A stable tetramer is not the only oligomeric state that mitochondrial single-stranded DNA binding proteins can adopt

Mitochondrial single-stranded DNA (ssDNA)-binding proteins (mtSSBs) are required for mitochondrial DNA replication and stability and are generally assumed to form homotetramers, and this species is proposed to be the one active for ssDNA binding. However, we recently reported that the mtSSB from Saccharomyces cerevisiae (ScRim1) forms homotetramers at high protein concentrations, whereas at low protein concentrations, it dissociates into dimers that bind ssDNA with high affinity. In this work, using a combination of analytical ultracentrifugation techniques and DNA binding experiments with fluorescently labeled DNA oligonucleotides, we tested whether the ability of ScRim1 to form dimers is unique among mtSSBs. Although human mtSSBs and those from Schizosaccharomyces pombe, Xenopus laevis, and Xenopus tropicalis formed stable homotetramers, the mtSSBs from Candida albicans and Candida parapsilosis formed stable homodimers. Moreover, the mtSSBs from Candida nivariensis and Candida castellii formed tetramers at high protein concentrations, whereas at low protein concentrations, they formed dimers, as did ScRim1. Mutational studies revealed that the ability to form either stable tetramers or dimers depended on a complex interplay of more than one amino acid at the dimer-dimer interface and the C-terminal unstructured tail. In conclusion, our findings indicate that mtSSBs can adopt different oligomeric states, ranging from stable tetramers to stable dimers, and suggest that a dimer of mtSSB may be a physiologically relevant species that binds to ssDNA in some yeast species.