A biophysical and structural analysis of DNA binding by oligomeric hSSB1 (NABP2/OBFC2B)

The oxidative modification of DNA can result in the loss of genome integrity and must be repaired to maintain overall genomic stability. We have recently demonstrated that human single stranded DNA binding protein 1 (hSSB1/NABP2/OBFC2B) plays a crucial role in the removal of 8-oxo-7,8-dihydro-guanine (8-oxoG), the most common form of oxidative DNA damage. The ability of hSSB1 to form disulphide-bonded tetramers and higher oligomers in an oxidative environment is critical for this process. In this study, we have used nuclear magnetic resonance (NMR) spectroscopy and surface plasmon resonance (SPR) experiments to determine the molecular details of ssDNA binding by oligomeric hSSB1. We reveal that hSSB1 oligomers can open up damaged dsDNA and interact with individual single strands; however, our data also show that oxidised bases are recognised in the same manner as undamaged DNA bases. NMR experiments provide evidence that oligomeric hSSB1 is able to bind longer ssDNA in both binding polarities using a distinct set of residues different to those of the related SSB from Escherichia coli. We further demonstrate that oligomeric hSSB1 recognises ssDNA with a significantly higher affinity than its monomeric form and propose structural models for oligomeric hSSB1-ssDNA interaction.