Simian virus Large T antigen interacts with the N-terminal domain of the 70 kD subunit of Replication Protein A in the same mode as multiple DNA damage response factors.

Simian virus 40 (SV40) serves as an important model organism for studying eukaryotic DNA replication. Its helicase， Large T-antigen (Tag)， is a multi-functional protein that interacts with multiple host proteins， including the ubiquitous ssDNA binding protein Replication Protein A (RPA). Tag recruits RPA， actively loads it onto the unwound DNA， and together they promote priming of the template. Although interactions of Tag with RPA have been mapped， no interaction between Tag and the N-terminal protein interaction domain of the RPA 70kDa subunit (RPA70N) has been reported. Here we provide evidence of direct physical interaction of Tag with RPA70N and map the binding sites using a series of pull-down and mutational experiments. In addition， a monoclonal anti-Tag antibody， the epitope of which overlaps with the binding site， blocks the binding of Tag to RPA70N. We use NMR chemical shift perturbation analysis to show that Tag uses the same basic cleft in RPA70N as multiple of DNA damage response proteins. Mutations in the binding sites of both RPA70N and Tag demonstrate that specific charge reversal substitutions in either binding partner strongly diminish the interaction. These results expand the known repertoire of contacts between Tag and RPA， which mediate the many critical roles of Tag in viral replication.