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  ( Sc Rim1) 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  Sc Rim1 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  Sc Rim1. 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.
Singh SP, Kukshal V, Galletto R