The Escherichia coli single-strand DNA binding protein (SSB) is essential to viability where it functions to regulate SSB interactome function. Here it binds to single-stranded DNA and to target proteins that comprise the interactome. The region of SSB that links these two essential protein functions is the intrinsically disordered linker. Key to linker function is the presence of three, conserved PXXP motifs that mediate binding to oligosaccharide-oligonucleotide binding folds (OB-fold) present in SSB and its interactome partners. Not surprisingly, partner OB-fold deletions eliminate SSB binding. Furthermore, single point mutations in either the PXXP motifs or, in the RecG OB-fold, obliterate SSB binding. The data also demonstrate that, and in contrast to the view currently held in the field, the C-terminal acidic tip of SSB is not required for interactome partner binding. Instead, we propose the tip has two roles.First, and consistent with the proposal of Dixon, to regulate the structure of the C-terminal domain in a biologically active conformation that prevents linkers from binding to SSB OB-folds until this interaction is required. Second, as a secondary binding domain. Finally, as OB-folds are present in SSB and many of its partners, we present the SSB interactome as the first family of OB-fold genome guardians identified in prokaryotes. K E Y W O R D SDNA helicase, OB-fold, PXXP motif, RecG, SH3 domain, single-strand binding protein, SSB interactome

Progressive fibrosis is a pathological hallmark of many chronic diseases responsible for organ failure. Although there is currently no therapy on the market that specifically targets fibrosis, the dynamic fibrogenic process is known to be regulated by multiple soluble mediators that may be therapeutically intervened. The failing hamster heart exhibits marked fibrosis and increased expression of secreted Frizzled-related protein 2 (sFRP2) amenable to reversal by mesenchymal stem cell (MSC) therapy. Given the previous demonstration that sFRP2-null mice subjected to myocardial infarction exhibited reduced fibrosis and improved function, we tested whether antibody-based sFRP2 blockade might counteract the fibrogenic pathway and repair cardiac injury. Cardiomyopathic hamsters were injected intraperitoneally twice a week each with 20 μg of sFRP2 antibody. Echocardiography, histology, and biochemical analyses were performed after 1 mo. sFRP2 antibody increased left ventricular ejection fraction from 40 ± 1.2 to 49 ± 6.5%, whereas saline and IgG control exhibited a further decline to 37 ± 0.9 and 31 ± 3.2%, respectively. Functional improvement is associated with a ∼50% reduction in myocardial fibrosis, ∼65% decrease in apoptosis, and ∼75% increase in wall thickness. Consistent with attenuated fibrosis, both MSC therapy and sFRP2 antibody administration significantly increased the activity of myocardial matrix metalloproteinase-2. Gene expression analysis of the hamster heart and cultured fibroblasts identified Axin2 as a downstream target, the expression of which was activated by sFRP2 but inhibited by therapeutic intervention. sFRP2 blockade also increased myocardial levels of VEGF and hepatocyte growth factor (HGF) along with increased angiogenesis. These findings highlight the pathogenic effect of dysregulated sFRP2, which may be specifically targeted for antifibrotic therapy.