kinase-like kinase inhibitors does not rescue the UV-induced destabilization of Cdc45-Claspin interactions, suggesting that the loss of the interaction between Cdc45 and Claspin occurs upstream of ataxia telangiectasia and Rad 3-related activation in the intra-S-phase checkpoint.
Subject(s)
Adaptor Proteins, Signal Transducing/metabolism , Cell Cycle Proteins/metabolism , Cell Cycle/physiology , Adaptor Proteins, Signal Transducing/genetics , Cell Cycle/genetics , Cell Cycle Proteins/genetics , Cell Line , DNA Damage/genetics , DNA Damage/radiation effects , DNA Replication/genetics , DNA Replication/radiation effects , HumansABSTRACT
BK virus (BKV) causes persistent and asymptomatic infections in most humans and is the etiologic agent of polyomavirus-associated nephropathy (PVAN) and other pathologies. Unfortunately, there are no animal models with which to study activation of BKV replication in the human kidney and the accompanying PVAN. Here we report studies of the restriction of BKV replication in murine cells and extracts and the cause(s) of this restriction. Upon infection of murine cells, BKV expressed large T antigen (TAg), but viral DNA replication and progeny were not detected. Transfection of murine cells with BKV TAg expression vectors also caused TAg expression without accompanying DNA replication. Analysis of the replication of DNAs containing chimeric BKV and murine polyomavirus origins revealed the importance of BKV core origin sequences and TAg for DNA replication. A sensitive assay was developed with purified BKV TAg that supported TAg-dependent BKV DNA replication with human but not with murine cell extracts. Addition of human replication proteins, DNA polymerase alpha-primase, replication protein A, or topoisomerase I to the murine extracts with BKV TAg did not rescue viral DNA replication. Notably, addition of murine extracts to human extracts inhibited BKV TAg-dependent DNA replication at a step prior to or during unwinding of the viral origin. These findings and differences in replication specificity between BKV TAg and the TAgs of simian virus 40 (SV40) and JC virus (JCV) and their respective origins implicate features of the BKV TAg and origin distinct from SV40 and JCV in restriction of BKV replication in murine cells.
