Insight into the Mechanism of Action of Prokaryotic SSB Interactomes.

To maintain genome stability, the coordinated actions of multiple proteins and protein complexes, which are collectively known as genome guardians, are required. In prokaryotes, one such 20-member genome guardian family known as the single-stranded DNA binding protein (SSB) interactome exists. Proteins within this essential family contain oligonucleotide/oligosaccharide-binding folds (OB-fold). These structurally conserved OB-folds bind to the intrinsically disordered linkers characteristic of SSB protein C-termini, resulting in partner regulation. The mechanism of binding employed is similar to that utilized by Src homology 3 domain (SH3) proteins in eukaryotes. Binding requires the interaction of conserved PXXP motifs in the SSB linker with the OB-fold in the partner. A second region of SSB C-termini, an 8-10 stretch of predominantly acidic amino acids functions to maintain the linker domain in a biologically active conformation, while simultaneously preventing it from adhering to the OB-folds of the SSB tetramer from which it emanates. In addition, this acidic domain also functions as a secondary binding site docking with a distal site in the partner, stabilizing the linker/OB-fold interactions. The interaction of an SSB with its partner proteins is genus-specific and results in the loading of partners onto the genome at various stages of the cell cycle thereby maintaining genome stability.