Physical and functional interactions between human mitochondrial single-stranded DNA-binding protein and tumour suppressor p53

Single-stranded DNA-binding proteins (SSB) form a class of proteins that bind preferentially single-stranded DNA with high affinity. They are involved in DNA metabolism in all organisms and serve a vital role in replication, recombination and repair of DNA. In this report, we identify human mitochondrial SSB (HmtSSB) as a novel protein-binding partner of tumour suppressor p53, in mitochondria. It binds to the transactivation domain (residues 161) of p53 via an extended binding interface, with dissociation constant of 12.7 ( 0.7) M. Unlike most binding partners reported to date, HmtSSB interacts with both TAD1 (residues 140) and TAD2 (residues 4161) subdomains of p53. HmtSSB enhances intrinsic 3-5 exonuclease activity of p53, particularly in hydrolysing 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) present at 3-end of DNA. Taken together, our data suggest that p53 is involved in DNA repair within mitochondria during oxidative stress. In addition, we characterize HmtSSB binding to ssDNA and p53 N-terminal domain using various biophysical measurements and we propose binding models for both.