The Study Of Structural Basis Of SOSS1Complex Assembly And Recognition Of SsDNA

DNA double-strand breaks (DSBs) are highly toxic and can cause genome rearrangement and cell death. Defective DSB repair can cause cancer, neurodegenerative disorders, immune deficiencies, cardiovascular disease and metabolic syndrome. One of the pathways to repair DSB is homologous recombination (HR), which involves the resection of DSBs to generate a3’-single-stranded DNA (ssDNA) overhang. ssDNA binding proteins(SSBs) are crucial for the protection of ssDNA and DSB signaling. Studies on them would provide fundamental insights into the mechanism of DSB repair. Recently, two ssDNA binding proteins SOSSB1(hSSB1) and SOSSB2(hSSB2) were reported to function at DSBs to form two separate heterotrimeric complexes with SOSSA and SOSSC, termed SOSSl and SOSS2, respectively. SOSS1and SOSS2sense ssDNA and promote DSB repair and checkpoint activation. However, until now, how SOSS1is assembled and recognizes ssDNA remains elusive. In this study, we reconstituted a SOSSl subcomplex containing the N-terminal half of SOSSA(SOSSAN) and full-length SOSSB1and SOSSC(designated as SOSSAN/B1/C) and determined the crystal structures of SOSSAN/B1/C in apo form and in complex with a35nt ssDNA as well as the structure of SOSSAN/B1in complex with a12nt ssDNA. These structures combined with functional analysis confirmed that SOSSA acts as a scaffold to bridge the interaction between SOSSB1and SOSSC and is indispensable for their stability and nuclear localization in cells; SOSSB1mediates the interaction between SOSS1and ssDNA through binding to SOSSA and ssDNA via two distinct surfaces; Whereas SOSSAN and SOSSC are not essential for ssDNA binding. These results reveal the structural basis of the SOSS1complex assembly and serve as a cornerstone for further elucidating the mechanism underlying longer ssDNA recognition by SOSS1and the HR pathway related SOSS1.